NCERT Textbook: Motion & Time Notes | Study Science & Technology for UPSC CSE - UPSC

 Page 1


Motion and Time
13
I
n Class VI, you learnt about different
types of motions. You learnt that a
motion could be along a straight line,
it could be circular or periodic. Can you
recall these three types of motions?
Table 13.1 gives some common
examples of motions. Identify the type
of motion in each case.
13.1 SLOW OR FAST
We know that some vehicles move faster
than others. Even the same vehicle may
move faster or slower at different times.
Make a list of ten objects moving along
a straight path. Group the motion of
these objects as slow and fast. How did
you decide which object is moving slow
and which one is moving fast?
If vehicles are moving on a road in
the same direction, we can easily tell
which one of them is moving faster than
the other. Let us look at the motion of
vehicles moving on a road.
Activity 13.1
Look at Fig. 13.1. It shows the position
of some vehicles moving on a road in
the same direction at some instant of
time. Now look at Fig. 13.2. It shows the
position of the same vehicles after some
time. From your observation of the two
figures, answer the following questions:
Which vehicle is moving the fastest
of all? Which one of them is moving the
slowest of all?
The distance moved by objects in a
given interval of time can help us to
decide which one is faster or slower. For
example, imagine that you have gone to
see off your friend at the bus stand.
Suppose you start pedalling your bicycle
at the same time as the bus begins to
Table 13.1 Some examples of
different types of motion
Example of Type of motion
motion Along a straight
line/circular/
periodic
Soldiers in a
march past
Bullock cart
moving on a
straight road
Hands of an
athlete in a race
Pedal of a bicycle
in motion
Motion of the Earth
around the Sun
Motion of a swing
Motion  of a
pendulum
It is common experience that the
motion of some objects is slow while that
of some others is fast.
2020-21
Page 2


Motion and Time
13
I
n Class VI, you learnt about different
types of motions. You learnt that a
motion could be along a straight line,
it could be circular or periodic. Can you
recall these three types of motions?
Table 13.1 gives some common
examples of motions. Identify the type
of motion in each case.
13.1 SLOW OR FAST
We know that some vehicles move faster
than others. Even the same vehicle may
move faster or slower at different times.
Make a list of ten objects moving along
a straight path. Group the motion of
these objects as slow and fast. How did
you decide which object is moving slow
and which one is moving fast?
If vehicles are moving on a road in
the same direction, we can easily tell
which one of them is moving faster than
the other. Let us look at the motion of
vehicles moving on a road.
Activity 13.1
Look at Fig. 13.1. It shows the position
of some vehicles moving on a road in
the same direction at some instant of
time. Now look at Fig. 13.2. It shows the
position of the same vehicles after some
time. From your observation of the two
figures, answer the following questions:
Which vehicle is moving the fastest
of all? Which one of them is moving the
slowest of all?
The distance moved by objects in a
given interval of time can help us to
decide which one is faster or slower. For
example, imagine that you have gone to
see off your friend at the bus stand.
Suppose you start pedalling your bicycle
at the same time as the bus begins to
Table 13.1 Some examples of
different types of motion
Example of Type of motion
motion Along a straight
line/circular/
periodic
Soldiers in a
march past
Bullock cart
moving on a
straight road
Hands of an
athlete in a race
Pedal of a bicycle
in motion
Motion of the Earth
around the Sun
Motion of a swing
Motion  of a
pendulum
It is common experience that the
motion of some objects is slow while that
of some others is fast.
2020-21
SCIENCE 144
move. The distance covered by you after
5 minutes would be much smaller than
that covered by the bus. Would you say
that the bus is moving faster than the
bicycle?
We often say that the faster vehicle
has a higher speed.  In a 100-metre race
it is easy to decide whose speed
is the highest. One who takes
shortest time to cover the
distance of 100 metres has the
highest speed.
13.2 SPEED
You are probably familiar with
the word speed. In the
examples given above, a higher
speed seems to indicate that a
given distance has been
covered in a shorter time, or a
larger distance covered in a
given time.
The most convenient way to
find out which of the two or
more objects is moving faster
is to compare the distances
moved by them in a unit time.
Thus, if we know the distance
covered by two buses in one
hour, we can tell which one is
faster. We call the distance
covered by an object in a unit
time as the speed of the object.
When we say that a car is
moving with a speed of 50
kilometres per hour, it implies
that it will cover a distance of
Fig. 13.2  Position of vehicles shown in
Fig. 13.1 after some time
Fig. 13.1  Vehicles moving in the same
direction on a road
50 kilometres in one hour. However, a
car seldom moves with a constant speed
for one hour. In fact, it starts moving
slowly and then picks up speed. So,
when we say that the car has a speed of
50 kilometres per hour, we usually
consider only the total distance covered
by it in one hour. We do not bother
whether the car has been moving with
2020-21
Page 3


Motion and Time
13
I
n Class VI, you learnt about different
types of motions. You learnt that a
motion could be along a straight line,
it could be circular or periodic. Can you
recall these three types of motions?
Table 13.1 gives some common
examples of motions. Identify the type
of motion in each case.
13.1 SLOW OR FAST
We know that some vehicles move faster
than others. Even the same vehicle may
move faster or slower at different times.
Make a list of ten objects moving along
a straight path. Group the motion of
these objects as slow and fast. How did
you decide which object is moving slow
and which one is moving fast?
If vehicles are moving on a road in
the same direction, we can easily tell
which one of them is moving faster than
the other. Let us look at the motion of
vehicles moving on a road.
Activity 13.1
Look at Fig. 13.1. It shows the position
of some vehicles moving on a road in
the same direction at some instant of
time. Now look at Fig. 13.2. It shows the
position of the same vehicles after some
time. From your observation of the two
figures, answer the following questions:
Which vehicle is moving the fastest
of all? Which one of them is moving the
slowest of all?
The distance moved by objects in a
given interval of time can help us to
decide which one is faster or slower. For
example, imagine that you have gone to
see off your friend at the bus stand.
Suppose you start pedalling your bicycle
at the same time as the bus begins to
Table 13.1 Some examples of
different types of motion
Example of Type of motion
motion Along a straight
line/circular/
periodic
Soldiers in a
march past
Bullock cart
moving on a
straight road
Hands of an
athlete in a race
Pedal of a bicycle
in motion
Motion of the Earth
around the Sun
Motion of a swing
Motion  of a
pendulum
It is common experience that the
motion of some objects is slow while that
of some others is fast.
2020-21
SCIENCE 144
move. The distance covered by you after
5 minutes would be much smaller than
that covered by the bus. Would you say
that the bus is moving faster than the
bicycle?
We often say that the faster vehicle
has a higher speed.  In a 100-metre race
it is easy to decide whose speed
is the highest. One who takes
shortest time to cover the
distance of 100 metres has the
highest speed.
13.2 SPEED
You are probably familiar with
the word speed. In the
examples given above, a higher
speed seems to indicate that a
given distance has been
covered in a shorter time, or a
larger distance covered in a
given time.
The most convenient way to
find out which of the two or
more objects is moving faster
is to compare the distances
moved by them in a unit time.
Thus, if we know the distance
covered by two buses in one
hour, we can tell which one is
faster. We call the distance
covered by an object in a unit
time as the speed of the object.
When we say that a car is
moving with a speed of 50
kilometres per hour, it implies
that it will cover a distance of
Fig. 13.2  Position of vehicles shown in
Fig. 13.1 after some time
Fig. 13.1  Vehicles moving in the same
direction on a road
50 kilometres in one hour. However, a
car seldom moves with a constant speed
for one hour. In fact, it starts moving
slowly and then picks up speed. So,
when we say that the car has a speed of
50 kilometres per hour, we usually
consider only the total distance covered
by it in one hour. We do not bother
whether the car has been moving with
2020-21
MOTION AND TIME 145
We can determine the speed of a given
object once we can measure the time
taken by it to cover a certain distance.
In Class VI you learnt how to measure
distances. But, how do we measure
time? Let us find out.
13.3 MEASUREMENT OF TIME
If you did not have a clock, how would
you decide what time of the day it is?
Have you ever wondered how our elders
could tell the approximate time of the
day by just looking at shadows?
How do we measure time interval of
a month? A year?
Our ancestors noticed that many
events in nature repeat themselves after
definite intervals of time. For example,
they found that the sun rises everyday
in the morning. The time between one
sunrise and the next was called a day.
Similarly, a month was measured from
one new moon to the next. A year was
fixed as the time taken by the earth to
complete one revolution of the sun.
Often we need to measure intervals
of time which are much shorter than a
day. Clocks or watches are perhaps the
most common time measuring devices.
Have you ever wondered how clocks and
watches measure time?
The working of clocks is rather
complex. But all of them make use of
some periodic motion. One of the most
well-known periodic motions is that of
a simple pendulum.
In everyday life we seldom find objects
moving with a constant speed over long
distances or for long durations of time.
If the speed of an object moving along
a straight line keeps changing, its
motion is said to be non-uniform. On
the other hand, an object moving along
a straight line with a constant speed
is said to be in uniform motion. In
this case, the average speed is the
same as the actual speed.
Fig. 13.3  Some common clocks
(b) Table clock
(c) Digital clock
(a) Wall clock
a constant speed or not during that
hour. The speed calculated here is
actually the average speed of the car. In
this book we shall use the term speed
for average speed. So, for us the speed
is the total distance covered divided
by the total time taken. Thus,
Total distance covered
Speed = 
Total time taken
2020-21
Page 4


Motion and Time
13
I
n Class VI, you learnt about different
types of motions. You learnt that a
motion could be along a straight line,
it could be circular or periodic. Can you
recall these three types of motions?
Table 13.1 gives some common
examples of motions. Identify the type
of motion in each case.
13.1 SLOW OR FAST
We know that some vehicles move faster
than others. Even the same vehicle may
move faster or slower at different times.
Make a list of ten objects moving along
a straight path. Group the motion of
these objects as slow and fast. How did
you decide which object is moving slow
and which one is moving fast?
If vehicles are moving on a road in
the same direction, we can easily tell
which one of them is moving faster than
the other. Let us look at the motion of
vehicles moving on a road.
Activity 13.1
Look at Fig. 13.1. It shows the position
of some vehicles moving on a road in
the same direction at some instant of
time. Now look at Fig. 13.2. It shows the
position of the same vehicles after some
time. From your observation of the two
figures, answer the following questions:
Which vehicle is moving the fastest
of all? Which one of them is moving the
slowest of all?
The distance moved by objects in a
given interval of time can help us to
decide which one is faster or slower. For
example, imagine that you have gone to
see off your friend at the bus stand.
Suppose you start pedalling your bicycle
at the same time as the bus begins to
Table 13.1 Some examples of
different types of motion
Example of Type of motion
motion Along a straight
line/circular/
periodic
Soldiers in a
march past
Bullock cart
moving on a
straight road
Hands of an
athlete in a race
Pedal of a bicycle
in motion
Motion of the Earth
around the Sun
Motion of a swing
Motion  of a
pendulum
It is common experience that the
motion of some objects is slow while that
of some others is fast.
2020-21
SCIENCE 144
move. The distance covered by you after
5 minutes would be much smaller than
that covered by the bus. Would you say
that the bus is moving faster than the
bicycle?
We often say that the faster vehicle
has a higher speed.  In a 100-metre race
it is easy to decide whose speed
is the highest. One who takes
shortest time to cover the
distance of 100 metres has the
highest speed.
13.2 SPEED
You are probably familiar with
the word speed. In the
examples given above, a higher
speed seems to indicate that a
given distance has been
covered in a shorter time, or a
larger distance covered in a
given time.
The most convenient way to
find out which of the two or
more objects is moving faster
is to compare the distances
moved by them in a unit time.
Thus, if we know the distance
covered by two buses in one
hour, we can tell which one is
faster. We call the distance
covered by an object in a unit
time as the speed of the object.
When we say that a car is
moving with a speed of 50
kilometres per hour, it implies
that it will cover a distance of
Fig. 13.2  Position of vehicles shown in
Fig. 13.1 after some time
Fig. 13.1  Vehicles moving in the same
direction on a road
50 kilometres in one hour. However, a
car seldom moves with a constant speed
for one hour. In fact, it starts moving
slowly and then picks up speed. So,
when we say that the car has a speed of
50 kilometres per hour, we usually
consider only the total distance covered
by it in one hour. We do not bother
whether the car has been moving with
2020-21
MOTION AND TIME 145
We can determine the speed of a given
object once we can measure the time
taken by it to cover a certain distance.
In Class VI you learnt how to measure
distances. But, how do we measure
time? Let us find out.
13.3 MEASUREMENT OF TIME
If you did not have a clock, how would
you decide what time of the day it is?
Have you ever wondered how our elders
could tell the approximate time of the
day by just looking at shadows?
How do we measure time interval of
a month? A year?
Our ancestors noticed that many
events in nature repeat themselves after
definite intervals of time. For example,
they found that the sun rises everyday
in the morning. The time between one
sunrise and the next was called a day.
Similarly, a month was measured from
one new moon to the next. A year was
fixed as the time taken by the earth to
complete one revolution of the sun.
Often we need to measure intervals
of time which are much shorter than a
day. Clocks or watches are perhaps the
most common time measuring devices.
Have you ever wondered how clocks and
watches measure time?
The working of clocks is rather
complex. But all of them make use of
some periodic motion. One of the most
well-known periodic motions is that of
a simple pendulum.
In everyday life we seldom find objects
moving with a constant speed over long
distances or for long durations of time.
If the speed of an object moving along
a straight line keeps changing, its
motion is said to be non-uniform. On
the other hand, an object moving along
a straight line with a constant speed
is said to be in uniform motion. In
this case, the average speed is the
same as the actual speed.
Fig. 13.3  Some common clocks
(b) Table clock
(c) Digital clock
(a) Wall clock
a constant speed or not during that
hour. The speed calculated here is
actually the average speed of the car. In
this book we shall use the term speed
for average speed. So, for us the speed
is the total distance covered divided
by the total time taken. Thus,
Total distance covered
Speed = 
Total time taken
2020-21
SCIENCE 146
A simple pendulum consists of a
small metallic ball or a piece of stone
suspended from a rigid stand by a
thread [Fig. 13.4 (a)]. The metallic ball
is called the bob of the pendulum.
Fig. 13.4 (a) shows the pendulum at
rest in its mean position. When the bob
of the pendulum is released after taking
it slightly to one side, it begins to move
to and fro [Fig. 13.4 (b)]. The to and fro
motion of a simple pendulum is an
example of a periodic or an oscillatory
motion.
The pendulum is said to have
completed one oscillation when its bob,
starting from its mean position O, moves
To set the pendulum in motion,
gently hold the bob and move it slightly
to one side. Make sure that the string
attached to the bob is taut while you
displace it. Now release the bob from its
displaced position. Remember that the
bob is not to be pushed when it is
released. Note the time on the clock
when the bob is at its mean position.
Instead of the mean position you may
note the time when the bob is at one of
its extreme positions. Measure the time
the pendulum takes to complete 20
oscillations. Record your observations
Fig. 13.4  (b) Different
positions of the bob of an
oscillating simple pendulum
Fig. 13.4  (a) A simple
pendulum
to A, to B and back to O. The
pendulum also completes one
oscillation when its bob moves
from one extreme position A to the
other extreme position B and
comes back to A. The time taken
by the pendulum to complete one
oscillation is called its time
period.
Activity 13.2
Set up a simple pendulum as
shown in Fig. 13.4 (a) with a
thread or string of length nearly
one metre. Switch off any fans
nearby. Let the bob of the
pendulum come to rest at its
mean position. Mark the mean
position of the bob on the floor
below it or on the wall behind it.
To measure the time period of
the pendulum we will need
a stopwatch. However, if a
stopwatch is not available, a table
clock or a wristwatch can be used.
A
O
B
2020-21
Page 5


Motion and Time
13
I
n Class VI, you learnt about different
types of motions. You learnt that a
motion could be along a straight line,
it could be circular or periodic. Can you
recall these three types of motions?
Table 13.1 gives some common
examples of motions. Identify the type
of motion in each case.
13.1 SLOW OR FAST
We know that some vehicles move faster
than others. Even the same vehicle may
move faster or slower at different times.
Make a list of ten objects moving along
a straight path. Group the motion of
these objects as slow and fast. How did
you decide which object is moving slow
and which one is moving fast?
If vehicles are moving on a road in
the same direction, we can easily tell
which one of them is moving faster than
the other. Let us look at the motion of
vehicles moving on a road.
Activity 13.1
Look at Fig. 13.1. It shows the position
of some vehicles moving on a road in
the same direction at some instant of
time. Now look at Fig. 13.2. It shows the
position of the same vehicles after some
time. From your observation of the two
figures, answer the following questions:
Which vehicle is moving the fastest
of all? Which one of them is moving the
slowest of all?
The distance moved by objects in a
given interval of time can help us to
decide which one is faster or slower. For
example, imagine that you have gone to
see off your friend at the bus stand.
Suppose you start pedalling your bicycle
at the same time as the bus begins to
Table 13.1 Some examples of
different types of motion
Example of Type of motion
motion Along a straight
line/circular/
periodic
Soldiers in a
march past
Bullock cart
moving on a
straight road
Hands of an
athlete in a race
Pedal of a bicycle
in motion
Motion of the Earth
around the Sun
Motion of a swing
Motion  of a
pendulum
It is common experience that the
motion of some objects is slow while that
of some others is fast.
2020-21
SCIENCE 144
move. The distance covered by you after
5 minutes would be much smaller than
that covered by the bus. Would you say
that the bus is moving faster than the
bicycle?
We often say that the faster vehicle
has a higher speed.  In a 100-metre race
it is easy to decide whose speed
is the highest. One who takes
shortest time to cover the
distance of 100 metres has the
highest speed.
13.2 SPEED
You are probably familiar with
the word speed. In the
examples given above, a higher
speed seems to indicate that a
given distance has been
covered in a shorter time, or a
larger distance covered in a
given time.
The most convenient way to
find out which of the two or
more objects is moving faster
is to compare the distances
moved by them in a unit time.
Thus, if we know the distance
covered by two buses in one
hour, we can tell which one is
faster. We call the distance
covered by an object in a unit
time as the speed of the object.
When we say that a car is
moving with a speed of 50
kilometres per hour, it implies
that it will cover a distance of
Fig. 13.2  Position of vehicles shown in
Fig. 13.1 after some time
Fig. 13.1  Vehicles moving in the same
direction on a road
50 kilometres in one hour. However, a
car seldom moves with a constant speed
for one hour. In fact, it starts moving
slowly and then picks up speed. So,
when we say that the car has a speed of
50 kilometres per hour, we usually
consider only the total distance covered
by it in one hour. We do not bother
whether the car has been moving with
2020-21
MOTION AND TIME 145
We can determine the speed of a given
object once we can measure the time
taken by it to cover a certain distance.
In Class VI you learnt how to measure
distances. But, how do we measure
time? Let us find out.
13.3 MEASUREMENT OF TIME
If you did not have a clock, how would
you decide what time of the day it is?
Have you ever wondered how our elders
could tell the approximate time of the
day by just looking at shadows?
How do we measure time interval of
a month? A year?
Our ancestors noticed that many
events in nature repeat themselves after
definite intervals of time. For example,
they found that the sun rises everyday
in the morning. The time between one
sunrise and the next was called a day.
Similarly, a month was measured from
one new moon to the next. A year was
fixed as the time taken by the earth to
complete one revolution of the sun.
Often we need to measure intervals
of time which are much shorter than a
day. Clocks or watches are perhaps the
most common time measuring devices.
Have you ever wondered how clocks and
watches measure time?
The working of clocks is rather
complex. But all of them make use of
some periodic motion. One of the most
well-known periodic motions is that of
a simple pendulum.
In everyday life we seldom find objects
moving with a constant speed over long
distances or for long durations of time.
If the speed of an object moving along
a straight line keeps changing, its
motion is said to be non-uniform. On
the other hand, an object moving along
a straight line with a constant speed
is said to be in uniform motion. In
this case, the average speed is the
same as the actual speed.
Fig. 13.3  Some common clocks
(b) Table clock
(c) Digital clock
(a) Wall clock
a constant speed or not during that
hour. The speed calculated here is
actually the average speed of the car. In
this book we shall use the term speed
for average speed. So, for us the speed
is the total distance covered divided
by the total time taken. Thus,
Total distance covered
Speed = 
Total time taken
2020-21
SCIENCE 146
A simple pendulum consists of a
small metallic ball or a piece of stone
suspended from a rigid stand by a
thread [Fig. 13.4 (a)]. The metallic ball
is called the bob of the pendulum.
Fig. 13.4 (a) shows the pendulum at
rest in its mean position. When the bob
of the pendulum is released after taking
it slightly to one side, it begins to move
to and fro [Fig. 13.4 (b)]. The to and fro
motion of a simple pendulum is an
example of a periodic or an oscillatory
motion.
The pendulum is said to have
completed one oscillation when its bob,
starting from its mean position O, moves
To set the pendulum in motion,
gently hold the bob and move it slightly
to one side. Make sure that the string
attached to the bob is taut while you
displace it. Now release the bob from its
displaced position. Remember that the
bob is not to be pushed when it is
released. Note the time on the clock
when the bob is at its mean position.
Instead of the mean position you may
note the time when the bob is at one of
its extreme positions. Measure the time
the pendulum takes to complete 20
oscillations. Record your observations
Fig. 13.4  (b) Different
positions of the bob of an
oscillating simple pendulum
Fig. 13.4  (a) A simple
pendulum
to A, to B and back to O. The
pendulum also completes one
oscillation when its bob moves
from one extreme position A to the
other extreme position B and
comes back to A. The time taken
by the pendulum to complete one
oscillation is called its time
period.
Activity 13.2
Set up a simple pendulum as
shown in Fig. 13.4 (a) with a
thread or string of length nearly
one metre. Switch off any fans
nearby. Let the bob of the
pendulum come to rest at its
mean position. Mark the mean
position of the bob on the floor
below it or on the wall behind it.
To measure the time period of
the pendulum we will need
a stopwatch. However, if a
stopwatch is not available, a table
clock or a wristwatch can be used.
A
O
B
2020-21
MOTION AND TIME 147
Table 13.2 Time period of a simple
pendulum
Length of the string = 100 cm
S.No. Time taken for 20 Time period
oscillations (s) (s)
1. 42 2.1
2.
3.
cells. These clocks are called quartz
clocks. The time measured by quartz
clocks is much more accurate than that
by the clocks available earlier.
Units of time and speed
The basic unit of time is a second. Its
symbol is s. Larger units of time are
minutes (min) and hours (h). You
already know how these units are related
to one another.
What would be the basic unit of
speed?
Since the speed is distance/time, the
basic unit of speed is m/s. Of course, it
could also be expressed in other units
such as m/min or km/h.
You must remember that the
symbols of all units are written in
singular. For example, we write 50 km
and not 50 kms, or 8 cm and not 8 cms.
Boojho is wondering how many
seconds there are in a day and how
many hours in a year. Can you help
him?
in Table 13.2. The first observation
shown is just a sample. Your
observations could be different from
this. Repeat this activity a few times and
record your observations. By dividing
the time taken for 20 oscillations by 20,
get the time taken for one oscillation, or
the time period of the pendulum.
Is the time period of your pendulum
nearly the same in all cases?
Note that a slight change in the
initial displacement does not affect the
time period of your pendulum.
Nowadays most clocks or watches
have an electric circuit with one or more
There is an interesting story about the discovery that the time period of a given
pendulum is constant. You might have heard the name of famous scientist
Galileo Galilie (A.D. 1564 –1642). It is said that once Galileo was sitting in a
church. He noticed that a lamp suspended from the ceiling with a chain was
moving slowly from one side to the other. He was surprised to find that his
pulse beat the same number of times during the interval in which the lamp
completed one oscillation. Galileo experimented with various pendulums to
verify his observation. He found that a pendulum of a given length takes always
the same time to complete one oscillation. This observation led to the
development of pendulum clocks. Winding clocks and wristwatches were
refinements of the pendulum clocks.
2020-21