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Problems and Solutions Section 2.1 (2.1 through 2.19)
2.1 The forced response of a single-degree-of-freedom, spring-mass system is modeled
by (assume the units are Newtons)
3
x(t) +12x(t) = 3cosωt 	
  
Compute the magnitude of the forced response for the two cases	
  ω	
  =2.1	
  rad/s and	
  ω	
  = 2.5
rad/sec. Comment on why one value is larger then the other.
Solution:
First divide through by the mass, 3 kg, to reveal the natural frequency.

x(t) + 4x(t) = cosωt 	
  
Thus f0 = 1 and ωn = 2 rad/s . Hence the magnitude of the forced response for ω	
  
=2.1	
  rad/s is given by equation (2.6):
X =
f0
ωn
2
− ω2
=
1
4 − 2.12
= −2.239 m
For 	
  ω	
  = 2.5 rad/sec this becomes
X =
f0
ωn
2
− ω2
=
1
4 − 2.52
= −0.444 m
	
  
The	
  value	
  of	
  the	
  magnitude	
  of	
  the	
  response	
  at	
  a	
  driving	
  frequency	
  of	
  2.5	
  
rad/s	
  is	
  much	
  smaller	
  then	
  that	
  at	
  2.1	
  rad/s	
  because	
  it	
  is	
  much	
  farther	
  away	
  
from	
  the	
  resonance	
  condition.
2.2 Consider the forced response of a single-degree-of-freedom, spring-mass system that
is modeled by (assume the units are Newtons)
3
x(t) +12x(t) = 3cosωt
Compute the total response of the system if the driving frequency is 2.5 rad/s and the
initial position and velocity are both zero.
Solution: Because the total response is asked for this problem differs from
problem 2.1, which only asks for the magnitude of the forced response. The total
response is given by equation 2.11 to be:
x t
( )=
v0
ωn
sin ωnt + x0 −
f0
ωn
2
− ω2
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
cos ωnt +
f0
ωn
2
− ω2
cos ωt
	
  
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
which for zero initial velocity and position becomes:
x t
( )= −
f0
ωn
2
− ω2
cos ωnt +
f0
ωn
2
− ω2
cos ωt
= 0.444cos2t − 0.444cos2.5t 	
  
2.3 To familiarize yourself with the nature of the forced response, plot the total response
of m
x t
( )+ kx t
( )= F 0 cosωt with ω = 2 rad/s, for a variety of values of the initial
conditions, values of f0 and values of ωn as given in the following chart:
	
  
Case	
   x0 	
   v0 	
   f0 	
   ωn	
  
1	
   0.1	
   0.1	
   0.1	
   1	
  
2	
   -­‐0.1	
   0.1	
   0.1	
   1	
  
3	
   0.1	
   0.1	
   1.0	
   1	
  
4	
   0.1	
   0.1	
   0.1	
   2.1	
  
5	
   1	
   0.1	
   0.1	
   1	
  
	
  
Solution: The total response is given by equation (2.11):
	
  
x t
( )=
v0
ωn
sin ωnt + x0 −
f0
ωn
2
− ω2
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
cos ωnt +
f0
ωn
2
− ω2
cos ωt 	
  
Insert the values of	
   x 0 ,	
  v0 ,	
  f0 ,	
  and	
  ωn for each of the five cases.	
  
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
2.4 Compute the response of a spring–mass system modeled by equation (2.2) to a force
of magnitude 23 N, driving frequency of twice the natural frequency, and initial
conditions given by x0 = 0 m and v0 = 0.2 m s. The mass of the system is 10 kg, the
spring stiffness is 1000 N m and the mass of the spring is considered and known to be 1
kg. What percent does the natural frequency change if the mass of the spring is not taken
into consideration?
Solution: Given: m sp = 1 kg, Example 1.4.4 yields that the effective mass is
m e = m +
msp
3
= 10 +
1
3
= 10.333 kg.
Thus the natural frequency, X and the coefficients in equation (2.11) for the
system now become
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
ωn
=
1000
10 + 1
3
= 9.837 rad/s, ω = 2ωn
=19.675 rad/s
X =
f0
ωn
2
−ω2
=
F0
/ me
ωn
2
−ω2
=
2.2258
9.8372
−19.6752
= −0.0077 m,
v0
ωn
= 0.02033 m
Thus the response as given by equation (2.11) is
x(t) = 0.02033sin9.837t + 7.7×10−3
(cos9.837t − cos19.675t) m
The frequency without the spring’s mass consider is given in example 2.1.1 as 10
rad/s. So the percent change by including the spring’s mass is
9.837 −10
10
⋅100 = −1.63%
So the mass of the spring reduced the natural frequency by 1.63 %.
2.5 Show that the solution x(t) =
f0
ωn
2
−ω2
[cos ω t – cos ωn t] can be written
x(t) =
f0
2(ωn
2
−ω2
)
sin
ωt +ωnt
2
sin
ωnt −ωt
2
.
Solution: Using the trig identity
cosu − cosv = 2sin
v + u
2
sin
v − u
2
Now let u = ωt and v = ωnt and substitute to get that
cosωt − cosωnt = 2sin
ωt +ωnt
2
sin
ωnt −ωt
2
Multiplying	
  by	
  
f0
ωn
2
−ω2
	
  then	
  yields	
  the	
  equation	
  (2.13).
2.6 A spring-mass system is driven from rest harmonically such that the displacement
response exhibits a beat of period of 0.2π s. The period of oscillation is measured to be
0.02π s. Calculate the natural frequency and the driving frequency of the system.
Solution: Given: Beat period: T
b
= 0.2π s.
The beat frequency is then
Tb =
2π
ωb
⇒ωb =
2π
Tb
=
2π
0.2π
= 10 rad/s = ωn −ω (1)
The period of oscillation is T0
= 0.02π s so that
T0 =
2π
ω0
⇒ω0 =
2π
T0
=
2π
0.02π
= 100 =
ωn +ω
2
⇒ωn +ω = 200 rad/s (2)
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
Solving these two equations in two unknowns yields: ωn = 105 rad/s and ω = 95 rad/s
2.7 An airplane wing modeled as a spring-mass system with natural frequency 40 Hz is
driven harmonically by the rotation of its engines at 39.9 Hz. Calculate the period of the
resulting beat.
Solution: Given: ωn = 2π (40) = 80π rad/s, ω = 2π (39.9) = 79.8π rad/s
Beat period: T b =
4π
ωn −ω
=
4π
80π − 79.8π
= 20 s.
2.8 Compute the total response of a spring-mass system with the following values: k 1000
N/m, m = 10 kg, subject to a harmonic force of magnitude F 0 = 100 N and frequency of
8.162 rad/s, and initial conditions given by x0 = 0.01 m and v 0 = 0.01 m/s. Plot the
response.
Solution: Given: k = 1000 N/m, m = 10 kg, F0=100 N, ω = 8.162 rad/s
x0=0.01m,	
  v0=0.01	
  m/s	
  
From	
  Eq.	
  (2.11):	
  	
  	
  
x(t) =
v0
ωn
sin ωnt + (x0 −
f0
ωn
2
−ω2
)cos ωnt +
f0
ωn
2
−ω2
cosωt 	
  
ωn =
k
m
=
1000
10
=10 rad / s 	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
   f0 =
F0
m
=
100
10
=10 N / m 	
  
In	
  Mathcad	
  the	
  solution	
  is	
  
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
 
	
  
	
  
2.9 Consider the system in Figure P2.9, write the equation of motion and calculate the
response assuming a) that the system is initially at rest, and b) that the system has
an initial displacement of 0.05 m.
Figure P2.9
Solution:	
  	
  The	
  equation	
  of	
  motion	
  is	
  
m 
x + k x =10sin10t 	
  
2,000 N/m
100 kg
x (t)
Friction-
free
surface
10 sin 10t N
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
Let	
  us	
  first	
  determine	
  the	
  general	
  solution	
  for	
  

x +ωn
2
x = f0 sinω t 	
  
Replacing the cosine function with a sine function in Eq. (2.4) and following the
same argument, the general solution is:
x(t) = A1 sinωnt + A2 cosωnt +
f0
ωn
2
−ω2
sinωt 	
  
Using	
  the	
  initial	
  conditions,	
   x(0) = x0 	
  and	
   
x(0) = v0 ,	
  a	
  general	
  expression	
  for	
  
the	
  response	
  of	
  a	
  spring-­‐mass	
  system	
  to	
  a	
  harmonic	
  (sine)	
  excitation	
  is:	
  
x(t) = (
v0
ωn
−
ω
ωn
⋅
f0
ωn
2
−ω2
)sinωnt + x0 cosωnt +
f0
ωn
2
−ω2
sinωt 	
  
Given:	
  	
  k=2000	
  N/m,	
  m=100	
  kg,	
  ω=10	
  rad/s,	
  
ωn
=
k
m
=
2000
100
= 20 rad/s = 4.472 rad/s	
  	
  	
  	
  	
  	
  	
  	
  	
  	
   f0 =
F0
m
=
10
100
= 0.1N/kg 	
  
a) x0	
  =	
  0	
  m,	
  v0	
  =	
  0	
  m/s	
  
Using	
  the	
  general	
  expression	
  obtained	
  above:	
  
x(t) = (0 −
10
20
⋅
0.1
20
2
−102
)sin 20t + 0 +
0.1
20
2
−102
sin10t 	
  
= 2.795×10−3
sin4.472t −1.25×10−3
sin10t 	
  
b) x0	
  =	
  0.05	
  m,	
  v0	
  =	
  0	
  m/s	
  
x(t) = (0 −
10
20
⋅
0.1
20
2
−102
)sin 20t + 0.05cos 20t +
0.1
20
2
−102
sin10t 	
  
= 0.002795sin4.472t + 0.05cos4.472t − 0.00125sin10t
= 5.01×10−2
sin(4.472t +1.515) −1.25×10−3
sin10t
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
2.10 Consider the system in Figure P2.10, write the equation of motion and calculate
the response assuming that the system is initially at rest for the values k1 =100
N/m, k2 = 500 N/m and m = 89 kg.
m 10 sin 10t N
k1 k2
Figure	
  P2.10	
  
Solution:	
  The	
  equation	
  of	
  motion	
  is	
  
m
x(t) + kx(t) = 10sin10t 	
   where	
   k =
1
1
k1
+
1
k2
	
  
The	
  general	
  expression	
  obtained	
  for	
  the	
  response	
  of	
  an	
  underdamped	
  spring-­‐
mass	
  system	
  to	
  a	
  harmonic	
  (sine)	
  input	
  in	
  Problem	
  2.7	
  was:	
  
x(t) = (
v0
ωn
−
ω
ωn
⋅
f0
ωn
2
−ω2
)sinωnt + x0 cosωnt +
f0
ωn
2
−ω2
sinωt 	
  
Substituting	
  the	
  following	
  values	
  
k	
  =	
  1/(1/100+1/500)=	
  83.333	
  N/m,	
  	
  	
  m	
  =	
  89	
  kg	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  ω	
  =	
  10	
  	
  rad/s	
  
ωn =
k
m
=
83.333
89
= 0.968 rad/s 	
  	
  	
  	
  	
  	
  	
  	
   f0 =
F0
m
=
10
89
= 0.112 N / kg 	
  
and	
  initial	
  conditions:	
  x0	
  =	
  0,	
  v0	
  =	
  0	
  
The	
  response	
  of	
  the	
  system	
  is	
  evaluated	
  as	
  
x(t) = 0.0117sin0.968t − 0.00113sin10t 	
  
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
2.11 Consider the system in Figure P2.11, write the equation of motion and calculate
the response assuming that the system is initially at rest for the values θ = 30°, k
= 1000 N/m and m = 50 kg.
Figure P2.11
	
   Solution:	
  Free	
  body	
  diagram:	
  
Assuming	
  x	
  =	
  0	
  to	
  be	
  at	
  equilibrium:	
  
Fx = m
x = −k(x + Δ) + mgsinθ
∑ + 90sin25t 	
   	
   	
   (1)	
  
where	
  	
  Δ	
  is	
  the	
  static	
  deflection	
  of	
  the	
  spring.	
  	
  From	
  static	
  equilibrium	
  in	
  the	
  x	
  
direction	
  yields	
  
−kΔ + mgsinθ 	
   	
   	
   	
   	
   (2)	
  
Substitution	
  of	
  (2)	
  onto	
  (1),	
  the	
  equation	
  of	
  motion	
  becomes	
  
m 
x + k x = 90sin2.5t 	
  
The	
  general	
  expression	
  for	
  the	
  response	
  of	
  a	
  mass-­‐spring	
  system	
  to	
  a	
  
harmonic	
  (sine)	
  excitation	
  (see	
  Problem	
  2.7)	
  is:	
  
x(t) = (
v0
ωn
−
ω
ωn
⋅
f0
ωn
2
−ω2
)sinωnt + x0 cosωnt +
f0
ωn
2
−ω2
sinωt 	
  
Given: v0 = 0, x0 = 0, ω = 2.5 rad/s
ωn =
k
m
=
1000
50
= 20 = 4.472 rad/s , f0 =
F0
m
=
90
50
=
9
5
N/kg
k
m
!
90 sin 2.5t N
m	
  
x	
  
mg	
  sin	
  θ	
  
F=90	
  sin	
  2.5	
  t	
  
Fs	
  
(Forces that are normal
to the x direction are
neglected)
θ	
  
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from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
So the response is:
x(t) = −0.0732sin4.472t + 0.1309sin2.5t
2.12 Compute the initial conditions such that the response of :
m 
x + kx = F 0 cosω t
oscillates at only one frequency (ω ).
	
  
Solution:	
  	
  From	
  Eq.	
  (2.11):	
  
x(t) =
v0
ωn
sin ωnt + (x0 −
f0
ωn
2
−ω2
)cos ωnt +
f0
ωn
2
−ω2
cosωt 	
  
For	
  the	
  response	
  of	
  m 
x + k x = F0 cos ω t 	
  to	
  have	
  only	
  one	
  frequency	
  content,	
  
namely,	
  of	
  the	
  frequency	
  of	
  the	
  forcing	
  function,	
  ω,	
  the	
  coefficients	
  of	
  the	
  first	
  
two	
  terms	
  are	
  set	
  equal	
  to	
  zero.	
  	
  This	
  yields	
  that	
  the	
  initial	
  conditions	
  have	
  to	
  
be	
  
x0 =
f0
ωn
2
−ω2
	
  	
  	
  and	
  	
  	
  v0 = 0 	
  
Then	
  the	
  solution	
  becomes	
  
x(t) =
f0
ωn
2
−ω2
cosωt 	
  
	
  
2.13	
   The	
  natural	
  frequency	
  of	
  a	
  65-­‐kg	
  person	
  illustrated	
  in	
  Figure	
  P.13	
  is	
  
measured	
  along	
  vertical,	
  or	
  longitudinal	
  direction	
  to	
  be	
  4.5	
  Hz.	
  	
  a)	
  What	
  is	
  the	
  
effective	
  stiffness	
  of	
  this	
  person	
  in	
  the	
  longitudinal	
  direction?	
  b)	
  If	
  the	
  person,	
  
1.8	
  m	
  in	
  length	
  and	
  0.58	
  m2	
  in	
  cross	
  sectional	
  area,	
  is	
  modeled	
  as	
  a	
  thin	
  bar,	
  
what	
  is	
  the	
  modulus	
  of	
  elasticity	
  for	
  this	
  system?	
  
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Figure	
  P2.13	
  Longitudinal	
  vibration	
  of	
  a	
  person	
  
Solution:	
  a)	
  First	
  change	
  the	
  frequency	
  in	
  Hz	
  to	
  rad/s:	
  
ωn
= 4.5
cycles
s
2π rad
cycles
= 9π rad/s.	
  Then	
  from	
  the	
  definition	
  of	
  natural	
  
frequency:	
  
k = mωn
2
= 65⋅(9π)2
= 5.196 ×104
N/m 	
  
b)	
  From	
  section	
  1.4,	
  the	
  value	
  of	
  the	
  stiffness	
  for	
  the	
  longitudinal	
  vibration	
  of	
  
a	
  beam	
  is	
  
k =
EA

⇒ E =
k
A
=
5.196 ×104
( )(1.8)
0.58
=1.613×105
N/m2
=1.613×105
Pa 	
  
	
  
2.14	
   If	
  the	
  person	
  in	
  Problem	
  2.13	
  is	
  standing	
  on	
  a	
  floor,	
  vibrating	
  at	
  4.49	
  Hz	
  with	
  
an	
  amplitude	
  of	
  1	
  N	
  (very	
  small),	
  what	
  longitudinal	
  displacement	
  would	
  the	
  
person	
  “feel”?	
  Assume	
  that	
  the	
  initial	
  conditions	
  are	
  zero.	
  	
  
Solution:	
  Using	
  equation	
  (2.12)	
  for	
  a	
  cosine	
  excitation	
  and	
  zero	
  initial	
  
conditions	
  yields	
  (converting	
  the	
  frequency	
  from	
  Hertz	
  to	
  rad/s	
  and	
  using	
  
the	
  value	
  of	
  k	
  calculated	
  in	
  2.11):	
  
X =
F0
m
1
ωn
2
−ω2
=
1
65
1
k
m
− (4.49⋅2π)2
=
1
65
1
5.196 ×104
65
− (4.49⋅2π)2
= 0.00443347 = 0.0043 m
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
 
2.14	
  	
   Vibration	
  of	
  body	
  parts	
  is	
  a	
  significant	
  problem	
  in	
  designing	
  machines	
  and	
  
structures.	
  	
  A	
  jackhammer	
  provides	
  a	
  harmonic	
  input	
  to	
  the	
  operator’s	
  arm.	
  	
  
To	
  model	
  this	
  situation,	
  treat	
  the	
  forearm	
  as	
  a	
  compound	
  pendulum	
  subject	
  
to	
  a	
  harmonic	
  excitation	
  (say	
  of	
  mass	
  6	
  kg	
  and	
  length	
  44.2	
  cm)	
  as	
  illustrated	
  
in	
  Figure	
  P2.14.	
  Consider	
  point	
  O	
  as	
  a	
  fixed	
  pivot.	
  Compute	
  the	
  maximum	
  
deflection	
  of	
  the	
  hand	
  end	
  of	
  the	
  arm	
  if	
  the	
  jackhammer	
  applies	
  a	
  force	
  of	
  10	
  
N	
  at	
  2	
  Hz.	
  	
  
	
  
Figure	
  P2.14	
  Vibration	
  model	
  of	
  a	
  forearm	
  driven	
  by	
  a	
  jackhammer	
  
	
   	
  
Solution:	
  Taking	
  moments	
  about	
  point	
  O	
  yields	
  (referring	
  to	
  Example	
  1.4.6	
  
for	
  the	
  inertial	
  of	
  a	
  compound	
  pendulum):	
  
m2
3

θ + mg

2
sinθ = FO
cosθ cosωt 	
  
Using	
  the	
  linear	
  approximation	
  for	
  sine	
  and	
  cosine	
  and	
  dividing	
  through	
  by	
  
the	
  inertia	
  yields:	
  

θ +
3g
2
θ =
3FO
m
cosωt 	
  
Thus	
  the	
  natural	
  frequency	
  is	
  
ωn
=
3g
2
=
3(9.81)
2(0.442)
= 5.77 rad/s (=0.92 Hz) 	
  
and	
  the	
  system	
  is	
  well	
  away	
  from	
  resonance.	
  	
  Referring	
  to	
  equation	
  (2.13),	
  
the	
  amplitude	
  for	
  zero	
  initial	
  conditions	
  is	
  (converting	
  the	
  driving	
  frequency	
  
from	
  2	
  Hertz	
  to	
  2(2π)	
  rad/s):	
  
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
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θ =
2 f0
ωn
2
−ω2
=
2
3F0

m2
⎛
⎝
⎜
⎞
⎠
⎟
3g
2
− (2⋅2π)2
= 0.182 rad 	
  
Note	
  that	
  sin(0.182)	
  =	
  0.181	
  so	
  the	
  approximation	
  made	
  above	
  is	
  valid.	
  	
  The	
  
maximum	
  linear	
  displacement	
  of	
  the	
  hand	
  end	
  of	
  the	
  arm	
  	
  is	
  just	
  
	
  
X = r θ = 0.442⋅0.182 = 0.08 m 	
  
	
  
	
  
	
  
2.15	
   An	
  airfoil	
  is	
  mounted	
  in	
  a	
  wind	
  tunnel	
  for	
  the	
  purpose	
  of	
  studying	
  the	
  
aerodynamic	
  properties	
  of	
  the	
  airfoil’s	
  shape.	
  	
  A	
  simple	
  model	
  of	
  this	
  is	
  
illustrated	
  in	
  Figure	
  P2.15	
  as	
  a	
  rigid	
  inertial	
  body	
  mounted	
  on	
  a	
  rotational	
  
spring,	
  fixed	
  to	
  the	
  floor	
  with	
  a	
  rigid	
  support.	
  Find	
  a	
  design	
  relationship	
  for	
  
the	
  spring	
  stiffness	
  k	
  in	
  terms	
  of	
  the	
  rotational	
  inertia,	
  J,	
  the	
  magnitude	
  of	
  the	
  
applied	
  moment,	
  M0,	
  and	
  the	
  driving	
  frequency,	
  ω,	
  that	
  will	
  keep	
  the	
  
magnitude	
  of	
  the	
  angular	
  deflection	
  less	
  then	
  5°.	
  	
  Assume	
  that	
  the	
  initial	
  
conditions	
  are	
  zero	
  and	
  that	
  the	
  driving	
  frequency	
  is	
  such	
  that	
  ωn
2
−ω2
> 0 .	
  
	
  
Figure	
  P2.15	
  Vibration	
  model	
  of	
  a	
  wing	
  in	
  a	
  wind	
  tunnel	
  
Solution:	
  Assuming	
  compatible	
  units,	
  the	
  equation	
  of	
  motion	
  is:	
  
J 
θ(t) + kθ(t) = M0
cosωt ⇒ 
θ(t) +
k
J
θ(t) =
M0
J
cosωt 	
  
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from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
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From	
  equation	
  (2.12)	
  the	
  maximum	
  deflection	
  for	
  zero	
  initial	
  conditions	
  is	
  
θmax
=
2M0
J
k
J
−ω2
< 5°
πrad
180°
=
π
36
rad
⇒
2M0
J
< (
k
J
−ω2
)
π
36
rad ⇒
36J
π
2M0
J
+
πω2
36
⎛
⎝
⎜
⎞
⎠
⎟ < k
	
  
2.16 The spar of an airplane wing is a relatively rigid beam extending along the length of
the wing inside the wing to provide strength. It is typical to model a spar as a cantilever
beam with the fixed end at the body of the aircraft. An example is given in Figure P2.16.
Using the modeling methods given in Section 1.5 determine a single-degree-of-freedom
model for the spar and compute its natural frequency. The spar here is modeled as a
cantilever beam of dimensions length 560 mm, width 38 mm and thickness 3.175 mm,
and has a mass of 13.975 grams. The beam’s Young's modulus is 10.29 GPa and its
shear modulus: 1.65 GPa.
MAKE THIS A BEAM MODELD AS A SDOF SM SYSTEM
Figure P2.16 A small, unmanned air vehicle with a rigid spar, modeled as a beam
Solution: This problem requires students to think back to Section 1.5 where an SDOF
model of a beam is given. The stiffness of the beam is given by equation (1.76) and in
Figure 1.26 as
k =
3EI
l3
where E is the beams elastic modulus, l is the length and I is the area moment of inertia of
a beam in the bending direction given by
I =
(thickness)(width)3
12
=
hb3
12
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from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying,
recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
This can be found on google or a strength of materials text. The formula for frequency
now requires the mass which will causes some students confusion. Looking at Figure
1.28, the effective mass of a system with a tip mass and substantial beam mass is results
in Equation (1.76) for the frequency:
ωn =
k
133
140
M + m
Combining these three expressions and letting the tip mass, m, be zero yields the
following expression for the frequency
ωn =
3E
l3
⋅
hb3
12
33
140
M
=
1.65×109
N/m2
(0.560m)3
⋅
(0.003175)(0.038)3
4
m4
33
140
⋅13.975×10−3
kg
= 352.459 rad/s ≈ 56.1 Hz
2.17 Compute the response of a shaft and disc system to an applied moment of
M = 10 sin 312 t
as indicated in Figure P2.17. Assume that the shaft is initially at rest (zero initial
conditions) and J = 0.5 kg m2
, the shear modulus is G = 8 × 1010
N/m2
, the shaft is 1 m
long, of diameter 5 cm and made of steel.
Figure P2.17
Solution: Summing moments the equation of motion is:
0.5
θ(t) +
GJp
l
θ(t) = 10sin312t

θ(t) +
GJp
0.5l
θ(t) = 20sin312t
The natural frequency (see also Example 1.5.1) is
ωn =
k
J
=
GJP
lJ
=
8×1010
N m
2
( ) π
32
5×10−2
m
( )
4
⎡
⎣
⎢
⎤
⎦
⎥
1 m
( ) 0.5 kg ⋅ m2
( )
= 313.33 rad/s
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settled down about them in the sunshine, Jos sat down willingly by
Stefeli. He first of all admired the neatly spread repast before him,
for he had never seen a meal set out so perfectly outdoors. The
swaying branches above gave them the needed shade.
“I am sure no one else could spread a table as well as you can,
Stefeli,” he said, full of admiration. Then he began to eat everything
Stefeli offered him with the heartiest appetite.
The afternoon passed as quickly as the morning, and when they
were strolling homewards in the bright evening light, Stefeli said,
“Oh, it was lovely today. I hope all the other days will be exactly like
it.”
“I hope so, too,” repeated Jos.
Vinzenz Lesa stood at his barn and watched his herd coming home.
Jos was running alongside of the cows, first on one side and then on
the other. This kept them walking in a neat row, instead of running
first here, then there.
“A quick, orderly boy,” said Vinzenz Lesa to himself, as he slowly met
the procession on its way to the stable.
A series of perfect days followed, and the children had the happiest
times. Stefeli’s wish seemed to be fulfilled, for she fairly beamed
with joy and well-being all day long, and Jos sang and yodelled
louder every day from sheer happiness.
“It’s just as if Vinzi were back since Jos is with us. Don’t you think
so, mother?” said Stefeli, coming home one day with cheeks flushed,
partly from delight and partly from the bright light of the setting sun.
Jos as usual had stayed outside with his uncle.
“Yes,” replied the mother. “As long as we can’t have Vinzi, I am glad
Jos takes his place. I like Jos as much as if he belonged to us.”
“I, too,” said Stefeli without hesitation. “But there are three times
when I had a better time with Vinzi. In the morning, in the evening,
and on rainy days. It was much nicer then when Vinzi was at home.”
The mother well understood what Stefeli alluded to, but she asked
the child to let Jos go on his way, as he was doing his duty and it
would be wrong of her to hinder him in that.
“I have tried already, but it does no good,” said Stefeli frankly.
She was forbidden to do so again. The mother was only too glad and
grateful to see how Jos occupied himself at those times, even if
Stefeli was displeased. He did so of his own free will, and she had
noticed that it had actually improved her husband’s temper.
Jos was always the first up in the morning, and if the stable, which
was his favorite resort, was still closed, he thought of something
which had to be set in order in the barn. In this way Vinzenz would
find him busy with hammering or mending something. The boy
always came to breakfast at the very last minute when the coffee
was put upon the table. He didn’t even notice how impatiently Stefeli
was waiting for him. In the evening after their return Jos could not
be lured away from his cows till the last one had returned from
watering and was comfortably bedded on the straw.
The mother had always set the steaming pot on the table before he
came in. Stefeli never could count on running over to the wild
strawberry bed with Jos, therefore.
On rainy days Jos always disappeared entirely. He knew in the early
morning what work had to be done that day in the fields, the hills,
the woods or on the trees. When it rained, he would quickly ask his
uncle, “Can I go with the man today?”
The other always agreed, “Why not, if you don’t mind the rain?”
Rain didn’t bother Jos at all. Everything that grew and brought in
fruit interested him so much that he wanted to see it and have a
hand in it if possible. Best of all, he liked it when the wood which
had been felled had to be brought home from the forest. Then the
horse was hitched to a large lumber wagon and Jos sat on the high
seat beside the hired man. As soon as they had left the house
behind them the man would say, “There, take them if you want to
drive.” Giving Jos the reins, he stretched out full length in the wagon
to sleep a little longer. Jos knew the horse well and the horse knew
him, so the matter went very smoothly. Jos had already helped in
every kind of farm work and knew exactly when things were in order
or when something was amiss. He took hold of everything with such
spirit and eagerness that he acquired an instinct for what was
needed when matters went wrong.
In the beginning Vinzenz Lesa had said to his wife in a bitter
tone,“Just look how he enjoys everything! He works with such an
eagerness, as if it were his own. Look at the pleasure he finds in it
all! And my own boy, for whom we work, has never even seen it.”
But everything had turned out to make Mrs. Lesa happy. Her
husband spoke every day in a different tone about Jos. He could say
quite cheerfully now: “The boy has four eyes in his head. He
discovers what I overlook myself and the man would never have
seen. He actually sees everything. I can entirely depend on him as
on no one else, and he is so handy that it is a pleasure to see him
work.”
Mrs. Lesa saw her husband’s growing delight in their young nephew
with hearty feelings of gratitude. He was in a much better humor
than he had been for years. When during the afternoon she was
sitting quietly and undisturbed in her room, her thoughts unerringly
dwelt upon Vinzi and she pondered about his future. She wondered
if he would return after a year and take up the life his father wished
him to, in which case he would never be really happy. Or if he had
definitely chosen a new path for himself, which would forever
estrange him from his father. She lost herself in imaginary
conversations with her son, and the slightest noise made her start
up, as she would think that her boy might have suddenly returned.
So it happened one day when her thoughts had been busy with Vinzi
that the housedoor was opened and the noise of several footsteps
neared the room. Mrs. Lesa had jumped up and running to the door
had quickly opened it.
“Old friends, who are looking for shelter in your house,” said a
vigorous man’s voice.
That same instant a slight hand grasped hers, then another. Only
then could she really see them.
“Welcome! Welcome, Alida, Hugo! Welcome, Mr. Thornau!” she
cried, heartily delighted. “Are you staying in our parts again?”
“Not exactly, but we are here for the present,” said Mr. Thornau,
returning her friendly handshake. “We’ll stay here, if you will receive
us. But if you say no, we’ll have to wander home again. Before
everything else, I must tell you that I am bringing you two children
who have lost their mother. They have asked me to come here, for
nowhere else will suit them.”
Mrs. Lesa was won over already. “The poor children,” she said to
herself, “and the poor father,” she added. Then she turned to the
latter.
“Mr. Thornau,” she said, “won’t you stay here till evening when my
husband comes home? Then we can talk it over.”
“And what does his wife say?” asked Mr. Thornau.
“Oh, she would love to take their mother’s place for a time, if she
could,” replied Mrs. Lesa.
“I am glad!” he exclaimed with satisfaction.
He was willing to wait for Mr. Lesa and settled meanwhile under the
shady walnut trees with his children.
Vinzenz Lesa looked with surprise at the guests he found waiting
before his house. But there was not much time for wondering,
because Mr. Thomau had gone up to him and eloquently informed
him of the reason for this visit. He was sure Mr. Lesa would not shut
his door against him and especially his children, who had declared
that they would not spend their summer anywhere in the world
except at Mr. Lesa’s residence. He had come with the firm hope that
Mr. Lesa would not turn them away and begged him not to propose
a house in the neighborhood they already knew, as the children
simply would not go back there. He had no intention, either, of
forcing them to it. It meant either Mr. Lesa’s house or going home
again. As it was impossible for him to stay with the children just
then, he could not leave them anywhere else alone.
Vinzenz Lesa was not displeased at hearing the gentleman’s words—
either his house or none. But still, one could not tell what these
children might bring into his home, for he hardly knew the people.
After reflecting a while he answered, “It falls to my wife; looking
after children is her affair.”
This suited him best. If his wife wanted to keep them, as he
suspected already, it would be his wife’s concern. If they got into
mischief, she would have to look out for them, for she had taken
them in.
“With pleasure,” replied Mr. Thornau. Quickly turning to Mrs. Lesa he
said, “Everything is settled, I suppose, for I think you have already
said yes in your heart.” Mrs. Lesa heartily shook Mr. Thornau’s hand,
for he had been right. She had long ago decided to keep the
motherless children and show them all the affection she was capable
of.
Mr. Thornau was anxious to take immediate leave, as he had
planned to journey on as soon as he knew his children were in good
hands. Then Mrs. Lesa consulted him about their life, what their
occupations and pleasures were to be. Her own children had always
led the most simple life, she told him. Mr. Thornau wished for
nothing better than to give them the life hers were leading. To be up
on the pasture from morning till evening would be the best for them
he could hope for. The rest of the time Mrs. Lesa could judge for
herself.
To his great disappointment Alida’s music lessons had been given up.
He had always enjoyed her playing. His chief wish was to see them
both come back to him sunburnt and with blooming cheeks, just as
her own lad looked.
Taking Mrs. Lesa a little apart, pointing to his sad-looking, pale boy,
he added, “I am sure I won’t have to specially recommend him to
your care. Just look at him! He never was very strong or happy, and
since he has lost his mother he is worse than ever. No life, no spirits,
no interest in anything! The doctor actually insisted on my sending
him away.”
Mr. Thornau quickly took farewell till the autumn, when he planned
to see them all.
Just then Jos and Stefeli, merrily chatting, came home with the
cows. Jos disappeared in the stable as usual and Stefeli slowly
approached the house. There was no hurry, for her mother was busy
in the kitchen and Jos would not come back yet for at least an hour.
Suddenly her slow steps turned into great leaps, for at the
strawberry hedge she had discovered two figures. Could it really be
true? And yet it was! With a cry of joy Stefeli ran over to Alida and
Hugo. Alida also greeted Stefeli with loud, continued expressions of
delight.
“But where is Vinzi?” asked Alida when the greetings were over.
Stefeli told her how long he had been away and that no one knew
when he would come back.
Alida looked extremely disappointed, but Stefeli always had a
consolation ready.
“I am sure he’ll come home while you are here. Jos is here, too, and
you’ll be sure to like him. Will you stay all summer long?”
Alida assented. “And we are to spend our days on the pasture,
because papa wants us to,” she added, “but it is a shame Vinzi won’t
be with us.”
Stefeli answered Alida that it was never dull up on the pasture. Her
father had prophesied a long stretch of good weather, and that
meant that they would be out of doors all day.
Mr. Lesa had been right. The merry little group could wander up day
after day to the sunny pasture, and Mrs. Lesa saw to it that a really
nourishing meal was always taken along. Alida was in perfect
raptures over this free life, hitherto unknown to her. What a blissful
beginning of the day to start off in the early morning, when she was
usually lying asleep in bed behind her heavy curtains! How delicious
the pure air was! All the birds in the trees sang and whistled so that
it sounded like a loud chorus of joy to God.
The two girls would start out together on little trips to explore the
interesting places on the large, wide pasture. Sometimes they would
seek berries or flowers; another time they went to the old wall
where the shining lizards sunned themselves, or listened attentively
when the children began to sing. Stefeli knew that if they were silent
or made the least movement, the little green creatures would quickly
slip away. Alida found it an especial treat to be able to sit down
anywhere on the sun-dried ground. She had not experienced this
before and it gave her constant pleasure. Stefeli was always ready to
settle down beside her, and everything furnished them with subjects
for lively conversation.
On the first morning, when Stefeli had promised to waken Alida
early, both girls stood fresh and full of enterprise before the barn,
waiting for Jos. He had to loosen the cows from their chains before
driving them out. Hugo had come down from his room, because his
father had wished him to go along to the pasture.
He looked so frail and tired that it hurt Mrs. Lesa to look at him. She
led him affectionately back to his room, and fixing the cushions on
his bed, told him to rest a little longer. There was no hurry for him to
go, for a little later on she would take him up and show him the way
herself. He would never have to start quite so early, if he did not
want to.
For the first time since he had lost his mother, Hugo felt himself
sheltered again by a mother’s affectionate care. From that day on a
great love for her began to fill his heart. She watched over him like a
mother and saw to it that everything was done for him that might do
him good. In these first days the quiet boy, who was still bearing a
great sorrow in his heart, spent many hours alone in Mrs. Lesa’s
company. He found great consolation in it and learned to feel such
confidence in her that he began to talk about his mother. She
listened with such sympathy that they always returned to that
subject when he was with her.
The comfort the boy found in her warm interest was soon apparent.
One day Hugo came down quite early into the gleaming sunshine.
He had never done it since that first day when he had looked so pale
and tired. He already seemed much stronger and to Mrs. Lesa’s joy
wished to go right up to the pasture. Till then he had preferred to sit
in the house till she sent him out and accompanied him part way.
Hugo found Jos alone on the pasture, singing and whistling while he
strolled about among the cows. Alida and Stefeli had gone on a little
trip of discovery. It seemed as if Hugo saw the beautiful creatures
who were grazing here and there, looking about them, for the first
time in his life. He began to ask Jos many questions, for after
watching them carefully he had noticed how much they differed in
their looks as well as in their ways. He had always thought that cows
were just cows, one like another. Jos was in his element now and
grew talkative, drawing Hugo’s attention to all the animals’ habits.
The subject proved so contagious that Hugo conceived a keen
interest in them and wanted to hear all about them. He only had to
ask to be told what he wished to know. Jos could describe them with
such keen vividness that Hugo grew most eager to share Jos’s
knowledge and to find pleasure in it. He soon knew what fodder was
the best for milk, which was made first into butter and then into
cheese, and how the milk had to be handled for that purpose. He
also learned that the Alpine herdsmen preferred Vinzenz Lesa’s milk
to any other because his cows were of the best stock and were so
immaculately kept.
The two were still talking eagerly when to their great astonishment
Stefeli came running toward them and spread out their mid-day
meal under the swaying ash tree. They had been so lost in their
conversation that they had not noticed how the time had flown. This
had suited Stefeli, too, because she had come back rather late from
her expedition. Alida also appeared and in the best of humor as the
four sat down to lunch. All of them felt especially merry, because
Hugo had never been so lively and gay.
“We’ll pretend we are a family,” suggested Alida, “and we must
always stay the same. Hugo is the proprietor of an estate and I am
his sister, the unmarried lady of the mansion. Jos is our manager.”
“Then Stefeli can be the mistress,” was Hugo’s proposition.
“But Hugo, she couldn’t be,” Jos cried out. “Stefeli cooks for us and
sets the table. She couldn’t be the gentleman’s wife, she must be my
wife.”
“Jos is right,” decided Alida, “we’ll have it that way, then.”
“You see the owner has a mother; that’s much better than a wife,”
said Hugo. “When Mrs. Lesa visits us some day, you’ll have to
receive her as my mother and prepare a great feast for her.”
This thought met great approval, and they began to plan
immediately for this feast. Alida invented such astonishing plans for
the celebration with torches and rockets that Jos said, “Then our
cows will all jump over the hedges from fright and the people at the
feast will have to climb after them in their festive jackets.”
All four burst into loud laughter at this picture, which brought a
sudden end to these extravagant plans. Their meal was ended and
they settled here and there in the shadows of the broad branches.
Soon all four were sound asleep, fanned by the leafy bows above
them. They slept as well as if they had been lying on regal couches.
On Hugo’s pale cheeks the air and sunshine, gently caressing him,
roused a faint rosy color.
Talking merrily, the whole company afterward came wandering down
from the pasture, aglow from the golden light of the setting sun. As
usual Jos went to the stable and Hugo disappeared with him.
“Oh, dear, now he begins it, too,” said Stefeli. “I only wonder what
he wants to do there still.”
“Just let him go,” remarked Alida, “I am glad of it. He is much
happier when he is with Jos. I have noticed that today.”
A
CHAPTER XI
THE OLD SONG ONCE MORE
UTUMN was here. In Lesa’s household it had come much too soon
for every one, and the children could not believe that their days
on the pasture were nearly over. Hugo and Alida would soon have to
go right into the heart of a large city far away. Jos would go home to
his mountain, while poor Stefeli would be left behind, unhappy and
alone.
“I don’t want to be always alone,” said Stefeli determinedly, when
they had discussed their prospects on one of their last days together.
“I’ll send a letter to Uncle Lorenz. You know he promised me
something.”
“I suppose Vinzi will soon come back,” said Alida, “and he’ll be
wonderful company for you. We would have had lots more fun if he
had been with us this summer. There were so many things I wanted
to talk to him about, things none of you could understand.”
The same day Vinzenz Lesa said to his wife, “I hope Lorenz won’t
come yet to fetch Jos home. It has been such a good year that we
still have our hands full. Things have to be done on every side and I
can’t do them without the boy. He has the best memory and such
good ideas; common sense, besides, enough for three, and a love
and interest in the work as if it were all for himself, instead of for us.
He just wants to keep the farm in good shape. I would give half the
place to have such a son. It would mean so much to me.”
“Let’s be glad he is still with us,” replied the wife, “and I think he’ll
stay yet for a while. Good Cousin Lorenz hasn’t even warned us yet.”
Mr. Delrick sent word from time to time, and a letter had come rather
lately with the usual good news. Vinzi was well and everything going
smoothly. Mr. Delrick seemed to be in no hurry for Vinzi’s return,
though the year of his absence was practically over.
A few days later when the children walked into the room one night a
large letter lay on the table.
“That’s from papa,” cried Alida, upon seeing the handwriting. It was
addressed to Mr. Lesa.
“Now everything is over,” said Hugo, who had come in also. “You’ll
find that we’ll have to go now, Alida.”
The children were quite frightened, for even while they had
discussed their departure, none of them had realized how near it
really was. Even Jos, who had joined them, made large, bewildered
eyes. He had never quite pictured to himself the end of their bliss,
and just lately least of all, when he and Hugo had been drawn so
closely to each other in friendship. Should this all be over for good
now, with their separation?
As soon as the father came in, he took up the letter, only to put it
aside again. He only read letters when there was nothing else to do,
and supper was more important. When Mrs. Lesa had brought it in
and had looked after every one’s needs, she asked her husband if
she should open the letter. She knew that the children, who had
recognized their father’s handwriting, were most anxious to hear it.
As he willingly agreed, she first read the message to herself. All she
told them, however, was that Mr. Thornau would arrive in a few days
to fetch his children home. They would hear about the rest the next
day. A deep silence followed. Soon the four children quietly stole
away from the table, one by one. They met outside under the large
walnut tree according to a previous plan. Here, under the old tree as
a staunch witness, they gave each other the solemn promise to do
everything in their power to reunite next summer. They longed to
spend as perfect a summer together again. When the children had
gone to bed and Vinzenz Lesa had lit his pipe, Mrs. Lesa sat down
beside him to read aloud Mr. Thornau’s letter. He was writing from
Dresden, where he had stopped a few days. He had gone there to
see Mr. Delrick, who had decided to join him on his trip to
Switzerland. This would give him an opportunity personally to bring
the parents news about their son. But, as Mr. Thornau’s time was
extremely limited, he unfortunately would have to take the last train
to Leuk on Sunday evening and leave again with the earliest train on
Monday. He therefore made them a proposal. If the Lesa family,
including their little daughter, would bring his children to Freiburg on
Sunday, they could all spend a pleasant day together in that city. As
Freiburg was Mr. Lesa’s former home, Mr. Thornau had a hope that it
was possible to persuade Mr. Lesa to go there. They might arrive
early on Sunday afternoon, and he and Mr. Delrick would meet them
at the train, as they expected to reach Freiburg earlier.
In conclusion he said that Mr. Lesa and his wife would, by doing him
this favor, crown their former kindness to his children. A refusal of his
expectations would mean no less to him than a hailstorm to Mr.
Lesa’s fields.
Vinzenz Lesa remained silent for quite a while. Then he asked, “Does
he write nothing about our boy?”
Mrs. Lesa answered that the only thing about Vinzi was what she had
read, namely, that Mr. Delrick was anxious to give them news about
the boy in person.
“Do you know why he doesn’t want to bring him?” asked Lesa
further.
“We can’t know that,” replied the wife.
“But we can guess, and I’ll tell you why. The reason is that Mr.
Delrick realizes that he mustn’t make him into a travelling musician.
He is in no hurry to bring him home, because he has turned his
thoughts further than ever from farming and he is afraid to tell me. I
know he is a good man, but he has made a mistake. Everybody
dislikes admitting such a thing.”
Vinzenz Lesa blew such thick clouds of smoke from his pipe that his
wife found it advisable to let them disperse a little. Then she said
cautiously, “I think we had better hear first what Mr. Delrick has to
say. It will be a blessing to be able to talk to him. What do you say
about going to Freiburg?”
“I wouldn’t think of going,” said her husband curtly. “You don’t
suppose I’ll go as far as that for no reason whatever. Why should I
go to Freiburg, as if I had nothing else to do?”
“Nobody who knows you could possibly think that,” said his wife.
“Besides, Mr. Thornau hasn’t fixed it on a Sunday for nothing. He
knows well enough you wouldn’t come during the week. He asks us
most politely, and it would have been easy for him to tell any other
person to fetch the children home. Apparently, he would value it very
much if we took the children to him. I must admit, Vinzenz, that I
have grown extremely fond of them. They have both clearly shown
me how they hate to leave. I’d love to go and put them into his
hands myself.”
“Why don’t you go with them, then?” he retorted, still showing an
obstinate disinclination in his voice. “You had better go to Freiburg.
You are sufficient escort for them.”
“No, Vinzenz, I won’t do it,” answered his wife with firm decision.
“You can see from the letter that Mr. Thornau wants you, too. You
also know that I couldn’t settle with Mr. Delrick what is further to be
done about the boy. We’ll either go together, or we won’t go at all. It
is for you to decide.”
The silence which followed proved too long for Mrs. Lesa and she
began again. “I don’t really see why we shouldn’t go to Freiburg as
long as they ask us to. Don’t you remember how delighted we used
to be as children when father and mother would take us there on a
Sunday? We were the happiest creatures on earth sitting in the high
carriage between them with such exciting adventures before us. We
really should do it for memory’s sake. We always went to church first
and heard the beautiful organ. Do you remember how your mother
insisted on doing this first of all? Wouldn’t it give you pleasure to go
back? It also would give you a splendid chance to run over to your
farm. You ought to have looked it over long ago, and you are sure to
find lots of things to attend to. Now that Jos is with us, you can leave
much more easily than afterward. You’ll have a lot of important
things to do there, and if nothing special takes you there, you’ll
never make up your mind to go. You had better take the journey
before something actually goes wrong and you might have reason to
blame yourself for not having gone before.”
The thought that the trip might have a good side had begun to take
hold of Vinzenz Lesa. Very deliberately, but still unwillingly, he
replied, “How did you think of all these things? I don’t see how we
could be away so long. It would take at least three days.”
“Not for everybody,” replied Mrs. Lesa promptly, for she had thought
out everything minutely. “We can easily attend to things here on
Sunday morning before leaving. We’ll be in Freiburg early in the
afternoon and that same evening I’ll travel home as far as I can with
the children, in order to get here as soon as possible on Monday
morning.”
“How many children do you expect to bring home? The two strangers
will have gone and you yourself have only one.”
This time the husband had found the words first.
“Only our two, Jos and Stefeli; I don’t mean to bring strangers
home,” his wife said calmly.
“What! You want to take Jos along, too?” said Vinzenz Lesa much
excited. “That would make it out of the question for me to go. If Jos
isn’t home to keep things in order, I won’t even budge from here!
Count on that.”
But Mrs. Lesa began to show him how easily their hired man could
look after the simple work in the stable which had to be done on
Sunday. Monday morning Jos would be back again. In this way
nothing would be neglected. “Just think, Vinzenz,” concluded his
wife, “what Jos has done for us. He has been with us a long while
and we have not done the slightest thing for him. I am sure it would
give him pleasure to go along. We owe it only to him if you have
time now to look over your other farm to see what has to be done
there. You can easily stay away a week, for I promise to look after
the farm if I have Jos, so you won’t need to grumble.”
Vinzenz Lesa was a just man, eager to act upon what he felt to be
right. He was glad to give Jos a well-earned holiday and to have the
opportunity besides of carrying out a needful undertaking. After
weighing the matter carefully, he finally declared, “All right, we’ll go
then, but tell Jos about it as soon as possible. He can plan things out
beforehand so everything will stay in order while he is gone.”
“Tomorrow,” said Mrs. Lesa, happy at having such pleasant prospects
to tell the children, especially her ever-willing Jos.
At the news of their coming expedition the children showed
unbounded delight. Alida and Stefeli jumped from sheer enthusiasm
and Hugo said, “Now everything isn’t quite over yet. It will be much
easier for us to go away if everybody comes along.”
Jos could hardly realize the treat that lay before him. It was too
wonderful to be taken to Freiburg and see the city and get a glimpse
of the fine country on the way. From joy and astonishment he could
scarcely say a word. It would be his first journey, for he had never
been away from his mountain except when he had come to Leuk.
Sunday arrived. The whole company travelled gaily across the
beautiful country which lay gleaming in the brightest sunshine. The
trip proved even more delightful than they had anticipated. Alida and
Stefeli never stopped talking, and Hugo kept busy either joining in
with them or pointing out the interesting things along the way, which
they would have overlooked on account of their constant chatter. Jos,
on the contrary, was so absorbed in the new, beautiful things on
every side that he never took his eyes from the landscape and
remained in the deepest silence, attentively gazing out.
Thus the hours flew by unnoticed. Quite surprised, the children
jumped up from their seats when the father said, “Look out, now, we
are nearly at Freiburg.”
In a few minutes the name was actually called out and the train
stopped. Mrs. Lesa looked out of the window expectantly to see if
the gentlemen had come to meet them. Sure enough, there they
were. But the mother uttered a low sigh. She had hoped till the last
moment that Mr. Delrick, as a surprise, had brought their boy with
him. But Vinzi wasn’t there.
Loud cries of joy beside her drew her attention to the two children,
who flung themselves upon their father. Mr. Thornau now held them
both in his arms and responded to their stormy greetings with
exclamations of joyous surprise. He held his son a short distance
from him and looked at him happily.
“And is this supposed to be my thin, frail Hugo, who had neither
spirit nor strength. You look as sunburnt as a woodsman, with your
fat cheeks and beaming eyes. Mrs. Lesa,” he called to the mother,
“what did you do with him? He is a different creature. And is this
stalwart gipsy girl really my Alida? The blood throbs through your
brown cheeks with health. Mrs. Lesa, Mrs. Lesa, how on earth did
you do it?”
“Oh, we know, we know how,” cried out the children simultaneously,
and immediately began to relate to the father how they had spent
their delicious summer days. It would have been impossible for Mrs.
Lesa to give any information now.
After greeting her husband, Mr. Delrick took Mrs. Lesa’s hand. Just is
if he had read from her eyes what was in her heart, he said, “Don’t
worry about Vinzi, Mrs. Lesa. He is perfectly well, and he is still a
great joy to me. The reason why he is not here is not a bad one, you
must believe me. Oh, here is my little Stefeli, too, and another old
friend besides,” he continued, turning toward the children. “I am glad
Jos came, too, for he must belong entirely to your family by now.”
“Yes, like our own,” said Mrs. Lesa, putting her arms around the boy.
“Thank God that we have him.”
“I’d like to ask what you think about a plan we made,” continued Mr.
Delrick. “I wonder what Mr. Lesa will say. Our friend, Mr. Thornau,
has asked us all to dinner today at a hotel quite near the beautiful
old Nicholas church. So we thought it would be pleasant to go there
first of all and then have a nice, quiet time together.”
Mrs. Lesa glanced at her husband, for she wanted him to decide. She
herself was most anxious to go to the church immediately. Old
memories had risen in Vinzenz Lesa’s mind as soon as he had set his
foot in Freiburg. As a child he had known nothing more wonderful
than to drive to Freiburg on a Sunday, and his little girl neighbor had
felt the same. He had always loved to enter the tall, quiet church
holding his mother’s hand. Whenever he heard the sounds of the
wonderful organ, they revealed to him a different world, one quite
apart from his daily life.
“I’d be only too glad to go to church first,” he began. “It is but
suitable that we should go, as it is Sunday today.”
At this the little company started off into the town. It was very still in
the large old church, and so dark and solemn that the children
stepped very lightly before settling down beside their parents.
Suddenly the organ began to play, and rich, powerful tones floated
through the quiet building. It seemed as if all the heavenly hosts
were singing a hymn of praise and joy for all the world.
RICH, POWERFUL TONES FLOATED THROUGH THE QUIET
CHURCH
Stefeli uttered a low cry and violently pulled her mother’s sleeve. “It
is Vinzi, mother,” she said in intense excitement. The mother also had
recognized her boy’s curly head at her first glance toward the organ.
It had so surprised and overcome her that she could barely keep
from sobbing aloud.
She wanted her husband to share her deep emotion. Quietly
touching him, she whispered, “Vinzenz, it is Vinzi.”
He gave no reply, nor looked up, seemingly unwilling to show he had
been moved. The music suddenly changed. From a low plaint it
swelled to a tremendous wail, is if a chorus of despairing creatures
were cast down by sorrow and contrition. In the midst of this, their
grief reached its height and changed into meek, ardent entreaties for
help and mercy. At that moment the heavens seemed to open and a
clear, lovely chant of angels brought down a message of love and
eternal joy. Just in the middle of the angels’ choir, a rich, clear voice
rang out and filled the church with the words:
“And the blessed song of mercy—”
Jos, upon hearing the well-known sounds, so much more glorious
and inspiring than he had ever imagined them, had been completely
carried away. At the ending where he had always joined in he could
not stop himself from lustily singing his words to the chorus of
angels.
When the last tone faded away a deep silence reigned in the church.
After a while both gentlemen rose. Vinzenz Lesa also got up, but he
had been obliged to wipe his eyes a number of times.
“How can you believe such a thing,” he said to his wife in a hoarse
voice in answer to the words she had whispered to him. “No one can
tell me that was Vinzi.”
Mr. Delrick stood immediately behind him. “Mr. Lesa,” he said, patting
him on the shoulder, “we don’t need to believe anything till we have
gone to the organ to see for ourselves.”
“He can certainly play, I know that,” said Mr. Thornau satisfied. “How
did you like it, Alida?”
“Oh, it was wonderful and I wish it were not over yet.”
“Come, let us go along, too. I’d love to see who played,” and taking
his daughter’s hand, Mr. Thornau followed the others up to the organ
loft. Mr. Delrick had quickly gone ahead, and before the others had
arrived, the same joyful melody greeted them again. Vinzenz Lesa,
upon entering, stood stock still. At the organ really sat his curly-
headed boy. As he was playing, the father could actually hear how he
produced those stirring melodies. Nothing helped now, and Lesa had
to wipe his eyes repeatedly.
Vinzi had ceased, and his mother finally let Stefeli’s hand go. The
child rushed to her brother and tenderly clung to him. Now the father
came up with Mr. Delrick.
“Mr. Lesa,” he said, “your son has played you the song of mercy, and
I hope you will also be merciful and forgive him if he does not want
to become a farmer.”
Lesa had grasped his son’s hand. “More than that, more than that,
Vinzi,” he finally said after a great effort to control himself. “You
make your parents proud of you and not what I used to think. I
never realized such a thing could happen. I never, never knew it.
When I used to come to Freiburg as a boy and heard this organ, I
thought that people who played it were creatures entirely set apart,
not human beings like one of us. You must be most thankful to Mr.
Delrick, boy, for we owe him everything. He found the way for you
and made this possible.”
“Not I, Mr. Lesa, not I,” Mr. Delrick remonstrated. “Pater Silvanus is
the one who knew what was in Vinzi and who knew what ought to
be done with him. Let us be grateful to him. You shall also know
right away that Vinzi is not losing time with a breadless art. He
already has an offer from a church in Dresden to play every Sunday.
I suppose you approve of that, Mr. Lesa? He’ll be able to keep on
studying at the same time and I think the longer the better. I hope it
will suit you to leave him with me for a few more years.”
“Whatever you deem wise, sir,” Lesa replied, unhesitatingly, perfectly
convinced that Mr. Delrick’s plans and his would evermore agree.
Vinzenz Lesa was pleased that his friend had kept a long silence
about Vinzi, until he could really prove his case.
One surprise after another was in store for Vinzi. When Jos suddenly
stepped up to him, Vinzi greeted his old friend with the greatest joy.
“So it was you, after all!” he exclaimed. “I heard you singing down in
the church and it nearly upset me. I had to think over and over
again: ‘Who knows our song and has a voice just like Jos’s?’ But I
thought you couldn’t be here, and here you are, after all.” In the
greatest surprise he greeted Alida, Hugo and Mr. Thornau. Alida
found so many things to talk over with Vinzi about their past and
future that Mr. Thornau found it wise to announce that the moment
for dinner had arrived. Then all their conversations could be
continued at leisure.
However much Mr. Lesa fought against it, he was taken to dinner at
the Zohringer Hotel, where he had not set his foot in his life before.
Unfortunately the hour of parting came much too soon for them all.
“You don’t know what I have to thank you for,” said Mr. Thornau to
Vinzenz Lesa at leave-taking. “My son, who never has shown the
slightest inclination nor wish for any occupation, has just informed
me that he has decided to become a landowner. After having already
chosen the manager for his estate, he insists on getting all his cows
from you. He says you have the finest that can be found anywhere,
and he knows all about the breed. My daughter has been fighting
against studying music for over a year and has not once opened her
beautiful piano in spite of all my remonstrances. Now she declares to
me that it is her highest wish to take up her lessons again. Hearing
her former pupil play so beautifully makes her ambitious to work
hard and catch up with him. I really mean it most seriously, Mr. Lesa,
when I say that my gratitude for everything that my children have
experienced in your house is much greater than I can say. I would
love to prove it to you. Don’t you ever expect to get to Hamburg, Mr.
Lesa?”
“No, I don’t believe so,” said the latter truthfully. “But we are glad if
the children have profited somewhat. We liked to have them and
would like them to come again.”
Mr. Thornau pressed Mrs. Lesa’s hand most gratefully, for his children
had told him so much about her that he knew she had been a real
mother to them. “Will you let the children come back again?” he
asked sincerely.
“It would make me quite unhappy if they didn’t come next summer,”
she said half sadly, half happily, “and I’ll look forward to their coming
every summer.”
Upon taking leave of Mr. Lesa, Mr. Delrick told him that they would
see each other again, as he was planning to spend Vinzi’s holidays in
Leuk. But first he intended to go to the Simplon in order to show
Pater Silvanus how his pupil had progressed. If the father had no
objection he would take Vinzi with him. Lesa replied that Mr. Delrick
should do whatever he thought best.
Vinzi, who was standing beside them, listened breathlessly. What had
his father said? How could he say this when he had forbidden him so
emphatically to go back to the mountain?
Anxious to know the truth, he asked a little timidly, “Father, can I
really go to the mountain with Mr. Delrick?”
“You may do whatever Mr. Delrick says,” replied the father.
Vinzi’s eyes sparkled with delight. He was to return to the mountain
in a few days and see every one he had loved so dearly. He would be
able to thank Pater Silvanus, as he fully realized how much he had to
be grateful for.
That same hour all separated. The children found it quite impossible
to get done with their leave-taking. Whenever they seemed to have
finished, they began again. But they were far from sad, as they had
the sure prospect of meeting again next summer. Vinzi was to be
with them also, as Mr. Delrick had promised to come to Leuk at the
beginning of his holidays. Mr. Thornau travelled in the direction of
Basle, and Vinzenz Lesa toward Bulle, near his paternal home. Mr.
Delrick and Mrs. Lesa, with the three children, journeyed toward
Lausanne on the lake of Geneva. Here they spent the night in order
to arrive home as soon as possible the next morning.
On the evening of the following day Vinzi went upstairs with his
mother. When they came to his little room, he said to her. “Oh,
mother, it is so lovely to be at home again; won’t you please come in
and say good-night to me the way you used to do?”
Before going to bed that night Stefeli had run outside, as if looking
for some one. She found Mr. Delrick sitting on the bench as usual,
just as she had expected.
“When you go to the mountain, Mr. Delrick, will you give somebody a
message from me?” she inquired, as soon as she had joined him.
After expressing his readiness to do so, Stefeli went on: “Won’t you
please tell Uncle Lorenz that Jos mustn’t go home yet. He promised
to leave him here till father sends him away, and I know that he’ll
never do that.”
Mr. Delrick promised faithfully to deliver her message. He could not
help smiling to himself when he remembered how similar were the
three messages he was asked to take. Mr. Lesa had begged his
cousin Lorenz to leave Jos with him, if possible, as he could hardly
live without the boy. A few moments later Jos had appeared, very
anxious to let his father know that he could not yet come home. So
much had still to be done before the winter, and then spring would
be here again. This year’s work could barely be finished before next
year’s labors would begin. His Uncle Vinzenz should really not be left
alone, for it was settled now that Vinzi would stay away for good. Mr.
Delrick said to himself with a smile that Cousin Lorenz would not be
able to gainsay this threefold request, especially against his own firm
promise.
Mrs. Lesa went into Vinzi’s room and sat down on his bed. This
vividly brought back to her the dreadful evening before he had
departed for the Simplon. It was the first time Vinzi had been left
alone with his mother since his return. He began to pour out his
heart to her and told her all about his life in Dresden. He had not
written, because his father had said that it was no good. He could
not relate enough of Mr. Delrick’s fatherly kindness to him in every
way. Vinzi was allowed to study many things, especially music. One
splendid teacher taught him to play the organ and the other gave
him instructions in the laws of music. From the latter he had learned
to understand these and all their marvels. This lesson had been a
great joy to him and he had been impatient for it every time.
“But my organ lesson was better yet,” Vinzi added. “Oh, mother, I
never realized one could look forward so much to anything. It was
always like a great feast to me.”
With boundless happiness the mother looked at the radiant eyes of
her boy. “And isn’t it part of your happiness, Vinzi, to live with Mr.
Delrick? It must be wonderful to belong to his home and be with him
so much!” she inquired.
“Yes, and not a small part of it either,” he cried out. “I am glad you
know him, otherwise you could have no idea how kind, affectionate
and helpful he is. Everybody in his house is good to me, too, for his
sake. Mrs. Wyneken, his housekeeper, who has charge of everything,
is as kind to me as if she were my grandmother. Frederick the butler,
and Minnie, the maid, both wait on me and won’t let me do anything
for myself. It is mostly because they all love to hear me play the
organ. Mrs. Wyneken says that Mr. Delrick has grown happy again
since I play it. He had lost every one in the world he loved and had
been very sad. He has a little organ in a large room downstairs. Mr.
Delrick asks me to play it for him every night. Sometimes we can
hardly stop. Now you know what my life in Dresden is like. But I
cannot say how much I look forward to my little hayloft on the
mountain under the stars.”
“Vinzi,” said the mother, “do you ever thank God for all the blessings
He has heaped upon you. Do you realize that it is He who has given
you everything?”
“Yes, I do, mother,” he replied, looking frankly into her eyes. “I never
forget how frightened and troubled I often was. Sometimes we sing
one of the songs you taught me in the evening, but I sing them quite
differently now. I used only to enjoy the tune, but now I thank and
praise God from the bottom of my heart.”
“If you should ever get into a difficult position, Vinzi,” the mother
concluded, “you must remember that God often means to work out
our good when we fear evil. When I sat on this bed before you went
away the first time, I should in my shortsightedness have done
everything to keep you home. And just think! That journey proved
the beginning of your great happiness.”
Vinzi had listened attentively. “It was the same with me,” he said. “I
thought it was the most dreadful thing that could happen to me and
it proved just the opposite. I’ll always remember that, mother.”
When the mother left Vinzi’s chamber after a hearty good-night, her
heart was filled with gratitude and bliss. Folding her hands she sent
up a heartfelt prayer of thanks to Heaven.
What blessings God had sent to her and her household. Her boy’s
inmost wish had been fulfilled and his future lay clearly before him.
The father was not only satisfied, but actually pleased. He did not
despise his Vinzi any more, because he realized that he could be
truly proud of the boy and he showed it quite openly in the way he
looked at him.
Vinzi had been away a long while and had lived in totally different
surroundings. He had learned a great deal, but had remained just as
affectionate and simple as when he had gone away. This made his
mother extremely happy, and she sent a fervent prayer to God to
keep him pious and childlike upon all the paths of his coming life.
The End
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Engineering Vibration 4th Edition Inman Solutions Manual

  • 1. Engineering Vibration 4th Edition Inman Solutions Manual download https://testbankdeal.com/product/engineering-vibration-4th- edition-inman-solutions-manual/ Find test banks or solution manuals at testbankdeal.com today!
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  • 5. Problems and Solutions Section 2.1 (2.1 through 2.19) 2.1 The forced response of a single-degree-of-freedom, spring-mass system is modeled by (assume the units are Newtons) 3 x(t) +12x(t) = 3cosωt   Compute the magnitude of the forced response for the two cases  ω  =2.1  rad/s and  ω  = 2.5 rad/sec. Comment on why one value is larger then the other. Solution: First divide through by the mass, 3 kg, to reveal the natural frequency.  x(t) + 4x(t) = cosωt   Thus f0 = 1 and ωn = 2 rad/s . Hence the magnitude of the forced response for ω   =2.1  rad/s is given by equation (2.6): X = f0 ωn 2 − ω2 = 1 4 − 2.12 = −2.239 m For  ω  = 2.5 rad/sec this becomes X = f0 ωn 2 − ω2 = 1 4 − 2.52 = −0.444 m   The  value  of  the  magnitude  of  the  response  at  a  driving  frequency  of  2.5   rad/s  is  much  smaller  then  that  at  2.1  rad/s  because  it  is  much  farther  away   from  the  resonance  condition. 2.2 Consider the forced response of a single-degree-of-freedom, spring-mass system that is modeled by (assume the units are Newtons) 3 x(t) +12x(t) = 3cosωt Compute the total response of the system if the driving frequency is 2.5 rad/s and the initial position and velocity are both zero. Solution: Because the total response is asked for this problem differs from problem 2.1, which only asks for the magnitude of the forced response. The total response is given by equation 2.11 to be: x t ( )= v0 ωn sin ωnt + x0 − f0 ωn 2 − ω2 ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ cos ωnt + f0 ωn 2 − ω2 cos ωt   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 6. which for zero initial velocity and position becomes: x t ( )= − f0 ωn 2 − ω2 cos ωnt + f0 ωn 2 − ω2 cos ωt = 0.444cos2t − 0.444cos2.5t   2.3 To familiarize yourself with the nature of the forced response, plot the total response of m x t ( )+ kx t ( )= F 0 cosωt with ω = 2 rad/s, for a variety of values of the initial conditions, values of f0 and values of ωn as given in the following chart:   Case   x0   v0   f0   ωn   1   0.1   0.1   0.1   1   2   -­‐0.1   0.1   0.1   1   3   0.1   0.1   1.0   1   4   0.1   0.1   0.1   2.1   5   1   0.1   0.1   1     Solution: The total response is given by equation (2.11):   x t ( )= v0 ωn sin ωnt + x0 − f0 ωn 2 − ω2 ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ cos ωnt + f0 ωn 2 − ω2 cos ωt   Insert the values of   x 0 ,  v0 ,  f0 ,  and  ωn for each of the five cases.   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 7. 2.4 Compute the response of a spring–mass system modeled by equation (2.2) to a force of magnitude 23 N, driving frequency of twice the natural frequency, and initial conditions given by x0 = 0 m and v0 = 0.2 m s. The mass of the system is 10 kg, the spring stiffness is 1000 N m and the mass of the spring is considered and known to be 1 kg. What percent does the natural frequency change if the mass of the spring is not taken into consideration? Solution: Given: m sp = 1 kg, Example 1.4.4 yields that the effective mass is m e = m + msp 3 = 10 + 1 3 = 10.333 kg. Thus the natural frequency, X and the coefficients in equation (2.11) for the system now become © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 8. ωn = 1000 10 + 1 3 = 9.837 rad/s, ω = 2ωn =19.675 rad/s X = f0 ωn 2 −ω2 = F0 / me ωn 2 −ω2 = 2.2258 9.8372 −19.6752 = −0.0077 m, v0 ωn = 0.02033 m Thus the response as given by equation (2.11) is x(t) = 0.02033sin9.837t + 7.7×10−3 (cos9.837t − cos19.675t) m The frequency without the spring’s mass consider is given in example 2.1.1 as 10 rad/s. So the percent change by including the spring’s mass is 9.837 −10 10 ⋅100 = −1.63% So the mass of the spring reduced the natural frequency by 1.63 %. 2.5 Show that the solution x(t) = f0 ωn 2 −ω2 [cos ω t – cos ωn t] can be written x(t) = f0 2(ωn 2 −ω2 ) sin ωt +ωnt 2 sin ωnt −ωt 2 . Solution: Using the trig identity cosu − cosv = 2sin v + u 2 sin v − u 2 Now let u = ωt and v = ωnt and substitute to get that cosωt − cosωnt = 2sin ωt +ωnt 2 sin ωnt −ωt 2 Multiplying  by   f0 ωn 2 −ω2  then  yields  the  equation  (2.13). 2.6 A spring-mass system is driven from rest harmonically such that the displacement response exhibits a beat of period of 0.2π s. The period of oscillation is measured to be 0.02π s. Calculate the natural frequency and the driving frequency of the system. Solution: Given: Beat period: T b = 0.2π s. The beat frequency is then Tb = 2π ωb ⇒ωb = 2π Tb = 2π 0.2π = 10 rad/s = ωn −ω (1) The period of oscillation is T0 = 0.02π s so that T0 = 2π ω0 ⇒ω0 = 2π T0 = 2π 0.02π = 100 = ωn +ω 2 ⇒ωn +ω = 200 rad/s (2) © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 9. Solving these two equations in two unknowns yields: ωn = 105 rad/s and ω = 95 rad/s 2.7 An airplane wing modeled as a spring-mass system with natural frequency 40 Hz is driven harmonically by the rotation of its engines at 39.9 Hz. Calculate the period of the resulting beat. Solution: Given: ωn = 2π (40) = 80π rad/s, ω = 2π (39.9) = 79.8π rad/s Beat period: T b = 4π ωn −ω = 4π 80π − 79.8π = 20 s. 2.8 Compute the total response of a spring-mass system with the following values: k 1000 N/m, m = 10 kg, subject to a harmonic force of magnitude F 0 = 100 N and frequency of 8.162 rad/s, and initial conditions given by x0 = 0.01 m and v 0 = 0.01 m/s. Plot the response. Solution: Given: k = 1000 N/m, m = 10 kg, F0=100 N, ω = 8.162 rad/s x0=0.01m,  v0=0.01  m/s   From  Eq.  (2.11):       x(t) = v0 ωn sin ωnt + (x0 − f0 ωn 2 −ω2 )cos ωnt + f0 ωn 2 −ω2 cosωt   ωn = k m = 1000 10 =10 rad / s                             f0 = F0 m = 100 10 =10 N / m   In  Mathcad  the  solution  is   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 10.       2.9 Consider the system in Figure P2.9, write the equation of motion and calculate the response assuming a) that the system is initially at rest, and b) that the system has an initial displacement of 0.05 m. Figure P2.9 Solution:    The  equation  of  motion  is   m  x + k x =10sin10t   2,000 N/m 100 kg x (t) Friction- free surface 10 sin 10t N © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 11. Let  us  first  determine  the  general  solution  for    x +ωn 2 x = f0 sinω t   Replacing the cosine function with a sine function in Eq. (2.4) and following the same argument, the general solution is: x(t) = A1 sinωnt + A2 cosωnt + f0 ωn 2 −ω2 sinωt   Using  the  initial  conditions,   x(0) = x0  and    x(0) = v0 ,  a  general  expression  for   the  response  of  a  spring-­‐mass  system  to  a  harmonic  (sine)  excitation  is:   x(t) = ( v0 ωn − ω ωn ⋅ f0 ωn 2 −ω2 )sinωnt + x0 cosωnt + f0 ωn 2 −ω2 sinωt   Given:    k=2000  N/m,  m=100  kg,  ω=10  rad/s,   ωn = k m = 2000 100 = 20 rad/s = 4.472 rad/s                     f0 = F0 m = 10 100 = 0.1N/kg   a) x0  =  0  m,  v0  =  0  m/s   Using  the  general  expression  obtained  above:   x(t) = (0 − 10 20 ⋅ 0.1 20 2 −102 )sin 20t + 0 + 0.1 20 2 −102 sin10t   = 2.795×10−3 sin4.472t −1.25×10−3 sin10t   b) x0  =  0.05  m,  v0  =  0  m/s   x(t) = (0 − 10 20 ⋅ 0.1 20 2 −102 )sin 20t + 0.05cos 20t + 0.1 20 2 −102 sin10t   = 0.002795sin4.472t + 0.05cos4.472t − 0.00125sin10t = 5.01×10−2 sin(4.472t +1.515) −1.25×10−3 sin10t © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 12. 2.10 Consider the system in Figure P2.10, write the equation of motion and calculate the response assuming that the system is initially at rest for the values k1 =100 N/m, k2 = 500 N/m and m = 89 kg. m 10 sin 10t N k1 k2 Figure  P2.10   Solution:  The  equation  of  motion  is   m x(t) + kx(t) = 10sin10t   where   k = 1 1 k1 + 1 k2   The  general  expression  obtained  for  the  response  of  an  underdamped  spring-­‐ mass  system  to  a  harmonic  (sine)  input  in  Problem  2.7  was:   x(t) = ( v0 ωn − ω ωn ⋅ f0 ωn 2 −ω2 )sinωnt + x0 cosωnt + f0 ωn 2 −ω2 sinωt   Substituting  the  following  values   k  =  1/(1/100+1/500)=  83.333  N/m,      m  =  89  kg                            ω  =  10    rad/s   ωn = k m = 83.333 89 = 0.968 rad/s                 f0 = F0 m = 10 89 = 0.112 N / kg   and  initial  conditions:  x0  =  0,  v0  =  0   The  response  of  the  system  is  evaluated  as   x(t) = 0.0117sin0.968t − 0.00113sin10t   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 13. 2.11 Consider the system in Figure P2.11, write the equation of motion and calculate the response assuming that the system is initially at rest for the values θ = 30°, k = 1000 N/m and m = 50 kg. Figure P2.11   Solution:  Free  body  diagram:   Assuming  x  =  0  to  be  at  equilibrium:   Fx = m x = −k(x + Δ) + mgsinθ ∑ + 90sin25t       (1)   where    Δ  is  the  static  deflection  of  the  spring.    From  static  equilibrium  in  the  x   direction  yields   −kΔ + mgsinθ           (2)   Substitution  of  (2)  onto  (1),  the  equation  of  motion  becomes   m  x + k x = 90sin2.5t   The  general  expression  for  the  response  of  a  mass-­‐spring  system  to  a   harmonic  (sine)  excitation  (see  Problem  2.7)  is:   x(t) = ( v0 ωn − ω ωn ⋅ f0 ωn 2 −ω2 )sinωnt + x0 cosωnt + f0 ωn 2 −ω2 sinωt   Given: v0 = 0, x0 = 0, ω = 2.5 rad/s ωn = k m = 1000 50 = 20 = 4.472 rad/s , f0 = F0 m = 90 50 = 9 5 N/kg k m ! 90 sin 2.5t N m   x   mg  sin  θ   F=90  sin  2.5  t   Fs   (Forces that are normal to the x direction are neglected) θ   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 14. So the response is: x(t) = −0.0732sin4.472t + 0.1309sin2.5t 2.12 Compute the initial conditions such that the response of : m  x + kx = F 0 cosω t oscillates at only one frequency (ω ).   Solution:    From  Eq.  (2.11):   x(t) = v0 ωn sin ωnt + (x0 − f0 ωn 2 −ω2 )cos ωnt + f0 ωn 2 −ω2 cosωt   For  the  response  of  m  x + k x = F0 cos ω t  to  have  only  one  frequency  content,   namely,  of  the  frequency  of  the  forcing  function,  ω,  the  coefficients  of  the  first   two  terms  are  set  equal  to  zero.    This  yields  that  the  initial  conditions  have  to   be   x0 = f0 ωn 2 −ω2      and      v0 = 0   Then  the  solution  becomes   x(t) = f0 ωn 2 −ω2 cosωt     2.13   The  natural  frequency  of  a  65-­‐kg  person  illustrated  in  Figure  P.13  is   measured  along  vertical,  or  longitudinal  direction  to  be  4.5  Hz.    a)  What  is  the   effective  stiffness  of  this  person  in  the  longitudinal  direction?  b)  If  the  person,   1.8  m  in  length  and  0.58  m2  in  cross  sectional  area,  is  modeled  as  a  thin  bar,   what  is  the  modulus  of  elasticity  for  this  system?   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 15. Visit https://testbankdead.com now to explore a rich collection of testbank, solution manual and enjoy exciting offers!
  • 16.   Figure  P2.13  Longitudinal  vibration  of  a  person   Solution:  a)  First  change  the  frequency  in  Hz  to  rad/s:   ωn = 4.5 cycles s 2π rad cycles = 9π rad/s.  Then  from  the  definition  of  natural   frequency:   k = mωn 2 = 65⋅(9π)2 = 5.196 ×104 N/m   b)  From  section  1.4,  the  value  of  the  stiffness  for  the  longitudinal  vibration  of   a  beam  is   k = EA  ⇒ E = k A = 5.196 ×104 ( )(1.8) 0.58 =1.613×105 N/m2 =1.613×105 Pa     2.14   If  the  person  in  Problem  2.13  is  standing  on  a  floor,  vibrating  at  4.49  Hz  with   an  amplitude  of  1  N  (very  small),  what  longitudinal  displacement  would  the   person  “feel”?  Assume  that  the  initial  conditions  are  zero.     Solution:  Using  equation  (2.12)  for  a  cosine  excitation  and  zero  initial   conditions  yields  (converting  the  frequency  from  Hertz  to  rad/s  and  using   the  value  of  k  calculated  in  2.11):   X = F0 m 1 ωn 2 −ω2 = 1 65 1 k m − (4.49⋅2π)2 = 1 65 1 5.196 ×104 65 − (4.49⋅2π)2 = 0.00443347 = 0.0043 m © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 17.   2.14     Vibration  of  body  parts  is  a  significant  problem  in  designing  machines  and   structures.    A  jackhammer  provides  a  harmonic  input  to  the  operator’s  arm.     To  model  this  situation,  treat  the  forearm  as  a  compound  pendulum  subject   to  a  harmonic  excitation  (say  of  mass  6  kg  and  length  44.2  cm)  as  illustrated   in  Figure  P2.14.  Consider  point  O  as  a  fixed  pivot.  Compute  the  maximum   deflection  of  the  hand  end  of  the  arm  if  the  jackhammer  applies  a  force  of  10   N  at  2  Hz.       Figure  P2.14  Vibration  model  of  a  forearm  driven  by  a  jackhammer       Solution:  Taking  moments  about  point  O  yields  (referring  to  Example  1.4.6   for  the  inertial  of  a  compound  pendulum):   m2 3  θ + mg  2 sinθ = FO cosθ cosωt   Using  the  linear  approximation  for  sine  and  cosine  and  dividing  through  by   the  inertia  yields:    θ + 3g 2 θ = 3FO m cosωt   Thus  the  natural  frequency  is   ωn = 3g 2 = 3(9.81) 2(0.442) = 5.77 rad/s (=0.92 Hz)   and  the  system  is  well  away  from  resonance.    Referring  to  equation  (2.13),   the  amplitude  for  zero  initial  conditions  is  (converting  the  driving  frequency   from  2  Hertz  to  2(2π)  rad/s):   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 18. θ = 2 f0 ωn 2 −ω2 = 2 3F0  m2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 3g 2 − (2⋅2π)2 = 0.182 rad   Note  that  sin(0.182)  =  0.181  so  the  approximation  made  above  is  valid.    The   maximum  linear  displacement  of  the  hand  end  of  the  arm    is  just     X = r θ = 0.442⋅0.182 = 0.08 m         2.15   An  airfoil  is  mounted  in  a  wind  tunnel  for  the  purpose  of  studying  the   aerodynamic  properties  of  the  airfoil’s  shape.    A  simple  model  of  this  is   illustrated  in  Figure  P2.15  as  a  rigid  inertial  body  mounted  on  a  rotational   spring,  fixed  to  the  floor  with  a  rigid  support.  Find  a  design  relationship  for   the  spring  stiffness  k  in  terms  of  the  rotational  inertia,  J,  the  magnitude  of  the   applied  moment,  M0,  and  the  driving  frequency,  ω,  that  will  keep  the   magnitude  of  the  angular  deflection  less  then  5°.    Assume  that  the  initial   conditions  are  zero  and  that  the  driving  frequency  is  such  that  ωn 2 −ω2 > 0 .     Figure  P2.15  Vibration  model  of  a  wing  in  a  wind  tunnel   Solution:  Assuming  compatible  units,  the  equation  of  motion  is:   J  θ(t) + kθ(t) = M0 cosωt ⇒  θ(t) + k J θ(t) = M0 J cosωt   © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 19. From  equation  (2.12)  the  maximum  deflection  for  zero  initial  conditions  is   θmax = 2M0 J k J −ω2 < 5° πrad 180° = π 36 rad ⇒ 2M0 J < ( k J −ω2 ) π 36 rad ⇒ 36J π 2M0 J + πω2 36 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ < k   2.16 The spar of an airplane wing is a relatively rigid beam extending along the length of the wing inside the wing to provide strength. It is typical to model a spar as a cantilever beam with the fixed end at the body of the aircraft. An example is given in Figure P2.16. Using the modeling methods given in Section 1.5 determine a single-degree-of-freedom model for the spar and compute its natural frequency. The spar here is modeled as a cantilever beam of dimensions length 560 mm, width 38 mm and thickness 3.175 mm, and has a mass of 13.975 grams. The beam’s Young's modulus is 10.29 GPa and its shear modulus: 1.65 GPa. MAKE THIS A BEAM MODELD AS A SDOF SM SYSTEM Figure P2.16 A small, unmanned air vehicle with a rigid spar, modeled as a beam Solution: This problem requires students to think back to Section 1.5 where an SDOF model of a beam is given. The stiffness of the beam is given by equation (1.76) and in Figure 1.26 as k = 3EI l3 where E is the beams elastic modulus, l is the length and I is the area moment of inertia of a beam in the bending direction given by I = (thickness)(width)3 12 = hb3 12 © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
  • 20. This can be found on google or a strength of materials text. The formula for frequency now requires the mass which will causes some students confusion. Looking at Figure 1.28, the effective mass of a system with a tip mass and substantial beam mass is results in Equation (1.76) for the frequency: ωn = k 133 140 M + m Combining these three expressions and letting the tip mass, m, be zero yields the following expression for the frequency ωn = 3E l3 ⋅ hb3 12 33 140 M = 1.65×109 N/m2 (0.560m)3 ⋅ (0.003175)(0.038)3 4 m4 33 140 ⋅13.975×10−3 kg = 352.459 rad/s ≈ 56.1 Hz 2.17 Compute the response of a shaft and disc system to an applied moment of M = 10 sin 312 t as indicated in Figure P2.17. Assume that the shaft is initially at rest (zero initial conditions) and J = 0.5 kg m2 , the shear modulus is G = 8 × 1010 N/m2 , the shaft is 1 m long, of diameter 5 cm and made of steel. Figure P2.17 Solution: Summing moments the equation of motion is: 0.5 θ(t) + GJp l θ(t) = 10sin312t  θ(t) + GJp 0.5l θ(t) = 20sin312t The natural frequency (see also Example 1.5.1) is ωn = k J = GJP lJ = 8×1010 N m 2 ( ) π 32 5×10−2 m ( ) 4 ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1 m ( ) 0.5 kg ⋅ m2 ( ) = 313.33 rad/s © 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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  • 22. settled down about them in the sunshine, Jos sat down willingly by Stefeli. He first of all admired the neatly spread repast before him, for he had never seen a meal set out so perfectly outdoors. The swaying branches above gave them the needed shade. “I am sure no one else could spread a table as well as you can, Stefeli,” he said, full of admiration. Then he began to eat everything Stefeli offered him with the heartiest appetite. The afternoon passed as quickly as the morning, and when they were strolling homewards in the bright evening light, Stefeli said, “Oh, it was lovely today. I hope all the other days will be exactly like it.” “I hope so, too,” repeated Jos. Vinzenz Lesa stood at his barn and watched his herd coming home. Jos was running alongside of the cows, first on one side and then on the other. This kept them walking in a neat row, instead of running first here, then there. “A quick, orderly boy,” said Vinzenz Lesa to himself, as he slowly met the procession on its way to the stable. A series of perfect days followed, and the children had the happiest times. Stefeli’s wish seemed to be fulfilled, for she fairly beamed with joy and well-being all day long, and Jos sang and yodelled louder every day from sheer happiness. “It’s just as if Vinzi were back since Jos is with us. Don’t you think so, mother?” said Stefeli, coming home one day with cheeks flushed, partly from delight and partly from the bright light of the setting sun. Jos as usual had stayed outside with his uncle. “Yes,” replied the mother. “As long as we can’t have Vinzi, I am glad Jos takes his place. I like Jos as much as if he belonged to us.” “I, too,” said Stefeli without hesitation. “But there are three times when I had a better time with Vinzi. In the morning, in the evening, and on rainy days. It was much nicer then when Vinzi was at home.”
  • 23. The mother well understood what Stefeli alluded to, but she asked the child to let Jos go on his way, as he was doing his duty and it would be wrong of her to hinder him in that. “I have tried already, but it does no good,” said Stefeli frankly. She was forbidden to do so again. The mother was only too glad and grateful to see how Jos occupied himself at those times, even if Stefeli was displeased. He did so of his own free will, and she had noticed that it had actually improved her husband’s temper. Jos was always the first up in the morning, and if the stable, which was his favorite resort, was still closed, he thought of something which had to be set in order in the barn. In this way Vinzenz would find him busy with hammering or mending something. The boy always came to breakfast at the very last minute when the coffee was put upon the table. He didn’t even notice how impatiently Stefeli was waiting for him. In the evening after their return Jos could not be lured away from his cows till the last one had returned from watering and was comfortably bedded on the straw. The mother had always set the steaming pot on the table before he came in. Stefeli never could count on running over to the wild strawberry bed with Jos, therefore. On rainy days Jos always disappeared entirely. He knew in the early morning what work had to be done that day in the fields, the hills, the woods or on the trees. When it rained, he would quickly ask his uncle, “Can I go with the man today?” The other always agreed, “Why not, if you don’t mind the rain?” Rain didn’t bother Jos at all. Everything that grew and brought in fruit interested him so much that he wanted to see it and have a hand in it if possible. Best of all, he liked it when the wood which had been felled had to be brought home from the forest. Then the horse was hitched to a large lumber wagon and Jos sat on the high seat beside the hired man. As soon as they had left the house behind them the man would say, “There, take them if you want to drive.” Giving Jos the reins, he stretched out full length in the wagon
  • 24. to sleep a little longer. Jos knew the horse well and the horse knew him, so the matter went very smoothly. Jos had already helped in every kind of farm work and knew exactly when things were in order or when something was amiss. He took hold of everything with such spirit and eagerness that he acquired an instinct for what was needed when matters went wrong. In the beginning Vinzenz Lesa had said to his wife in a bitter tone,“Just look how he enjoys everything! He works with such an eagerness, as if it were his own. Look at the pleasure he finds in it all! And my own boy, for whom we work, has never even seen it.” But everything had turned out to make Mrs. Lesa happy. Her husband spoke every day in a different tone about Jos. He could say quite cheerfully now: “The boy has four eyes in his head. He discovers what I overlook myself and the man would never have seen. He actually sees everything. I can entirely depend on him as on no one else, and he is so handy that it is a pleasure to see him work.” Mrs. Lesa saw her husband’s growing delight in their young nephew with hearty feelings of gratitude. He was in a much better humor than he had been for years. When during the afternoon she was sitting quietly and undisturbed in her room, her thoughts unerringly dwelt upon Vinzi and she pondered about his future. She wondered if he would return after a year and take up the life his father wished him to, in which case he would never be really happy. Or if he had definitely chosen a new path for himself, which would forever estrange him from his father. She lost herself in imaginary conversations with her son, and the slightest noise made her start up, as she would think that her boy might have suddenly returned. So it happened one day when her thoughts had been busy with Vinzi that the housedoor was opened and the noise of several footsteps neared the room. Mrs. Lesa had jumped up and running to the door had quickly opened it.
  • 25. “Old friends, who are looking for shelter in your house,” said a vigorous man’s voice. That same instant a slight hand grasped hers, then another. Only then could she really see them. “Welcome! Welcome, Alida, Hugo! Welcome, Mr. Thornau!” she cried, heartily delighted. “Are you staying in our parts again?” “Not exactly, but we are here for the present,” said Mr. Thornau, returning her friendly handshake. “We’ll stay here, if you will receive us. But if you say no, we’ll have to wander home again. Before everything else, I must tell you that I am bringing you two children who have lost their mother. They have asked me to come here, for nowhere else will suit them.” Mrs. Lesa was won over already. “The poor children,” she said to herself, “and the poor father,” she added. Then she turned to the latter. “Mr. Thornau,” she said, “won’t you stay here till evening when my husband comes home? Then we can talk it over.” “And what does his wife say?” asked Mr. Thornau. “Oh, she would love to take their mother’s place for a time, if she could,” replied Mrs. Lesa. “I am glad!” he exclaimed with satisfaction. He was willing to wait for Mr. Lesa and settled meanwhile under the shady walnut trees with his children. Vinzenz Lesa looked with surprise at the guests he found waiting before his house. But there was not much time for wondering, because Mr. Thomau had gone up to him and eloquently informed him of the reason for this visit. He was sure Mr. Lesa would not shut his door against him and especially his children, who had declared that they would not spend their summer anywhere in the world except at Mr. Lesa’s residence. He had come with the firm hope that Mr. Lesa would not turn them away and begged him not to propose
  • 26. a house in the neighborhood they already knew, as the children simply would not go back there. He had no intention, either, of forcing them to it. It meant either Mr. Lesa’s house or going home again. As it was impossible for him to stay with the children just then, he could not leave them anywhere else alone. Vinzenz Lesa was not displeased at hearing the gentleman’s words— either his house or none. But still, one could not tell what these children might bring into his home, for he hardly knew the people. After reflecting a while he answered, “It falls to my wife; looking after children is her affair.” This suited him best. If his wife wanted to keep them, as he suspected already, it would be his wife’s concern. If they got into mischief, she would have to look out for them, for she had taken them in. “With pleasure,” replied Mr. Thornau. Quickly turning to Mrs. Lesa he said, “Everything is settled, I suppose, for I think you have already said yes in your heart.” Mrs. Lesa heartily shook Mr. Thornau’s hand, for he had been right. She had long ago decided to keep the motherless children and show them all the affection she was capable of. Mr. Thornau was anxious to take immediate leave, as he had planned to journey on as soon as he knew his children were in good hands. Then Mrs. Lesa consulted him about their life, what their occupations and pleasures were to be. Her own children had always led the most simple life, she told him. Mr. Thornau wished for nothing better than to give them the life hers were leading. To be up on the pasture from morning till evening would be the best for them he could hope for. The rest of the time Mrs. Lesa could judge for herself. To his great disappointment Alida’s music lessons had been given up. He had always enjoyed her playing. His chief wish was to see them both come back to him sunburnt and with blooming cheeks, just as her own lad looked.
  • 27. Taking Mrs. Lesa a little apart, pointing to his sad-looking, pale boy, he added, “I am sure I won’t have to specially recommend him to your care. Just look at him! He never was very strong or happy, and since he has lost his mother he is worse than ever. No life, no spirits, no interest in anything! The doctor actually insisted on my sending him away.” Mr. Thornau quickly took farewell till the autumn, when he planned to see them all. Just then Jos and Stefeli, merrily chatting, came home with the cows. Jos disappeared in the stable as usual and Stefeli slowly approached the house. There was no hurry, for her mother was busy in the kitchen and Jos would not come back yet for at least an hour. Suddenly her slow steps turned into great leaps, for at the strawberry hedge she had discovered two figures. Could it really be true? And yet it was! With a cry of joy Stefeli ran over to Alida and Hugo. Alida also greeted Stefeli with loud, continued expressions of delight. “But where is Vinzi?” asked Alida when the greetings were over. Stefeli told her how long he had been away and that no one knew when he would come back. Alida looked extremely disappointed, but Stefeli always had a consolation ready. “I am sure he’ll come home while you are here. Jos is here, too, and you’ll be sure to like him. Will you stay all summer long?” Alida assented. “And we are to spend our days on the pasture, because papa wants us to,” she added, “but it is a shame Vinzi won’t be with us.” Stefeli answered Alida that it was never dull up on the pasture. Her father had prophesied a long stretch of good weather, and that meant that they would be out of doors all day.
  • 28. Mr. Lesa had been right. The merry little group could wander up day after day to the sunny pasture, and Mrs. Lesa saw to it that a really nourishing meal was always taken along. Alida was in perfect raptures over this free life, hitherto unknown to her. What a blissful beginning of the day to start off in the early morning, when she was usually lying asleep in bed behind her heavy curtains! How delicious the pure air was! All the birds in the trees sang and whistled so that it sounded like a loud chorus of joy to God. The two girls would start out together on little trips to explore the interesting places on the large, wide pasture. Sometimes they would seek berries or flowers; another time they went to the old wall where the shining lizards sunned themselves, or listened attentively when the children began to sing. Stefeli knew that if they were silent or made the least movement, the little green creatures would quickly slip away. Alida found it an especial treat to be able to sit down anywhere on the sun-dried ground. She had not experienced this before and it gave her constant pleasure. Stefeli was always ready to settle down beside her, and everything furnished them with subjects for lively conversation. On the first morning, when Stefeli had promised to waken Alida early, both girls stood fresh and full of enterprise before the barn, waiting for Jos. He had to loosen the cows from their chains before driving them out. Hugo had come down from his room, because his father had wished him to go along to the pasture. He looked so frail and tired that it hurt Mrs. Lesa to look at him. She led him affectionately back to his room, and fixing the cushions on his bed, told him to rest a little longer. There was no hurry for him to go, for a little later on she would take him up and show him the way herself. He would never have to start quite so early, if he did not want to. For the first time since he had lost his mother, Hugo felt himself sheltered again by a mother’s affectionate care. From that day on a great love for her began to fill his heart. She watched over him like a mother and saw to it that everything was done for him that might do
  • 29. him good. In these first days the quiet boy, who was still bearing a great sorrow in his heart, spent many hours alone in Mrs. Lesa’s company. He found great consolation in it and learned to feel such confidence in her that he began to talk about his mother. She listened with such sympathy that they always returned to that subject when he was with her. The comfort the boy found in her warm interest was soon apparent. One day Hugo came down quite early into the gleaming sunshine. He had never done it since that first day when he had looked so pale and tired. He already seemed much stronger and to Mrs. Lesa’s joy wished to go right up to the pasture. Till then he had preferred to sit in the house till she sent him out and accompanied him part way. Hugo found Jos alone on the pasture, singing and whistling while he strolled about among the cows. Alida and Stefeli had gone on a little trip of discovery. It seemed as if Hugo saw the beautiful creatures who were grazing here and there, looking about them, for the first time in his life. He began to ask Jos many questions, for after watching them carefully he had noticed how much they differed in their looks as well as in their ways. He had always thought that cows were just cows, one like another. Jos was in his element now and grew talkative, drawing Hugo’s attention to all the animals’ habits. The subject proved so contagious that Hugo conceived a keen interest in them and wanted to hear all about them. He only had to ask to be told what he wished to know. Jos could describe them with such keen vividness that Hugo grew most eager to share Jos’s knowledge and to find pleasure in it. He soon knew what fodder was the best for milk, which was made first into butter and then into cheese, and how the milk had to be handled for that purpose. He also learned that the Alpine herdsmen preferred Vinzenz Lesa’s milk to any other because his cows were of the best stock and were so immaculately kept. The two were still talking eagerly when to their great astonishment Stefeli came running toward them and spread out their mid-day meal under the swaying ash tree. They had been so lost in their
  • 30. conversation that they had not noticed how the time had flown. This had suited Stefeli, too, because she had come back rather late from her expedition. Alida also appeared and in the best of humor as the four sat down to lunch. All of them felt especially merry, because Hugo had never been so lively and gay. “We’ll pretend we are a family,” suggested Alida, “and we must always stay the same. Hugo is the proprietor of an estate and I am his sister, the unmarried lady of the mansion. Jos is our manager.” “Then Stefeli can be the mistress,” was Hugo’s proposition. “But Hugo, she couldn’t be,” Jos cried out. “Stefeli cooks for us and sets the table. She couldn’t be the gentleman’s wife, she must be my wife.” “Jos is right,” decided Alida, “we’ll have it that way, then.” “You see the owner has a mother; that’s much better than a wife,” said Hugo. “When Mrs. Lesa visits us some day, you’ll have to receive her as my mother and prepare a great feast for her.” This thought met great approval, and they began to plan immediately for this feast. Alida invented such astonishing plans for the celebration with torches and rockets that Jos said, “Then our cows will all jump over the hedges from fright and the people at the feast will have to climb after them in their festive jackets.” All four burst into loud laughter at this picture, which brought a sudden end to these extravagant plans. Their meal was ended and they settled here and there in the shadows of the broad branches. Soon all four were sound asleep, fanned by the leafy bows above them. They slept as well as if they had been lying on regal couches. On Hugo’s pale cheeks the air and sunshine, gently caressing him, roused a faint rosy color. Talking merrily, the whole company afterward came wandering down from the pasture, aglow from the golden light of the setting sun. As usual Jos went to the stable and Hugo disappeared with him.
  • 31. “Oh, dear, now he begins it, too,” said Stefeli. “I only wonder what he wants to do there still.” “Just let him go,” remarked Alida, “I am glad of it. He is much happier when he is with Jos. I have noticed that today.”
  • 32. A CHAPTER XI THE OLD SONG ONCE MORE UTUMN was here. In Lesa’s household it had come much too soon for every one, and the children could not believe that their days on the pasture were nearly over. Hugo and Alida would soon have to go right into the heart of a large city far away. Jos would go home to his mountain, while poor Stefeli would be left behind, unhappy and alone. “I don’t want to be always alone,” said Stefeli determinedly, when they had discussed their prospects on one of their last days together. “I’ll send a letter to Uncle Lorenz. You know he promised me something.” “I suppose Vinzi will soon come back,” said Alida, “and he’ll be wonderful company for you. We would have had lots more fun if he had been with us this summer. There were so many things I wanted to talk to him about, things none of you could understand.” The same day Vinzenz Lesa said to his wife, “I hope Lorenz won’t come yet to fetch Jos home. It has been such a good year that we still have our hands full. Things have to be done on every side and I can’t do them without the boy. He has the best memory and such good ideas; common sense, besides, enough for three, and a love and interest in the work as if it were all for himself, instead of for us. He just wants to keep the farm in good shape. I would give half the place to have such a son. It would mean so much to me.” “Let’s be glad he is still with us,” replied the wife, “and I think he’ll stay yet for a while. Good Cousin Lorenz hasn’t even warned us yet.” Mr. Delrick sent word from time to time, and a letter had come rather lately with the usual good news. Vinzi was well and everything going
  • 33. smoothly. Mr. Delrick seemed to be in no hurry for Vinzi’s return, though the year of his absence was practically over. A few days later when the children walked into the room one night a large letter lay on the table. “That’s from papa,” cried Alida, upon seeing the handwriting. It was addressed to Mr. Lesa. “Now everything is over,” said Hugo, who had come in also. “You’ll find that we’ll have to go now, Alida.” The children were quite frightened, for even while they had discussed their departure, none of them had realized how near it really was. Even Jos, who had joined them, made large, bewildered eyes. He had never quite pictured to himself the end of their bliss, and just lately least of all, when he and Hugo had been drawn so closely to each other in friendship. Should this all be over for good now, with their separation? As soon as the father came in, he took up the letter, only to put it aside again. He only read letters when there was nothing else to do, and supper was more important. When Mrs. Lesa had brought it in and had looked after every one’s needs, she asked her husband if she should open the letter. She knew that the children, who had recognized their father’s handwriting, were most anxious to hear it. As he willingly agreed, she first read the message to herself. All she told them, however, was that Mr. Thornau would arrive in a few days to fetch his children home. They would hear about the rest the next day. A deep silence followed. Soon the four children quietly stole away from the table, one by one. They met outside under the large walnut tree according to a previous plan. Here, under the old tree as a staunch witness, they gave each other the solemn promise to do everything in their power to reunite next summer. They longed to spend as perfect a summer together again. When the children had gone to bed and Vinzenz Lesa had lit his pipe, Mrs. Lesa sat down beside him to read aloud Mr. Thornau’s letter. He was writing from Dresden, where he had stopped a few days. He had gone there to see Mr. Delrick, who had decided to join him on his trip to
  • 34. Switzerland. This would give him an opportunity personally to bring the parents news about their son. But, as Mr. Thornau’s time was extremely limited, he unfortunately would have to take the last train to Leuk on Sunday evening and leave again with the earliest train on Monday. He therefore made them a proposal. If the Lesa family, including their little daughter, would bring his children to Freiburg on Sunday, they could all spend a pleasant day together in that city. As Freiburg was Mr. Lesa’s former home, Mr. Thornau had a hope that it was possible to persuade Mr. Lesa to go there. They might arrive early on Sunday afternoon, and he and Mr. Delrick would meet them at the train, as they expected to reach Freiburg earlier. In conclusion he said that Mr. Lesa and his wife would, by doing him this favor, crown their former kindness to his children. A refusal of his expectations would mean no less to him than a hailstorm to Mr. Lesa’s fields. Vinzenz Lesa remained silent for quite a while. Then he asked, “Does he write nothing about our boy?” Mrs. Lesa answered that the only thing about Vinzi was what she had read, namely, that Mr. Delrick was anxious to give them news about the boy in person. “Do you know why he doesn’t want to bring him?” asked Lesa further. “We can’t know that,” replied the wife. “But we can guess, and I’ll tell you why. The reason is that Mr. Delrick realizes that he mustn’t make him into a travelling musician. He is in no hurry to bring him home, because he has turned his thoughts further than ever from farming and he is afraid to tell me. I know he is a good man, but he has made a mistake. Everybody dislikes admitting such a thing.” Vinzenz Lesa blew such thick clouds of smoke from his pipe that his wife found it advisable to let them disperse a little. Then she said cautiously, “I think we had better hear first what Mr. Delrick has to
  • 35. say. It will be a blessing to be able to talk to him. What do you say about going to Freiburg?” “I wouldn’t think of going,” said her husband curtly. “You don’t suppose I’ll go as far as that for no reason whatever. Why should I go to Freiburg, as if I had nothing else to do?” “Nobody who knows you could possibly think that,” said his wife. “Besides, Mr. Thornau hasn’t fixed it on a Sunday for nothing. He knows well enough you wouldn’t come during the week. He asks us most politely, and it would have been easy for him to tell any other person to fetch the children home. Apparently, he would value it very much if we took the children to him. I must admit, Vinzenz, that I have grown extremely fond of them. They have both clearly shown me how they hate to leave. I’d love to go and put them into his hands myself.” “Why don’t you go with them, then?” he retorted, still showing an obstinate disinclination in his voice. “You had better go to Freiburg. You are sufficient escort for them.” “No, Vinzenz, I won’t do it,” answered his wife with firm decision. “You can see from the letter that Mr. Thornau wants you, too. You also know that I couldn’t settle with Mr. Delrick what is further to be done about the boy. We’ll either go together, or we won’t go at all. It is for you to decide.” The silence which followed proved too long for Mrs. Lesa and she began again. “I don’t really see why we shouldn’t go to Freiburg as long as they ask us to. Don’t you remember how delighted we used to be as children when father and mother would take us there on a Sunday? We were the happiest creatures on earth sitting in the high carriage between them with such exciting adventures before us. We really should do it for memory’s sake. We always went to church first and heard the beautiful organ. Do you remember how your mother insisted on doing this first of all? Wouldn’t it give you pleasure to go back? It also would give you a splendid chance to run over to your farm. You ought to have looked it over long ago, and you are sure to find lots of things to attend to. Now that Jos is with us, you can leave
  • 36. much more easily than afterward. You’ll have a lot of important things to do there, and if nothing special takes you there, you’ll never make up your mind to go. You had better take the journey before something actually goes wrong and you might have reason to blame yourself for not having gone before.” The thought that the trip might have a good side had begun to take hold of Vinzenz Lesa. Very deliberately, but still unwillingly, he replied, “How did you think of all these things? I don’t see how we could be away so long. It would take at least three days.” “Not for everybody,” replied Mrs. Lesa promptly, for she had thought out everything minutely. “We can easily attend to things here on Sunday morning before leaving. We’ll be in Freiburg early in the afternoon and that same evening I’ll travel home as far as I can with the children, in order to get here as soon as possible on Monday morning.” “How many children do you expect to bring home? The two strangers will have gone and you yourself have only one.” This time the husband had found the words first. “Only our two, Jos and Stefeli; I don’t mean to bring strangers home,” his wife said calmly. “What! You want to take Jos along, too?” said Vinzenz Lesa much excited. “That would make it out of the question for me to go. If Jos isn’t home to keep things in order, I won’t even budge from here! Count on that.” But Mrs. Lesa began to show him how easily their hired man could look after the simple work in the stable which had to be done on Sunday. Monday morning Jos would be back again. In this way nothing would be neglected. “Just think, Vinzenz,” concluded his wife, “what Jos has done for us. He has been with us a long while and we have not done the slightest thing for him. I am sure it would give him pleasure to go along. We owe it only to him if you have time now to look over your other farm to see what has to be done
  • 37. there. You can easily stay away a week, for I promise to look after the farm if I have Jos, so you won’t need to grumble.” Vinzenz Lesa was a just man, eager to act upon what he felt to be right. He was glad to give Jos a well-earned holiday and to have the opportunity besides of carrying out a needful undertaking. After weighing the matter carefully, he finally declared, “All right, we’ll go then, but tell Jos about it as soon as possible. He can plan things out beforehand so everything will stay in order while he is gone.” “Tomorrow,” said Mrs. Lesa, happy at having such pleasant prospects to tell the children, especially her ever-willing Jos. At the news of their coming expedition the children showed unbounded delight. Alida and Stefeli jumped from sheer enthusiasm and Hugo said, “Now everything isn’t quite over yet. It will be much easier for us to go away if everybody comes along.” Jos could hardly realize the treat that lay before him. It was too wonderful to be taken to Freiburg and see the city and get a glimpse of the fine country on the way. From joy and astonishment he could scarcely say a word. It would be his first journey, for he had never been away from his mountain except when he had come to Leuk. Sunday arrived. The whole company travelled gaily across the beautiful country which lay gleaming in the brightest sunshine. The trip proved even more delightful than they had anticipated. Alida and Stefeli never stopped talking, and Hugo kept busy either joining in with them or pointing out the interesting things along the way, which they would have overlooked on account of their constant chatter. Jos, on the contrary, was so absorbed in the new, beautiful things on every side that he never took his eyes from the landscape and remained in the deepest silence, attentively gazing out. Thus the hours flew by unnoticed. Quite surprised, the children jumped up from their seats when the father said, “Look out, now, we are nearly at Freiburg.” In a few minutes the name was actually called out and the train stopped. Mrs. Lesa looked out of the window expectantly to see if
  • 38. the gentlemen had come to meet them. Sure enough, there they were. But the mother uttered a low sigh. She had hoped till the last moment that Mr. Delrick, as a surprise, had brought their boy with him. But Vinzi wasn’t there. Loud cries of joy beside her drew her attention to the two children, who flung themselves upon their father. Mr. Thornau now held them both in his arms and responded to their stormy greetings with exclamations of joyous surprise. He held his son a short distance from him and looked at him happily. “And is this supposed to be my thin, frail Hugo, who had neither spirit nor strength. You look as sunburnt as a woodsman, with your fat cheeks and beaming eyes. Mrs. Lesa,” he called to the mother, “what did you do with him? He is a different creature. And is this stalwart gipsy girl really my Alida? The blood throbs through your brown cheeks with health. Mrs. Lesa, Mrs. Lesa, how on earth did you do it?” “Oh, we know, we know how,” cried out the children simultaneously, and immediately began to relate to the father how they had spent their delicious summer days. It would have been impossible for Mrs. Lesa to give any information now. After greeting her husband, Mr. Delrick took Mrs. Lesa’s hand. Just is if he had read from her eyes what was in her heart, he said, “Don’t worry about Vinzi, Mrs. Lesa. He is perfectly well, and he is still a great joy to me. The reason why he is not here is not a bad one, you must believe me. Oh, here is my little Stefeli, too, and another old friend besides,” he continued, turning toward the children. “I am glad Jos came, too, for he must belong entirely to your family by now.” “Yes, like our own,” said Mrs. Lesa, putting her arms around the boy. “Thank God that we have him.” “I’d like to ask what you think about a plan we made,” continued Mr. Delrick. “I wonder what Mr. Lesa will say. Our friend, Mr. Thornau, has asked us all to dinner today at a hotel quite near the beautiful
  • 39. old Nicholas church. So we thought it would be pleasant to go there first of all and then have a nice, quiet time together.” Mrs. Lesa glanced at her husband, for she wanted him to decide. She herself was most anxious to go to the church immediately. Old memories had risen in Vinzenz Lesa’s mind as soon as he had set his foot in Freiburg. As a child he had known nothing more wonderful than to drive to Freiburg on a Sunday, and his little girl neighbor had felt the same. He had always loved to enter the tall, quiet church holding his mother’s hand. Whenever he heard the sounds of the wonderful organ, they revealed to him a different world, one quite apart from his daily life. “I’d be only too glad to go to church first,” he began. “It is but suitable that we should go, as it is Sunday today.” At this the little company started off into the town. It was very still in the large old church, and so dark and solemn that the children stepped very lightly before settling down beside their parents. Suddenly the organ began to play, and rich, powerful tones floated through the quiet building. It seemed as if all the heavenly hosts were singing a hymn of praise and joy for all the world.
  • 40. RICH, POWERFUL TONES FLOATED THROUGH THE QUIET CHURCH Stefeli uttered a low cry and violently pulled her mother’s sleeve. “It is Vinzi, mother,” she said in intense excitement. The mother also had
  • 41. recognized her boy’s curly head at her first glance toward the organ. It had so surprised and overcome her that she could barely keep from sobbing aloud. She wanted her husband to share her deep emotion. Quietly touching him, she whispered, “Vinzenz, it is Vinzi.” He gave no reply, nor looked up, seemingly unwilling to show he had been moved. The music suddenly changed. From a low plaint it swelled to a tremendous wail, is if a chorus of despairing creatures were cast down by sorrow and contrition. In the midst of this, their grief reached its height and changed into meek, ardent entreaties for help and mercy. At that moment the heavens seemed to open and a clear, lovely chant of angels brought down a message of love and eternal joy. Just in the middle of the angels’ choir, a rich, clear voice rang out and filled the church with the words: “And the blessed song of mercy—” Jos, upon hearing the well-known sounds, so much more glorious and inspiring than he had ever imagined them, had been completely carried away. At the ending where he had always joined in he could not stop himself from lustily singing his words to the chorus of angels. When the last tone faded away a deep silence reigned in the church. After a while both gentlemen rose. Vinzenz Lesa also got up, but he had been obliged to wipe his eyes a number of times. “How can you believe such a thing,” he said to his wife in a hoarse voice in answer to the words she had whispered to him. “No one can tell me that was Vinzi.” Mr. Delrick stood immediately behind him. “Mr. Lesa,” he said, patting him on the shoulder, “we don’t need to believe anything till we have gone to the organ to see for ourselves.” “He can certainly play, I know that,” said Mr. Thornau satisfied. “How did you like it, Alida?” “Oh, it was wonderful and I wish it were not over yet.”
  • 42. “Come, let us go along, too. I’d love to see who played,” and taking his daughter’s hand, Mr. Thornau followed the others up to the organ loft. Mr. Delrick had quickly gone ahead, and before the others had arrived, the same joyful melody greeted them again. Vinzenz Lesa, upon entering, stood stock still. At the organ really sat his curly- headed boy. As he was playing, the father could actually hear how he produced those stirring melodies. Nothing helped now, and Lesa had to wipe his eyes repeatedly. Vinzi had ceased, and his mother finally let Stefeli’s hand go. The child rushed to her brother and tenderly clung to him. Now the father came up with Mr. Delrick. “Mr. Lesa,” he said, “your son has played you the song of mercy, and I hope you will also be merciful and forgive him if he does not want to become a farmer.” Lesa had grasped his son’s hand. “More than that, more than that, Vinzi,” he finally said after a great effort to control himself. “You make your parents proud of you and not what I used to think. I never realized such a thing could happen. I never, never knew it. When I used to come to Freiburg as a boy and heard this organ, I thought that people who played it were creatures entirely set apart, not human beings like one of us. You must be most thankful to Mr. Delrick, boy, for we owe him everything. He found the way for you and made this possible.” “Not I, Mr. Lesa, not I,” Mr. Delrick remonstrated. “Pater Silvanus is the one who knew what was in Vinzi and who knew what ought to be done with him. Let us be grateful to him. You shall also know right away that Vinzi is not losing time with a breadless art. He already has an offer from a church in Dresden to play every Sunday. I suppose you approve of that, Mr. Lesa? He’ll be able to keep on studying at the same time and I think the longer the better. I hope it will suit you to leave him with me for a few more years.” “Whatever you deem wise, sir,” Lesa replied, unhesitatingly, perfectly convinced that Mr. Delrick’s plans and his would evermore agree.
  • 43. Vinzenz Lesa was pleased that his friend had kept a long silence about Vinzi, until he could really prove his case. One surprise after another was in store for Vinzi. When Jos suddenly stepped up to him, Vinzi greeted his old friend with the greatest joy. “So it was you, after all!” he exclaimed. “I heard you singing down in the church and it nearly upset me. I had to think over and over again: ‘Who knows our song and has a voice just like Jos’s?’ But I thought you couldn’t be here, and here you are, after all.” In the greatest surprise he greeted Alida, Hugo and Mr. Thornau. Alida found so many things to talk over with Vinzi about their past and future that Mr. Thornau found it wise to announce that the moment for dinner had arrived. Then all their conversations could be continued at leisure. However much Mr. Lesa fought against it, he was taken to dinner at the Zohringer Hotel, where he had not set his foot in his life before. Unfortunately the hour of parting came much too soon for them all. “You don’t know what I have to thank you for,” said Mr. Thornau to Vinzenz Lesa at leave-taking. “My son, who never has shown the slightest inclination nor wish for any occupation, has just informed me that he has decided to become a landowner. After having already chosen the manager for his estate, he insists on getting all his cows from you. He says you have the finest that can be found anywhere, and he knows all about the breed. My daughter has been fighting against studying music for over a year and has not once opened her beautiful piano in spite of all my remonstrances. Now she declares to me that it is her highest wish to take up her lessons again. Hearing her former pupil play so beautifully makes her ambitious to work hard and catch up with him. I really mean it most seriously, Mr. Lesa, when I say that my gratitude for everything that my children have experienced in your house is much greater than I can say. I would love to prove it to you. Don’t you ever expect to get to Hamburg, Mr. Lesa?” “No, I don’t believe so,” said the latter truthfully. “But we are glad if the children have profited somewhat. We liked to have them and
  • 44. would like them to come again.” Mr. Thornau pressed Mrs. Lesa’s hand most gratefully, for his children had told him so much about her that he knew she had been a real mother to them. “Will you let the children come back again?” he asked sincerely. “It would make me quite unhappy if they didn’t come next summer,” she said half sadly, half happily, “and I’ll look forward to their coming every summer.” Upon taking leave of Mr. Lesa, Mr. Delrick told him that they would see each other again, as he was planning to spend Vinzi’s holidays in Leuk. But first he intended to go to the Simplon in order to show Pater Silvanus how his pupil had progressed. If the father had no objection he would take Vinzi with him. Lesa replied that Mr. Delrick should do whatever he thought best. Vinzi, who was standing beside them, listened breathlessly. What had his father said? How could he say this when he had forbidden him so emphatically to go back to the mountain? Anxious to know the truth, he asked a little timidly, “Father, can I really go to the mountain with Mr. Delrick?” “You may do whatever Mr. Delrick says,” replied the father. Vinzi’s eyes sparkled with delight. He was to return to the mountain in a few days and see every one he had loved so dearly. He would be able to thank Pater Silvanus, as he fully realized how much he had to be grateful for. That same hour all separated. The children found it quite impossible to get done with their leave-taking. Whenever they seemed to have finished, they began again. But they were far from sad, as they had the sure prospect of meeting again next summer. Vinzi was to be with them also, as Mr. Delrick had promised to come to Leuk at the beginning of his holidays. Mr. Thornau travelled in the direction of Basle, and Vinzenz Lesa toward Bulle, near his paternal home. Mr. Delrick and Mrs. Lesa, with the three children, journeyed toward
  • 45. Lausanne on the lake of Geneva. Here they spent the night in order to arrive home as soon as possible the next morning. On the evening of the following day Vinzi went upstairs with his mother. When they came to his little room, he said to her. “Oh, mother, it is so lovely to be at home again; won’t you please come in and say good-night to me the way you used to do?” Before going to bed that night Stefeli had run outside, as if looking for some one. She found Mr. Delrick sitting on the bench as usual, just as she had expected. “When you go to the mountain, Mr. Delrick, will you give somebody a message from me?” she inquired, as soon as she had joined him. After expressing his readiness to do so, Stefeli went on: “Won’t you please tell Uncle Lorenz that Jos mustn’t go home yet. He promised to leave him here till father sends him away, and I know that he’ll never do that.” Mr. Delrick promised faithfully to deliver her message. He could not help smiling to himself when he remembered how similar were the three messages he was asked to take. Mr. Lesa had begged his cousin Lorenz to leave Jos with him, if possible, as he could hardly live without the boy. A few moments later Jos had appeared, very anxious to let his father know that he could not yet come home. So much had still to be done before the winter, and then spring would be here again. This year’s work could barely be finished before next year’s labors would begin. His Uncle Vinzenz should really not be left alone, for it was settled now that Vinzi would stay away for good. Mr. Delrick said to himself with a smile that Cousin Lorenz would not be able to gainsay this threefold request, especially against his own firm promise. Mrs. Lesa went into Vinzi’s room and sat down on his bed. This vividly brought back to her the dreadful evening before he had departed for the Simplon. It was the first time Vinzi had been left alone with his mother since his return. He began to pour out his heart to her and told her all about his life in Dresden. He had not
  • 46. written, because his father had said that it was no good. He could not relate enough of Mr. Delrick’s fatherly kindness to him in every way. Vinzi was allowed to study many things, especially music. One splendid teacher taught him to play the organ and the other gave him instructions in the laws of music. From the latter he had learned to understand these and all their marvels. This lesson had been a great joy to him and he had been impatient for it every time. “But my organ lesson was better yet,” Vinzi added. “Oh, mother, I never realized one could look forward so much to anything. It was always like a great feast to me.” With boundless happiness the mother looked at the radiant eyes of her boy. “And isn’t it part of your happiness, Vinzi, to live with Mr. Delrick? It must be wonderful to belong to his home and be with him so much!” she inquired. “Yes, and not a small part of it either,” he cried out. “I am glad you know him, otherwise you could have no idea how kind, affectionate and helpful he is. Everybody in his house is good to me, too, for his sake. Mrs. Wyneken, his housekeeper, who has charge of everything, is as kind to me as if she were my grandmother. Frederick the butler, and Minnie, the maid, both wait on me and won’t let me do anything for myself. It is mostly because they all love to hear me play the organ. Mrs. Wyneken says that Mr. Delrick has grown happy again since I play it. He had lost every one in the world he loved and had been very sad. He has a little organ in a large room downstairs. Mr. Delrick asks me to play it for him every night. Sometimes we can hardly stop. Now you know what my life in Dresden is like. But I cannot say how much I look forward to my little hayloft on the mountain under the stars.” “Vinzi,” said the mother, “do you ever thank God for all the blessings He has heaped upon you. Do you realize that it is He who has given you everything?” “Yes, I do, mother,” he replied, looking frankly into her eyes. “I never forget how frightened and troubled I often was. Sometimes we sing one of the songs you taught me in the evening, but I sing them quite
  • 47. differently now. I used only to enjoy the tune, but now I thank and praise God from the bottom of my heart.” “If you should ever get into a difficult position, Vinzi,” the mother concluded, “you must remember that God often means to work out our good when we fear evil. When I sat on this bed before you went away the first time, I should in my shortsightedness have done everything to keep you home. And just think! That journey proved the beginning of your great happiness.” Vinzi had listened attentively. “It was the same with me,” he said. “I thought it was the most dreadful thing that could happen to me and it proved just the opposite. I’ll always remember that, mother.” When the mother left Vinzi’s chamber after a hearty good-night, her heart was filled with gratitude and bliss. Folding her hands she sent up a heartfelt prayer of thanks to Heaven. What blessings God had sent to her and her household. Her boy’s inmost wish had been fulfilled and his future lay clearly before him. The father was not only satisfied, but actually pleased. He did not despise his Vinzi any more, because he realized that he could be truly proud of the boy and he showed it quite openly in the way he looked at him. Vinzi had been away a long while and had lived in totally different surroundings. He had learned a great deal, but had remained just as affectionate and simple as when he had gone away. This made his mother extremely happy, and she sent a fervent prayer to God to keep him pious and childlike upon all the paths of his coming life. The End
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