Work and simple_machines

What is work?
 In science, the word

work has a different
meaning than you may be familiar with.
 The scientific definition of work is: using a
force to move an object a distance (when
both the force and the motion of the object
are in the same direction.)

2

Work or Not?


According to the
scientific definition,
what is work and what
is not?




a teacher lecturing to
her class
a mouse pushing a
piece of cheese with
its nose across the
floor
3

Work or Not?


According to the
scientific definition,
what is work and what
is not?




a teacher lecturing to
her class
a mouse pushing a
piece of cheese with
its nose across the
floor
4

What’s work?
 A scientist delivers a speech to an

audience of his peers.
 A body builder lifts 350 pounds above his
head.
 A mother carries her baby from room to
room.


A father pushes a baby in a carriage.

 A woman carries a 20 kg grocery bag to

her car?

6

What’s work?
 A scientist delivers a speech to an

audience of his peers. No
 A body builder lifts 350 pounds above his
head. Yes
 A mother carries her baby from room to
room. No


A father pushes a baby in a carriage. Yes

 A woman carries a 20 km grocery bag to

her car? No

7

Formula for work
Work = Force x Distance
 The unit of force is newtons
 The unit of distance is meters
 The unit of work is newton-meters
 One newton-meter is equal to one joule
 So, the unit of work is a

joule
8

W=FD
Work = Force x
Distance
Calculate: If a
man pushes a
concrete block 10
meters with a
force of 20 N, how
much work has he
done?
9

W=FD
Work = Force x
Distance
Calculate: If a
man pushes a
concrete block 10
meters with a
force of 20 N, how
much work has he
done? 200 joules
(W = 20N x 10m)
10

Power
 Power is the rate at which work is done.
 Power = Work*/Time

(force x distance)

*

 The unit of power is the watt.

11

Check for Understanding
1.Two physics students, Ben and Bonnie, are in
the weightlifting room. Bonnie lifts the 50 kg
barbell over her head (approximately .60 m) 10
times in one minute; Ben lifts the 50 kg barbell
the same distance over his head 10 times in 10
seconds.
Which student does the most work?
Which student delivers the most power?
Explain your answers.
12

Ben and Bonnie do
the same amount of work;
they apply the same force
to lift the same barbell the
same distance above
their heads.
Yet, Ben is the most
powerful since he does
the same work in less
time.
Power and time are
inversely proportional.

13

2. How much power will it take to move a
10 kg mass at an acceleration of 2 m/s/s a
distance of 10 meters in 5 seconds? This
problem requires you to use the formulas
for force, work, and power all in the correct
order.
Force=Mass x Acceleration
Work=Force x Distance
Power = Work/Time
14

2. How much power will it take to move a 10 kg mass at
an acceleration of 2 m/s/s a distance of 10 meters in 5
seconds? This problem requires you to use the formulas
for force, work, and power all in the correct order.
Force=Mass x Acceleration
Force=10 x 2
Force=20 N
Work=Force x Distance
Work = 20 x 10
Work = 200 Joules
Power = Work/Time
Power = 200/5
Power = 40 watts

15

History of Work

Before engines and motors were invented, people had
to do things like lifting or pushing heavy loads by hand.
Using an animal could help, but what they really needed
were some clever ways to either make work easier or
faster.
16

Simple Machines
Ancient people invented simple machines
that would help them overcome resistive
forces and allow them to do the desired work
against those forces.

17

Simple Machines


The six simple machines are:







Lever
Wheel and Axle
Pulley
Inclined Plane
Wedge
Screw

18

Simple Machines
 A machine is a device that helps make

work easier to perform by accomplishing
one or more of the following functions:





transferring a force from one place to another,
changing the direction of a force,
increasing the magnitude of a force, or
increasing the distance or speed of a force.

19

Mechanical Advantage
 It is useful to think about a machine in

terms of the input force (the force you
apply) and the output force (force which
is applied to the task).
 When a machine takes a small input force
and increases the magnitude of the output
force, a mechanical advantage has
been produced.
20

Mechanical Advantage







Mechanical advantage is the ratio of output force divided
by input force. If the output force is bigger than the input
force, a machine has a mechanical advantage greater
than one.
If a machine increases an input force of 10 pounds to an
output force of 100 pounds, the machine has a
mechanical advantage (MA) of 10.
In machines that increase distance instead of force, the
MA is the ratio of the output distance and input distance.
MA = output/input

21

No machine can increase both
the magnitude and the
distance of a force at the same
time.

22

The Lever





A lever is a rigid bar that
rotates around a fixed
point called the fulcrum.
The bar may be either
straight or curved.
In use, a lever has both
an effort (or applied) force
and a load (resistant
force).

23

The 3 Classes of Levers


The class of a lever is
determined by the
location of the effort
force and the load
relative to the fulcrum.

24

To find the MA of a lever, divide the output force by the input force, or
divide the length of the resistance arm by the length of the effort arm.

26

First Class Lever


In a first-class lever the fulcrum is located
at some point between the effort and
resistance forces.




Common examples of first-class levers
include crowbars, scissors, pliers, tin snips
and seesaws.
A first-class lever always changes the
direction of force (I.e. a downward effort force
on the lever results in an upward movement of
the resistance force).
27

Fulcrum is between EF (effort) and RF (load)
Effort moves farther than Resistance.
Multiplies EF and changes its direction
28

Second Class Lever





With a second-class lever, the load is located
between the fulcrum and the effort force.
Common examples of second-class levers
include nut crackers, wheel barrows, doors, and
bottle openers.
A second-class lever does not change the
direction of force. When the fulcrum is located
closer to the load than to the effort force, an
increase in force (mechanical advantage)
results.
29

RF (load) is between fulcrum and EF
Effort moves farther than Resistance.
Multiplies EF, but does not change its direction
30

Third Class Lever
 With a third-class lever, the effort force is

applied between the fulcrum and the
resistance force.




Examples of third-class levers include
tweezers, hammers, and shovels.
A third-class lever does not change the
direction of force; third-class levers always
produce a gain in speed and distance and a
corresponding decrease in force.

31

EF is between fulcrum and RF (load)
Does not multiply force
Resistance moves farther than Effort.
Multiplies the distance the effort force travels
32

Wheel and Axle




The wheel and axle is a
simple machine
consisting of a large
wheel rigidly secured to a
smaller wheel or shaft,
called an axle.
When either the wheel or
axle turns, the other part
also turns. One full
revolution of either part
causes one full revolution
of the other part.
33

Pulley









A pulley consists of a grooved wheel that
turns freely in a frame called a block.
A pulley can be used to simply change the
direction of a force or to gain a mechanical
advantage, depending on how the pulley is
arranged.
A pulley is said to be a fixed pulley if it does
not rise or fall with the load being moved. A
fixed pulley changes the direction of a force;
however, it does not create a mechanical
advantage.
A moveable pulley rises and falls with the
load that is being moved. A single moveable
pulley creates a mechanical advantage;
however, it does not change the direction of
a force.
The mechanical advantage of a moveable
pulley is equal to the number of ropes that
support the moveable pulley.

34

Inclined Plane


An inclined plane is
an even sloping
surface. The inclined
plane makes it easier
to move a weight from
a lower to higher
elevation.

35

Inclined Plane




The mechanical
advantage of an inclined
plane is equal to the
length of the slope
divided by the height of
the inclined plane.
While the inclined plane
produces a mechanical
advantage, it does so by
increasing the distance
through which the force
must move.
36

Although it takes less force for car A to get to the top of the ramp,
all the cars do the same amount of work.

A

B

C
37

Inclined Plane


A wagon trail on a steep
hill will often traverse
back and forth to reduce
the slope experienced by
a team pulling a heavily
loaded wagon.



This same technique is
used today in modern
freeways which travel
winding paths through
steep mountain passes.
38

Wedge



The wedge is a modification of
the inclined plane. Wedges are
used as either separating or
holding devices.



A wedge can either be
composed of one or two
inclined planes. A double
wedge can be thought of as
two inclined planes joined
together with their sloping
surfaces outward.

39

Screw




The screw is also a
modified version of
the inclined plane.
While this may be
somewhat difficult to
visualize, it may help
to think of the threads
of the screw as a type
of circular ramp (or
inclined plane).
40

MA of an screw can be calculated by dividing the number of
turns per inch.

41

Efficiency


We said that the input force times the distance equals the
output force times distance, or:
Input Force x Distance = Output Force x Distance
However, some output force is lost due to friction.



The comparison of work input to work output is called
efficiency.



No machine has 100 percent efficiency due to friction.
43

Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying
bricks and placing them on the wall of a building being constructed,
or a project supervisor observing and recording the progress of the
workers from an observation booth.

2. How much work is done in pushing an object 7.0 m across a floor
with a force of 50 N and then pushing it back to its original position?
How much power is used if this work is done in 20 sec?

3. Using a single fixed pulley, how heavy a load could you lift?

44

Practice Questions
4. Give an example of a machine in which friction is both an
advantage and a disadvantage.
5. Why is it not possible to have a machine with 100%
efficiency?

6. What is effort force? What is work input? Explain the
relationship between effort force, effort distance, and
work input.
45

Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying bricks
and placing them on the wall of a building being constructed, or a project
supervisor observing and recording the progress of the workers from an
observation booth. Work is defined as a force applied to an object, moving
that object a distance in the direction of the applied force. The bricklayer is
doing more work.
2. How much work is done in pushing an object 7.0 m across a floor with a
force of 50 N and then pushing it back to its original position? How much
power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700
N-m or J; Power = 700 N-m/20 sec = 35 W
3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed
lift?
pulley has a mechanical advantage of one, it will only change the direction
of the force applied to it. You would be able to lift a load equal to your own
weight, minus the negative effects of friction.

46

Practice Questions
4. Give an example of a machine in which friction is both an advantage and a

disadvantage. One answer might be the use of a car jack. Advantage of
friction: It allows a car to be raised to a desired height without slipping.
Disadvantage of friction: It reduces efficiency.

5. Why is it not possible to have a machine with 100% efficiency? Friction
lowers the efficiency of a machine. Work output is always less than work
input, so an actual machine cannot be 100% efficient.
6. What is effort force? What is work input? Explain the relationship between
effort force, effort distance, and work input. The effort force is the force
applied to a machine. Work input is the work done on a machine. The work
input of a machine is equal to the effort force times the distance over which
the effort force is exerted.

47

Work and simple_machines

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