Motion and its description

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Motion and its Description
SCIENCE AND TECHNOLOGY
9
MOTION AND ITS DESCRIPTION
Youmusthaveseennumberofthingsinmotion.Forexamplecar,bicycle,busmoving
on a road, train moving on rails, aeroplane flying in the sky, blades of an electric
fan and a child on a swing. What makes things move?Are all the motions similar?
You might have seen that some move along straight line, some along curved path
andsometoandfrofromafixedposition.Howandwhythesemotionsaredifferent?
You will find answers to all such questions in this lesson. Besides studying about
various types of motions, you will learn how to describe a motion. For this we will
try to understand the concepts of distance, displacement, velocity and acceleration.
We will also learn how these concepts are related with each other as well as with
time. How a body moving with constant speed can acquire acceleration will also be
discussed in this lesson.
OBJECTIVES
After completing this lesson you will be able to:
explain the concept of motion and distinguish between rest and motion;
describe various types of motion – rectilinear, circular, rotational and
oscillatory;
define distance, displacement, speed, average speed, velocity and
acceleration;
describe uniform and uniformly accelerated motion in one dimension;
draw and interpret the distance time graphs and velocity time graphs;
establish relationship among displacement, speed, average speed, velocity
and acceleration;
apply these equations to make daily life situation convenient and
explain the circular motion.

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9.1 MOTION AND REST
If you observe a moving bus you will notice that the position of bus is changing with
time. What does this mean? This means that the bus is in motion. Now suppose you
are sitting in a bus moving parallel to another bus moving in the same direction with
same speed.You will observe that the position of the other bus with respect to your
bus is not changing with time. In this case the other bus seems to be at rest with
respect to your bus. However, both the buses are moving with respect to
surroundings. Thus, an object in motion can be at rest with respect to one observer
whereas for another observer, the same object may be in motion. Thus we can say
that the motion is relative.
Letusunderstandtheconceptofrelativemotion.Supposeyouaresittinginavehicle
waiting for traffic signal and the vehicle beside you just starts moving, you will feel
that your vehicle is moving backward.
Suppose Chintu and Golu are going to the market. Golu is running and Chintu is
walkingbehindhim.Thedistancebetweenthetwowillgoonincreasing,thoughboth
are moving in the same direction.To Golu it will appear that Chintu is moving away
from him. To Chintu also, it will appear that Golu is moving ahead and away from
him. This is also an example of relative motion. See Fig. 9.1.
Fig. 9.1 An example of relative motion
Think and Do
One day, Nimish while standing on the bank of a river in the evening observed
boats were approaching the bank, vehicles passing on the bridge, cattle going
away from the bank of the river towards the village, moon rising in the sky, birds
flying and going back to their nests, etc. Can you list some thoughts that could
be emerging in the mind of the Nimish. What type of world Nimish has around
him?
Chintu Golu

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A
C
B
D
E
We can conclude that motion is a continuous change in the position of the object
with respect to the observer. Suppose you are moving towards your friend standing
in a field. In what way are you in motion?Are you in motion if you are observing
yourself?Isyourfriendinmotionwithrespecttoyou?Areyouinmotionwithrespect
to your friend? Now you may have understood that observer with respect to itself
can not be in motion. Thus, you are moving towards the object with respect to your
friendandyourfriendismovingtowardsyouwithrespecttoyouinoppositedirection.
In other words the change in position of the object with respect to observer decides
whether object is in motion. This change should also be continuous. Let us take an
interesting example to understand the concept of motion. There are five players
participating in 200 metre race event. They are running in their lanes as shown in
the Fig. 9.2. The players A, B, C, D and E runs 2, 3, 4, 3, 2 metre respectively
in one second. Can you help the player to understand that which player is in motion
withrespecttowhichplayerandwhichplayerisatrestwithrespecttowhichplayer?
Fill your responses in the table given below.
Fig. 9.2
Table 9.1
Observer player Player in motion Player at rest Remark
A B, C, D E E is in rest with respect to
A because change in
position of A and E in
1second is zero while in
other cases is not.
B
C
D
Now you will be able to help Nimish to answer some of his questions.

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9.1.1 Types of Motion
In our daily life we see many objects moving. Some objects moving in straight line
and some are not. For example, a ball rolls on a horizontal surface, a stone falling
from a building, and a runner on 100 m race track. In all these examples, you may
notice that the position of moving objects is changing with respect to time along a
straight line. This type of motion is called motion in a straight line or rectilinear
motion.
Fig. 9.3 Example of rectilinear motion
Can you think at least two more other example of such motions. You might have
observed the motion of time hands of a clock, motion of child sitting on a merry-
go-round, motion of the blades of an electric fan. In such a motion, an object follows
a circular path during motion. This type of motion is called circular motion.
ACTIVITY 9.1
(A) Suspend a small stone with a string (of length less than your height) with the
helpofyourhand.Displacethestoneasidefromthepositionofrestandrelease.
(B) Let the stone comes to rest and bring it to the point of suspension with the help
of your hand and release it.
(a) Ball rolling on horizontal surface
(b) Stone falling by hand
(c) A runner on a 100 m race track

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(A)
(B)
(C)
(C) Now hold the stone firmly in your hand and whirl it over your head.
Write in table given below, what type of motion of stone you have observed in all
the above three cases with justification.
Table 9.2
Case Type of motion Justification
A
B
C
(A) A person suspend the stone attached to a string, (B) A person oscillate the stone
attached to a string, (C) A person whirling the stone attached to a string
Fig. 9.4 (A), (B) (C)
Have you ever noticed that the motion of the branches of a tree? They move to and
fro from their central positions (position of rest). Such type of motion is called
oscillatory motion. In such a motion, an object oscillates about a point often called
position of rest or equilibrium position. The motion of swing and pendulum of wall
clock are also oscillating motions. Can you think about the motion of the needle of
a sewing machine? What type of motion is it? Now you can distinguish some of the
motions viewed by Nimish.

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9.2 DISTANCE AND DISPLACEMENT
For a moving object two points are significant. One is the point of start or origin
where from the object starts its motion and the other is the point where it reaches
after certain interval of time. Points of start and destination are connected by a path
taken by the object during its motion. The length of the path followed by object is
called distance. There may be a number of paths between the point of start and the
point of destination. Hence the object may cover different distances between same
point of start and destination. The unit of distance is metre (m) or kilometre (km).
ACTIVITY 9.2
An object moves from pointAto B along three different paths. Measure the distance
travelled by object along these three paths.
Fig. 9.5
In any motion, you will notice that object gets displaced while it changes its position
continuously. The change in position of the object is called displacement.
Basically, it is the shortest distance between initial and final position of the object.
The path followed by the object between initial and final positions may or may not
be straight line. Hence, the length of the path does not always represent the
displacement.
ACTIVITY 9.3
In the following cases measure the distance and displacement and write their values
in the table given below:
(a) A body moves from A to B
(b) A body moves from A to B then comes to C

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(c) A body goes from position A to B and comes back to position A
(d) A body goes from posion A to B and then C
(e) A body moves from position A to B along a circular arc
Fig. 9.6
Table 9.3
Case Distance Displacement
(i)
(ii)
(iii)
(iv)
(v)
Now you can conclude that:
(a) displacement is smaller or equal to the distance.
(b) displacement is equal to distance, if body moves along a straight line path and
does not change its direction.
(c) if a body does not move along a straight line path its displacement is less than
the distance.
(d) displacement can be zero but distance can not be zero.
(e) magnitudeofdisplacementistheminimumdistancebetweenfinalpositionand
initialposition.
(f) distance is the length of the path followed by the body.
(g) distance is path dependent while displacement is position dependent.
Can you now, suggest a situation in which the distance is twice the displacement?

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9.2.1 Graphical Representation of Distance and Displacement
Distance and displacement can also be shown by graphical representation. To draw
a graph, follow the following steps:
(i) Analyse the range of variables (maximum and minimum values).
(ii) Select the suitable scale to represent the data on the graph line adequately.
(iii) Take independent quantity on x-axis and dependent quantity on y-axis.
Take distance on x-axis and displacement on y-axis. You know that for a motion
along a straight line without changing its direction the distance is always equal to the
displacement. If you draw the graph, you will find that the graph line is a straight
line passing through origin making an angle of 45° with distance axis as shown in
Fig 9.7.
Fig. 9.7
Let us take another situation where an object moving from one position to another
and coming back to the same position. In this case the graph line will be a straight
linemakinganangleof45°withdistanceaxisuptoitsmaximumvalueandthencomes
to zero as shown in Fig. 9.8.
Fig. 9.8

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Now you can infer that:
If graph line is a straight line making an angle of 45° with x-axis or y-axis, the
motion is straight line motion and distance is equal to the displacement.
For same value of displacement, the distance travelled can be different.
If graph line does not make an angle of 45° with x-axis or y-axis, the motion
will not be straight line motion.
When an object moves along a circular path, the maximum displacement is equal
to the diameter of the circular path and the distance travelled by object keeps on
increasing with time as shown in Fig. 9.9.
Fig. 9.9
INTEXT QUESTIONS 9.1
Choose the correct answer in the followings:
1. For an object moving along a straight line without changing its direction the
(a) distance travelled > displacement
(b) distance travelled < displacement
(c) distance travelled = displacement
(d) distance is not zero but displacement is zero
2. In a circular motion the distance travelled is
(a) always > displacement
(b) always < displacements
(c) always = displacement
(d) zero when displacement is zero

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3. Two persons start from position A and reach to
position B by two different paths ACB and AB
respectively as shown in Fig. 9.10.
(a) Their distances travelled are same
(b) Their displacement are same
(c) The displacement of I > the displacement of II
(d) The distance travelled by I < distance travelled by II
4. In respect of the top point of the bicycle wheel of radius R moving along a
straightroad,whichofthefollowingholdsgoodduringhalfofthewheelrotation.
(a) distance = displacement
(b) distance < displacement
(c) displacement = 2R
(d) displacement = πR
5. An object thrown vertically upward to the height of 20 m comes to the hands
of the thrower in 10 second. The displacement of the object is
(a) 20 m (b) 40 m (c) Zero (d) 60 m
6. Draw a distance-displacement graph for an object in uniform circular motion on
a track of radius 14 m.
9.3 UNIFORM AND NON-UNIFORM MOTION
Let us analyze the data of the motion of two objectsAand B given in the table 9.4.
Table 9.4
Time in seconds (t) 0 10 20 30 40 50
Position of A (x1 in metre) 0 4 8 12 16 20
Position of object B (x2 in metre) 0 4 12 12 12 20
DoyoufindanydifferencebetweenthemotionofobjectAandB?Obviouslyobjects
AandBstartmovingatthesametimefromrestandbothobjectstravelequaldistance
in equal time. However, the object A has same rate of change in its position and
objectBhasdifferentrateofchangeinposition.Themotioninwhichanobjectcovers
equal distance in equal interval of time is called uniform motion whereas the motion
in which distance covered by object is not equal in equal interval of time is called
non-uniform motion. Thus, the motion of object A is uniform and of object B is non-
uniform.You can draw the position-time graph for the motion of object A and B and
observe the nature of the graph for both types of motion.
For the uniform motion of object A the graph is a straight line graph and for non-
uniform motion of object B the graph is not a straight line as shown in the Fig. 9.11.
Fig. 9.10

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Fig. 9.11 Graph representing uniform and non-uniform motion
9.3.1 Speed
Whileyouplanyourjourneytovisitaplaceofyourinterestyouintendtothinkabout
timeofjourneysothatyoucanarrangeneedfulthingslikeeatablesetc.forthatperiod
of time. How will you do it? For this you would like to know how far you have
to reach and how fast you can cover the destination. The measure of how fast motion
can take place is the speed. Speed can be defined as the distance travelled by
a body in unit time.
Thus speed =
Distance travelled
time taken
Its SI unit is metre per second which is written as ms–1. The other commonly used
unit is km h–1.
i.e., 1 kmh–1 = –11000 m 5
ms
60 60 s 18
=
×
ACTIVITY 9.4
Here position of four bodies A, B, C and D are given after equal interval of time
i.e. 2 s. Identify the nature of the motion of the bodies as uniform and non-uniform
motion.
Table 9.5
Time (s) → Bodies ↓ 0 2 4 6 8
positions (m) → A 0 4 8 12 16
B 0 8 8 10 12
C 4 8 12 16 20
D 0 6 12 16 20

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To identify the nature of the motion you can make a table as given below
Table 9.6
Time taken by body (s) → 2 – 0 = 2 4 – 2 = 2 6 – 4 = 2 8 – 6 = 2
Distance covered by
body (m) ↓
A 4 – 0 = 4 8 – 4 = 4 12 – 8 = 4 16 – 12 = 4
B 8 – 0 = 8 8 – 8 = 0 10 – 8 = 2 12 – 10 = 2
C 8 – 4 = 4 12 – 8 = 4 16 – 12 = 4 20 – 16 = 4
D 8 – 4 = 4 12 – 6 = 6 16 – 12 = 4 20 – 16 = 4
From the above table you can conclude that body A and C travel equal distances
in equal interval of time so their motion is uniform. But the distances travelled by
body B and D for equal intervals of time are not equal, hence their motion is non-
uniformmotion.
To analyze the motion as uniform motion or non-uniform motion, the motion can be
represented by graph. The position-time graph of all the four bodies A, B, C and
D is shown in Fig. 9.12.
Fig. 9.12
Now you can see that the bodies having uniform motion e.g. A and C have their
graphlinestraightandthebodieshavingnon-uniformmotiondonothavetheirposition
time graph line straight. In this graphical representation on axis 1 div = 1s and on
y-axis 1 div = 2m.

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Agraphdrawnfordifferentdistancestravelledbyobjectwithrespecttotimeiscalled
distance-time graph as shown in Fig. 9.13.
Fig. 9.13
In Fig. 9.13 distance travelled in 10 s is 22 m. Therefore, the speed of the object
=
( )
( )
22 m
10 s
= 2.2 ms–1
This motion can be represented by another way i.e., speed =
AB
OB
. This ratio is also
known as slope of the graph line. Thus the speed is the slope of position-time graph.
Example 9.1 An object moves along a rectangular path of sides 20 m and 40 m
respectively. It takes 30 minutes to complete two rounds. What is the speed of the
object?
Solution:
( )2 2 20 40 mDistance travelled
time taken 30 60 s
× +
=
×
–14
ms
30
=
9.3.2 Velocity
If you are asked to reach a destination and you are provided three, four paths of
differentlengths,whichofthepathwouldyouprefer?Obviously,thepathofshortest
length but not always. This is also called displacement. In the previous section you
havelearntaboutdistance.Whenmotionisalongtheshortestpath,itisdirectedfrom
thepointofstarttothepointoffinish.Howfastthismotionisdeterminesthevelocity.
The velocity is the ratio of length of the shortest path i.e. displacement to the time
taken

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velocity =
Displacement
Time taken
Velocity has same unit as the unit of speed i.e., ms–1 (S.I. unit) or kmh–1.
The shortest path or the displacement is directed from initial position of the object
to the final position of the object. Hence, the velocity is also directed from initial
position of the object to the final position of the object. Thus we can say that the
velocity has direction. Speed does not have direction because it depends upon the
totaldistancetravelledbytheobjectirrespectiveofthedirection.Thequantitieswhich
have direction are called vector and which do not have direction are called scalar
quantity. Thus, velocity can also be expressed as
velocity =
Change in position
Time taken
ACTIVITY 9.5
Observe the motion of an object in the following situations. Find speed and velocity
ineachsituationandcommentoverthesituationwhichyoufinddifferentfromother.
Ohject moves from A to B in time 10 s on the scale 1 cm = 10 m
Object moves from A to B than to C in 10 s on the scale 1 cm = 10 m
Object moves from A to B than to C in 20 s on the scale 1 cm = 10 m
Object completes a round of radius 7 m in 10 s
Fig. 9.14

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Now you will be able to distinguish the speed and velocity. Magnitude of
instantaneous velocity is the speed. Now you can understand the importance of
preplanning your journey to save time, effort and fuel etc.
Example 9.2 In a rectangular field of sides 60 m and 80 m respectively two formers
start moving from the same point and takes same time i.e. 30 minutes to reach
diagonally opposite point along two different paths as shown in Fig. 9.15. Find the
velocity and speed of both the formers.
Fig. 9.15
Solution: The displacement of both the former in same i.e.,
2 260 80+ = 3600 6400 10000+ = = 100 m
∴ Velocity A and B, v =
displacement
time taken
=
100 m 1
30 60s 18
=
×
ms–1
speed of A =
( )80+60 mDistance travelled
time taken 30×60s
=
=
140
3800
ms–1 =14
18
ms–1
and speed of B =
Distance travelled 100s
time taken 30×60s
= =
1
18
ms–1
Note: In this example you can appreciate that the velocity of both the formers is
same but not the speed.
9.3.3 Average speed and average velocity
Speed during a certain interval of time can not be used to determine total distance
coveredingiventimeofthejourneyandalsothetimetakentocoverthetotaldistance

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ofjourney.Itisbecauseabodydoesnotalwaystravelequaldistanceinequalinterval
of time. In most of the cases the body travels non-uniformly. Thus, in case of non-
uniform motion to determine average speed is quite useful. The average speed can
be determined by the ratio of total distance covered to the total time taken.
Average speed =
total distance covered
total time taken
Similarly in case of average velocity in place of total distance covered you can take
totaldisplacement.
∴ Average speed =
total displacement
total time taken
Let us take few examples to understand the average speed and average velocity.
Example 9.3 If a body covers 50 m distance in 30 s and next 100 m in 45 s then
= 50 + 100 = 150 m
and total time taken = 30 + 45 = 75 s
∴ Average speed =
150 m
75 s
= 2ms–1
Example 9.4 If an object moves with the speed of 10 ms–1 for 10 s and with
8 ms–1 for 20 s, then total distance covered will be the sum of distance covered
in 10 s and the distance covered in 20 s = 10 × 10 + 8 × 20 = 260 m
∴ The average speed =
total time taken
=
( )
260 m 260 m
10+20 s 30 s
=
= 8.66 ms–1
Example 9.5 If a body moves 50 m with the speed of 5 ms–1 and then 60 m with
speed of 6 ms-1, then total distance covered
= 50 + 60 = 110 m
and total time taken will be the sum of time taken for 50 m and 60 m = 20 s
Thus, average speed =
total time taken
=
110 m
20 s
= 5.5 ms–1

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Example 9.6 If an object moves 30 m toward north in 10 s and then 40 m eastward
in next 10s, The displacement of the object will be OB
= 2 230 40+ = 900 1600+ = 2500
= 50 m
∴ The average velocity =
total displacement covered
total time taken
=
( )
50 m 50 m
= = 2.5
10+10 s 20 s
ms–1
Fig 9.16
Example 9.7 If an object moves along a circular track of radius 14 m and complete
one round in 20 s then for one complete round total displacement is zero and the
average velocity will also be zero.
From these examples you can conclude that:
(i) Instantaneous speed is the magnitude of instantaneous velocity but average
speed is not the magnitude of average velocity.
(ii) Average velocity is less than or equal to the average speed.
(iii) Average velocity can be zero but not average speed.
1. Some of the quantities are given in column I. Their corresponding values are
written in column II but not in same order. You have to match these values
corresponding to the values given in column I:
Column I Column II
(a) 1 kmh–1 (i) 20 ms–1
(b) 18 kmh–1 (ii) 10 ms–1
(c) 72 kmh–1 (iii) 5/18 ms–1
(d) 36 kmh–1 (iv) 5 ms–1

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2. A cyclist moves along the path shown in the diagram and takes 20 minutes from
point A to point B. Find the distance, displacement and speed of the cyclist.
Fig. 9.17
3. Identify the situation for which speed and average speed of the objects are equal.
(i) Freelyfallingball
(ii) Second or minute needle of a clock
(iii) Motion of a ball on inclined plane
(iv) Train going from Delhi to Mumbai
(v) When object moves with uniform speed
4. The distance-time graph of the motion of an object is given. Find the average
speed and maximum speed of the object during the motion.
Fig. 9.18
5. The distance travelled by an object at different times is given in the table below.
Draw a distance-time graph and calculate the average speed of the object. State
whether the motion of the object is uniform or non-uniform.
Table 9.7
Time (s) → 0 10 20 30 40 50
Distance (m) → 0 2 4 6 8 10

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6. A player completes his half of the race in 60 minutes and next half of the race
in 40 minutes. If he covers a total distance of 1200 m, find his average speed.
7. A train has to cover a distance of 1200 km in 16 h. The first 800 km are covered
by the train in 10 h. What should be the speed of the train to cover the rest
of the distance? Also find the average speed of the train.
8. A bird flies from a tree A to the tree B with the speed of 40 km h–1 and returns
to tree A from tree B with the speed of 60 km h–1. What is the average speed
of the bird during this journey?
9. Three players P, Q and R reach from point A to B in same time by following
three paths shown in the Fig. 9.19. Which of the player has more speed, which
has covered more distance?
Fig. 9.19
9.4. GRAPHICAL REPRESENTATION OF MOTION
Itshowsthechangeinonequantitycorrespondingtoanotherquantityinthegraphical
representation.
9.4.1 Position-time Graph
Itiseasytoanalyzeandunderstandmotionofanobjectifitisrepresentedgraphically.
To draw graph of the motion of an object, its position at different times are shown
on y-axis and time on x-axis. For example, positions of an object at different times
are given in Table 9.8.
Table 9.8 Position of different objects at different time
Time(s) 0 1 2 3 4 5 6 7 8 9 10
Position(m) 0 10 20 30 40 50 60 70 80 90 100
In order to plot position-time graph for data given in Table 9.8, we represent time
on horizontal axis and position on vertical axis drawn on a graph paper. Next, we
choose a suitable scale for this.

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Forexample,inFig.9.20onedivisiononhorizontalaxisrepresents1softimeinterval
and one division on vertical axis represents in 10 m, respectively. If we join different
points representing corresponding position time data, we get straight line as shown
inFig.9.20.Thislinerepresentstheposition-timegraphofthemotioncorresponding
to data given in Table 9.8.
Fig. 9.20 Position-time graph for the motion of a particle on the basis of
data given in table
We note from the data that displacement of the object in 1st second, 2nd second,.....,
10th second is the same i.e., 10 m. In 10 second, the displacement is 100 m.
Therefore, velocity is
100 m
10 s
= 10 ms–1 for the whole course of motion. Velocity
during 1st second = 10 ms–1 and so on.
Thus, velocity is constant i.e., equal to 10 ms–1 throughout the motion. The motion
of an object in which velocity is constant, is called uniform motion.
As you see Fig. 9.20, for uniform motion position-time graph is a straight line.
Like position-time graph, you can also plot displacement-time graph. Displacement
is represented on the vertical axis and time interval on the horizontal axis. Since
displacement in each second is 10 m for data in table, the same graph (Fig. 9.20)
alsorepresentsthedisplacement-timegraphifverticalaxisislabeledasdisplacement.
For good understanding you can observe the following graphs.

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(A) Uniform motion (B) Object is at rest
(C) Non-uniform motion, rate of change (D) Non-uniform motion, rate of change
in position is increasing in position is decreasing
Fig. 9.21 Graph (A), (B), (C), (D)
9.4.2 Velocity-Time Graph
Take time on the horizontal axis and velocity on the vertical axis on a graph paper.
Let one division on horizontal axis represent 1 s and one division on vertical axis
represent 10 ms–1. Plotting the data in Table 9.9 gives us the graph as shown in
Fig. 9.22.
Table 9.9 Velocity-time data of objects A and B
Time(s) 0 1 2 3 4 5 6 7 8
Velocity of A (ms-1) 0 10 20 30 40 50 60 70 80
Velocity of B (ms-1) 0 10 10 10 10 10 10 10 10
Fig. 9.22 Velocity-time graph for the motion of object A and B on the
basis of data given in table

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Lines OR and PQ represent the motion of object A and B respectively. Thus, we
see that the velocity-time graph of motion represented in Table 9.9 is a straight line
and parallel to time axis for object B. This is so because the velocity is constant
throughout the motion. The motion is uniform. Consider the area under the graph
in Fig. 9.22 for object B.
Area = (8s) × (10 ms–1) = 80 m. This is equal to the displacement of the object
B in 8 s.
Area under velocity-time graph = Displacement of the object during that time
interval
Similarly for object A area under the graph in Fig. 9.22.
= ( ) ( )
1
8 s 80 0
2
× − ms–1
= ( ) ( )
1
8 80 m
2
× = 320 m
This is equal to the displacement of object A in 8 s.
Though, we obtained this result for object B for a simple case of uniform motion,
it is general result.
Let x be displacement of an object in time t, moving with uniform velocity v, then
x = vt (for uniform motion)
You may have seen the motion of objects moving differently. Can you think what
make this difference? Observe the motion of a ball on a floor. The ball slows down
and finally comes to rest. This means that the velocity during different time intervals
ofmotionisdifferent.Inotherwordsvelocityisnotconstant.Suchamotioniscalled
accelerated motion.
9.5 ACCELERATION
In the previous section we have learnt about the non-uniform motion in which the
changeinvelocityindifferentintervalsofmotionisdifferent.Thischangeinvelocity
with time is called acceleration. Thus, the acceleration of an object is defined as
the change in velocity divided by the time interval during which this occurs.
Acceleration =
Change in velocity
Time interval
Itsunitisms–2.Itisspecifiedbydirection.Itsdirectionisalongthedirectionofchange
in velocity. Suppose the velocity of an object changes from 10 ms–1 to 30 ms–1 in
a time interval of 2 s.
Fig. 9.23 Changing velocity

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The acceleration, a =
–1 –130 ms – 10 ms
2.0s
= 10 ms–2
This means that the object accelerates in +x direction and its velocity increases at
a rate of 10 ms–1 in every second.
If the acceleration of an object during its motion is constant, we say that object is
moving with uniform acceleration. The velocity-time graph of such a motion is
straight line inclined to the time axis as shown in Fig. 9.24.
For a given time interval, if the final velocity is more than the initial velocity, then
accordingtoFig.9.24,theaccelerationwillbepositive.However,ifthefinalvelocity
is less than the initial velocity, the acceleration will be negative.
Fig. 9.24 velocity-time graph of an object moving with uniform acceleration
When velocity of the object is constant, acceleration will be zero. Thus, for uniform
motion,theaccelerationiszeroandfornon-uniformmotion,theaccelerationisnon-
zero.
Example9.8 Findthedistanceanddisplacementfromthegivenvelocity-timegraph
in Fig. 9.25.
Fig. 9.25

Notes
Moving Things
214
Solution:
Distancetravelled = Area of ΔOAB + Area of ΔBCD
=
1
2
(25) × (20) +
1
2
(10) × (20)
= 250 + 100 = 350 m
Displacement = Area of ΔOAB – Area of ΔBCD
=
1
2
(25) × (20) –
1
2
(10) × (20)
= 250 – 100 = 150 m
Example9.9Fromthegivenvelocity-timegraphobtaintheacceleration-timegraph.
Fig. 9.26
Solution: From the given graph acceleration for 0 – 10 s time interval
=
15 0
10 0
−
−
= 1.5 ms–2
acceleration for 10 – 20s time interval in same as for 20 – 30s time interval
=
20 15 5
30 10 20
−
=
−
= 0.25 ms–2
acceleration for 30 – 40s time interval =
20 20
40 30
−
−
= 0

Notes
MODULE - 3
Moving Things
215
acceleration for 40 – 50 and 50 – 60s interval =
30 20 10
60 40 20
−
=
−
= 0.5 ms–2
For all the above time intervals the acceleration-time graph can be drawn as shown
in Fig. 9.27.
Fig. 9.27
1. Describe the motion of an object shown in Fig. 9.28.
Fig. 9.28 Position-time graph of an object

Notes
Moving Things
216
2. Compare the velocity of two objects where motion is shown in Fig. 9.29.
Fig. 9.29 Position-time graph for object A and B.
3. Draw the graph for the motion of object A and B on the basis of data given in
Table 9.10.
Table 9.10
Time(s) 0 10 20 30 40 50
Position (m) for A 0 5 5 5 5 5
Position (m) for B 0 2 4 6 8 10
4. A car accelerates from rest uniformly and attains a maximum velocity of
2 ms–1 in 5 seconds. In next 10 seconds it slows down uniformly and comes
to rest at the end of 10th second. Draw a velocity-time graph for the motion.
Calculate from the graph (i) acceleration, (ii) retardation, and (iii) distance
travelled.
5. A body moving with a constant speed of 10 ms–1 suddenly reverses its direction
of motion at the 5th second and comes to rest in next 5 second. Draw a position-
time graph of the motion to represent this situation.
9.6 EQUATIONS OF MOTION
Consider an object moving with uniform acceleration, a. Let u be the initial velocity
(at time t = 0), v, velocity after time t and S, displacement during this time interval.
There are certain relationships between these quantities. Let us find out.
We know that
Acceleration =
Chnage in velocity
Time interval
∴ a =
v u
t
−
or v = u + at ...(9.1)

Notes
MODULE - 3
Moving Things
217
This is called as the first equation of motion.
Also, we know that
Displacement = (average velocity) × (time interval)
or s =
2
v u
t
+⎛ ⎞
⎜ ⎟
⎝ ⎠
=
2
u at u
t
+ +⎛ ⎞
⎜ ⎟
⎝ ⎠
( )v u at= +∵
or s = 21
2
ut at+ ...(9.2)
This is called the second equation of motion.
If object starts from rest, u = 0 and
s = 21
0
2
t at× +
or s = 21
2
at
Thus, we see that the displacement of an object undergoing a constant acceleration
is proportional to t2, while the displacement of an object with constant velocity (zero
acceleration) is proportional to t.
Now, if we take
v u
a
t
−
= and .
2
v u
s t
+⎛ ⎞
= ⎜ ⎟
⎝ ⎠
and multiply them, we find that
a.s =
( )
2
v u v u
t
t
− +⎛ ⎞
⎜ ⎟
⎝ ⎠
=
2 2
2
v u−
or 2a.s = v2 – u2
or v2 = u2 + 2as ...(9.3)
This is called as third equation of motion. In case of motion under gravity ‘a’ can
be replaced by ‘g’.
1. Aballisthrownstraightupwardswithaninitialvelocity19.6ms–1.Itwascaught
at the same distance above the ground from which it was thrown:
(i) How high does the ball rise?
(ii) How long does the ball remain in air? (g = 9.8 ms–2)

Notes
Moving Things
218
2. A brick is thrown vertically upwards with the velocity of 192.08 ms–1to the
labourer at the height of 9.8 m. What are its velocity and acceleration when it
reaches the labourer?
3. A body starts its motion with a speed of 10 ms–1 and accelerates for 10 s with
10 ms–2. What will be the distance covered by the body in 10 s?
4. A car starts from rest and covers a distance of 50 m in 10 s and 100 m in next
10 s. What is the average speed of the car?
9.7 UNIFORM CIRCULAR MOTION
Youmayhaveseenthe motionof the bicycle ona straightlevelroad.Doallmovable
parts of the bicycle move alike? If not, then how are they moving differently? Does
the peddling make a difference in these motions? Like Nimish, number of questions
you may have in your mind. Let us try to answer these questions. Bicycle is moving
on a straight road so its motion is rectilinear motion.
Fig. 9.30 Bicycle moving on a road
Now look at the wheels of the bicycle.Any point on the wheel of the bicycle always
remains at a constant distance from the axis of the wheel and moves around the fixed
point i.e., axis of the wheel. On the basis of this description of motion of the wheel
you can decide very obviously that this motion is circular motion.
Similarly,canyouthinkaboutthemotionoftheflywheelofthebicycle?Duringnon-
peddling, there is no circular motion of flywheel and it moves in a straight line thus,
its motion is rectilinear motion. But during the peddling its motion is circular motion
can you think about the motion of any part of the bicycle which has two types of
motion at the same time?Yes, during the circular motion of the wheel or flywheel,
they are also advancing in forward direction on a straight road. Thus, there motion
is circular motion as well as rectilinear motion at the same time.
Now consider the motion of an object along a circular track of radius R through
four points A, B, C and D on the track as shown in Fig. 9.31. If object completes
each round of motion in same time, than it covers equal distance in equal interval
oftimeanditsmotionwillbeuniformmotion.Sinceduringthisuniformmotionequal
distance is being covered in equal interval of time, therefore, the ratio of distance

Notes
MODULE - 3
Moving Things
219
Fig. 9.31 Circular motion
covered to the time taken i.e., speed will remain constant. It means in uniform
circular motion speed remains constant.
Now think about velocity, velocity remains
along the direction of motion. In Fig. 9.31
you can see the direction of motion changes
at every point as shown at point A, B, C and
D. Since there is a change in direction of
motion, therefore, the direction of velocity
also changes. We can say that in uniform
circular motion, velocity changes due to
changeindirectionofmotionandthemotion
of the object is accelerated motion. This
acceleration is due to change in the direction
of motion. But in this motion speed remains constant. How interested this motion
is because a body moving with constant speed acquires acceleration.
Think and Do
K I L O M E T R E T O
S P E E D T O N C N E
O N D I S T A A N O E
P D I S P L A C D I A
A N S V E L O C I T Y
T A P P E E R C S A N
K A L U D I N E T R A
T E A M Y O Y L A E D
M A C H I N E E N L L
E P E P T A D R C E K
T O M F T R E A E C D
R N E N G I N T G C Q
E E N K L O M E T A R
Intheabovewordgrididentifythemeaningfulwords,relatedtodescriptionofmotion,
in horizontal or vertical columns in sequence and define them (at least three).

Notes
Moving Things
220
1. In circular motion the point around which body moves
(a) always remain in rest
(b) always remain in motion
(c) may or may not be in motion
(d) remaininoscillatorymotion
2. Inuniformcircularmotion
(a) speed remain constant
(b) velocityremainconstant
(c) speed and velocity both remain constant
(d) neither speed nor velocity remain constant
3. A point on a blade of a ceiling fan has
(a) alwaysuniformcircularmotion
(b) always uniformly accelerated circular motion
(c) may be uniform or non-uniform circular motion
(d) variable accelerated circular motion
WHAT YOU HAVE LEARNT
If a body stays at the same position with time, it is at rest.
If the body changes its position with time, it is in motion.
Motion is said to be rectilinear if the body moves in the same straight line all
the time. e.g., a car moving in straight line on a level road.
The motion is said to be circular if the body moves on a circular path; e.g. the
motion of the tip of the second needle of a watch.
The total path length covered by a moving body is the distance travelled by it.
The distance between the final and initial position of a body is called its
displacement.
Distance travelled in unit time is called speed, whereas, displacement per unit
time is called velocity.

Notes
MODULE - 3
Moving Things
221
Position-time graph of a body moving in a straight line with constant speed is
a straight line sloping with time axis. The slope of the line gives the velocity of
the object in motion.
Velocity-time graph of a body in straight line with constant speed is a straight
line parallel to time axis.Area under the graph gives distance travelled.
Velocity-time graph of a body in straight line with constant acceleration is a
straight line sloping with the time axis. The slop of the line gives acceleration.
For uniformly accelerated motion
v = u + at
s = ut +
1
2
at2
and v2 = u2 + 2as
where u=initialvelocity,v=finalvelocity,ands=distancetravelledintseconds
TERMINAL EXERCISE
1. An object initially at rest moves for t seconds with a constant acceleration a.
The average speed of the object during this time interval is
(a)
2
a t⋅
; (b) 2a t⋅ ; (c)
21
2
a t⋅ ; (d)
21
2
a t⋅
2. Acarstartsfromrestwithauniformaccelerationof4ms–2.Thedistancetravelled
in metres at the ends of 1s, 2s, 3s and 4s are respectively,
(a) 4, 8, 16, 32 (b) 2, 8, 18, 32
(c) 2, 6, 10, 14 (d) 4, 16, 32, 64
3. Does the direction of velocity decide the direction of acceleration?
4. Establish the relation between acceleration and distance travelled by the body
5. Explain whether or not the following particles have acceleration:
(i) a particle moving in a straight line with constant speed, and
(ii) a particle moving on a curve with constant speed.
6. Consider the following combination of signs for velocity and acceleration of an
object with respect to a one dimensional motion along x-axis and give example
from real life situation for each case:

Notes
Moving Things
222
Table 9.11
Velocity Acceleration Example
(a)Positive Positive Ball rolling down on a slope
likeslideorramp
(b)Positive Negative
(c)Positive Zero
(d)Negative Positive
(e)Negative Negative
(f)Negative Zero
(g)zero Positive
(h) Zero Negative
7. Acartravellinginitiallyat7ms–1 acceleratesattherateof8.0ms–2 foraninterval
of 2.0 s. What is its velocity at the end of the 2 s?
8. Acar travelling in a straight line has a velocity of 5.0 ms–1 at some instant.After
4.0s,itsvelocityis8.0ms–1.Whatisitsaverageaccelerationinthistimeinterval?
9. The velocity-time graph for an object moving along a straight line has shown in
Fig. 3.32. Find the average acceleration of this object during the time interval
0 to 5.0 s, 5.0 s to 15.0 s and 0 to 20.0 s.
Fig. 9.32
10. The velocity of an automobile changes over a period of 8 s as shown in the table
givenbelow:

Notes
MODULE - 3
Moving Things
223
Table 9.12
Time (s) Velocity (ms–1) Time (s) Velocity (ms–1)
0.0 0.0 5.0 20.0
1.0 4.0 6.0 20.0
2.0 8.0 7.0 20.0
3.0 12.0 8.0 20.0
4.0 16.0
(i) Plot the velocity-time graph of motion.
(ii) Determine the distance the car travels during the first 2 s.
(iii) What distance does the car travel during the first 4 s?
(iv) What distance does the car travel during the entire 8 s?
(v) Find the slope of the line between t = 5.0 s and t = 7.0 s. What does
the slope indicate?
(vi) Find the slope of the line between t = 0 s to t = 4 s. What does this slope
represent?
11. The position-time data of a car is given in the table given below:
Table 9.13
Time (s) Position (m) Time (s) Position (m)
0 0 25 150
5 100 30 112.5
10 200 35 75
15 200 40 37.5
20 200 45 0
(i) Plot the position-time graph of the car.
(ii) Calculate average velocity of the car during first 10 seconds.
(iii) Calculate the average velocity between t = 10 s to t = 20 s.
(iv) Calculate the average velocity between t = 20 s and t = 25 s. What can
you say about the direction of the motion of car?
12. An object is dropped from the height of 19.6 m. Draw the displacement-time
graph for time when object reach the ground. Also find velocity of the object
when it touches the ground.

Notes
Moving Things
224
13. An object is dropped from the height of 19.6 m. Find the distance travelled by
object in last second of its journey.
14. Show that for a uniformly accelerated motion starting from velocity u and
acquiring velocity v has average velocity equal to arithmetic mean of the initial
(u) and final velocity (v).
15. Findthedistance,averagespeed,displacement,averagevelocityandacceleration
of the object whose motion is shown in the graph (Fig. 9.33).
Fig. 9.33
16. A body accelerates from rest and attains a velocity of 10 ms-1 in 5 s. What is
its acceleration?
ANSWERS TO INTEXT QUESTIONS
9.1
1. (c) 2. (a) 3. (b) 4. (a) 5. (c)
6.
Fig. 9.34

Notes
MODULE - 3
Moving Things
225
9.2
1. (a)(iii) (b) (iv) (c) (i) (d) (ii)
2. Distance = 140 m,Displacement = 100 m, Speed = 7 ms–1
3. When object moves with uniform speed
4. 2ms–1, 5 ms–1
5. Average speed = 0.2 ms–1, motion is uniform motion
Fig. 9.35
6. 0.2 ms–1 7. 63 km h–1 8. 48 km h–1 9. R, R
9.3
1. For first five seconds object moves with constant speed i.e. 2ms–1. From 5 to
15 second it remains at rest and then from 15 to 20 seconds it moves with
constant speed 2 ms–1.
The motion of the object is not uniform.
2. Velocity of object A is 4 times the velocity of B.
3.
Fig. 9.36

Notes
Moving Things
226
4.
Fig. 9.37
(i) a = 0.4 ms–2, (ii) –a = 0.4 ms–2, (iii) 10 m
5.
Fig. 9.38
9.4
1. (i) 19.6 m, (ii) 4 s 2. Zero and 9.8 ms–2
3. 600 m 4. 7.5 ms–1
9.5
1. (a) 2. (a) 3. (b)

Motion and its description

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