Chapter 8
MOTION
In everyday life, we see some objects at rest Activity ______________ 8.1
and others in motion. Birds fly, fish swim,
• Discuss whether the walls of your
blood flows through veins and arteries, and
classroom are at rest or in motion.
cars move. Atoms, molecules, planets, stars
and galaxies are all in motion. We often Activity ______________ 8.2
perceive an object to be in motion when its
• Have you ever experienced that the
position changes with time. However, there train in which you are sitting appears
are situations where the motion is inferred to move while it is at rest?
through indirect evidences. For example, we • Discuss and share your experience.
infer the motion of air by observing the
movement of dust and the movement of leaves Think and Act
and branches of trees. What causes the
We sometimes are endangered by the
phenomena of sunrise, sunset and changing
motion of objects around us, especially
of seasons? Is it due to the motion of the
if that motion is erratic and
earth? If it is true, why don’t we directly uncontrolled as observed in a flooded
perceive the motion of the earth? river, a hurricane or a tsunami. On the
An object may appear to be moving for other hand, controlled motion can be a
one person and stationary for some other. For service to human beings such as in the
the passengers in a moving bus, the roadside generation of hydro-electric power. Do
trees appear to be moving backwards. A you feel the necessity to study the
person standing on the road–side perceives erratic motion of some objects and
the bus alongwith the passengers as moving. learn to control them?
However, a passenger inside the bus sees his
fellow passengers to be at rest. What do these 8.1 Describing Motion
observations indicate? We describe the location of an object by
Most motions are complex. Some objects specifying a reference point. Let us
may move in a straight line, others may take understand this by an example. Let us
a circular path. Some may rotate and a few assume that a school in a village is 2 km north
others may vibrate. There may be situations of the railway station. We have specified the
involving a combination of these. In this position of the school with respect to the
chapter, we shall first learn to describe the railway station. In this example, the railway
motion of objects along a straight line. We station is the reference point. We could have
shall also learn to express such motions also chosen other reference points according
through simple equations and graphs. Later, to our convenience. Therefore, to describe the
we shall discuss ways of describing circular position of an object we need to specify a
motion. reference point called the origin.
2020-21
,8.1.1 MOTION ALONG A STRAIGHT LINE while the magnitude of displacement = 35 km.
Thus, the magnitude of displacement (35 km)
The simplest type of motion is the motion is not equal to the path length (85 km).
along a straight line. We shall first learn to Further, we will notice that the magnitude of
describe this by an example. Consider the the displacement for a course of motion may
motion of an object moving along a straight be zero but the corresponding distance
path. The object starts its journey from O covered is not zero. If we consider the object
which is treated as its reference point to travel back to O, the final position concides
(Fig. 8.1). Let A, B and C represent the with the initial position, and therefore, the
position of the object at different instants. At displacement is zero. However, the distance
first, the object moves through C and B and covered in this journey is OA + AO = 60 km +
reaches A. Then it moves back along the same 60 km = 120 km. Thus, two different physical
path and reaches C through B. quantities — the distance and the
Fig. 8.1: Positions of an object on a straight line path
The total path length covered by the object displacement, are used to describe the overall
is OA + AC, that is 60 km + 35 km = 95 km. motion of an object and to locate its final
This is the distance covered by the object. To position with reference to its initial position
describe distance we need to specify only the at a given time.
numerical value and not the direction of
motion. There are certain quantities which Activity ______________ 8.3
are described by specifying only their • Take a metre scale and a long rope.
numerical values. The numerical value of a • Walk from one corner of a basket-ball
physical quantity is its magnitude. From this court to its oppposite corner along its
example, can you find out the distance of the sides.
final position C of the object from the initial • Measure the distance covered by you
position O? This difference will give you the and magnitude of the displacement.
numerical value of the displacement of the • What dif ference would you notice
object from O to C through A. The shortest between the two in this case?
distance measured from the initial to the final
position of an object is known as the
Activity ______________ 8.4
displacement. • Automobiles are fitted with a device
Can the magnitude of the displacement that shows the distance travelled. Such
be equal to the distance travelled by an a device is known as an odometer. A
object? Consider the example given in car is driven from Bhubaneshwar to
New Delhi. The difference between the
(Fig. 8.1). For motion of the object from O to
final reading and the initial reading of
A, the distance covered is 60 km and the
the odometer is 1850 km.
magnitude of displacement is also 60 km. • Find the magnitude of the displacement
During its motion from O to A and back to B, between Bhubaneshwar and New Delhi
the distance covered = 60 km + 25 km = 85 km by using the Road Map of India.
MOTION 99
2020-21
, Q
uestions Table 8.1
1. An object has moved through a Time Distance Distance
distance. Can it have zero travelled by travelled by
displacement? If yes, support object A in m object B in m
your answer with an example. 9:30 am 10 12
2. A farmer moves along the
9:45 am 20 19
boundary of a square field of side
10:00 am 30 23
10 m in 40 s. What will be the
magnitude of displacement of the 10:15 am 40 35
farmer at the end of 2 minutes 20 10:30 am 50 37
seconds from his initial position? 10:45 am 60 41
3. Which of the following is true for 11:00 am 70 44
displacement?
(a) It cannot be zero.
(b) Its magnitude is greater than 8.2 Measuring the Rate of Motion
the distance travelled by the
object.
8.1.2 U NIFORM MOTION AND N O N -
UNIFORM MOTION
Consider an object moving along a straight
line. Let it travel 5 m in the first second,
5 m more in the next second, 5 m in the
third second and 5 m in the fourth second.
In this case, the object covers 5 m in each
second. As the object covers equal distances
(a)
in equal intervals of time, it is said to be in
uniform motion. The time interval in this
motion should be small. In our day-to-day
life, we come across motions where objects
cover unequal distances in equal intervals
of time, for example, when a car is moving
on a crowded street or a person is jogging
in a park. These are some instances of
non-uniform motion.
Activity ______________ 8.5
• The data regarding the motion of two
different objects A and B are given in
Table 8.1.
• Examine them carefully and state
whether the motion of the objects is
(b)
uniform or non-uniform.
Fig. 8.2
100 SCIENCE
2020-21
MOTION
In everyday life, we see some objects at rest Activity ______________ 8.1
and others in motion. Birds fly, fish swim,
• Discuss whether the walls of your
blood flows through veins and arteries, and
classroom are at rest or in motion.
cars move. Atoms, molecules, planets, stars
and galaxies are all in motion. We often Activity ______________ 8.2
perceive an object to be in motion when its
• Have you ever experienced that the
position changes with time. However, there train in which you are sitting appears
are situations where the motion is inferred to move while it is at rest?
through indirect evidences. For example, we • Discuss and share your experience.
infer the motion of air by observing the
movement of dust and the movement of leaves Think and Act
and branches of trees. What causes the
We sometimes are endangered by the
phenomena of sunrise, sunset and changing
motion of objects around us, especially
of seasons? Is it due to the motion of the
if that motion is erratic and
earth? If it is true, why don’t we directly uncontrolled as observed in a flooded
perceive the motion of the earth? river, a hurricane or a tsunami. On the
An object may appear to be moving for other hand, controlled motion can be a
one person and stationary for some other. For service to human beings such as in the
the passengers in a moving bus, the roadside generation of hydro-electric power. Do
trees appear to be moving backwards. A you feel the necessity to study the
person standing on the road–side perceives erratic motion of some objects and
the bus alongwith the passengers as moving. learn to control them?
However, a passenger inside the bus sees his
fellow passengers to be at rest. What do these 8.1 Describing Motion
observations indicate? We describe the location of an object by
Most motions are complex. Some objects specifying a reference point. Let us
may move in a straight line, others may take understand this by an example. Let us
a circular path. Some may rotate and a few assume that a school in a village is 2 km north
others may vibrate. There may be situations of the railway station. We have specified the
involving a combination of these. In this position of the school with respect to the
chapter, we shall first learn to describe the railway station. In this example, the railway
motion of objects along a straight line. We station is the reference point. We could have
shall also learn to express such motions also chosen other reference points according
through simple equations and graphs. Later, to our convenience. Therefore, to describe the
we shall discuss ways of describing circular position of an object we need to specify a
motion. reference point called the origin.
2020-21
,8.1.1 MOTION ALONG A STRAIGHT LINE while the magnitude of displacement = 35 km.
Thus, the magnitude of displacement (35 km)
The simplest type of motion is the motion is not equal to the path length (85 km).
along a straight line. We shall first learn to Further, we will notice that the magnitude of
describe this by an example. Consider the the displacement for a course of motion may
motion of an object moving along a straight be zero but the corresponding distance
path. The object starts its journey from O covered is not zero. If we consider the object
which is treated as its reference point to travel back to O, the final position concides
(Fig. 8.1). Let A, B and C represent the with the initial position, and therefore, the
position of the object at different instants. At displacement is zero. However, the distance
first, the object moves through C and B and covered in this journey is OA + AO = 60 km +
reaches A. Then it moves back along the same 60 km = 120 km. Thus, two different physical
path and reaches C through B. quantities — the distance and the
Fig. 8.1: Positions of an object on a straight line path
The total path length covered by the object displacement, are used to describe the overall
is OA + AC, that is 60 km + 35 km = 95 km. motion of an object and to locate its final
This is the distance covered by the object. To position with reference to its initial position
describe distance we need to specify only the at a given time.
numerical value and not the direction of
motion. There are certain quantities which Activity ______________ 8.3
are described by specifying only their • Take a metre scale and a long rope.
numerical values. The numerical value of a • Walk from one corner of a basket-ball
physical quantity is its magnitude. From this court to its oppposite corner along its
example, can you find out the distance of the sides.
final position C of the object from the initial • Measure the distance covered by you
position O? This difference will give you the and magnitude of the displacement.
numerical value of the displacement of the • What dif ference would you notice
object from O to C through A. The shortest between the two in this case?
distance measured from the initial to the final
position of an object is known as the
Activity ______________ 8.4
displacement. • Automobiles are fitted with a device
Can the magnitude of the displacement that shows the distance travelled. Such
be equal to the distance travelled by an a device is known as an odometer. A
object? Consider the example given in car is driven from Bhubaneshwar to
New Delhi. The difference between the
(Fig. 8.1). For motion of the object from O to
final reading and the initial reading of
A, the distance covered is 60 km and the
the odometer is 1850 km.
magnitude of displacement is also 60 km. • Find the magnitude of the displacement
During its motion from O to A and back to B, between Bhubaneshwar and New Delhi
the distance covered = 60 km + 25 km = 85 km by using the Road Map of India.
MOTION 99
2020-21
, Q
uestions Table 8.1
1. An object has moved through a Time Distance Distance
distance. Can it have zero travelled by travelled by
displacement? If yes, support object A in m object B in m
your answer with an example. 9:30 am 10 12
2. A farmer moves along the
9:45 am 20 19
boundary of a square field of side
10:00 am 30 23
10 m in 40 s. What will be the
magnitude of displacement of the 10:15 am 40 35
farmer at the end of 2 minutes 20 10:30 am 50 37
seconds from his initial position? 10:45 am 60 41
3. Which of the following is true for 11:00 am 70 44
displacement?
(a) It cannot be zero.
(b) Its magnitude is greater than 8.2 Measuring the Rate of Motion
the distance travelled by the
object.
8.1.2 U NIFORM MOTION AND N O N -
UNIFORM MOTION
Consider an object moving along a straight
line. Let it travel 5 m in the first second,
5 m more in the next second, 5 m in the
third second and 5 m in the fourth second.
In this case, the object covers 5 m in each
second. As the object covers equal distances
(a)
in equal intervals of time, it is said to be in
uniform motion. The time interval in this
motion should be small. In our day-to-day
life, we come across motions where objects
cover unequal distances in equal intervals
of time, for example, when a car is moving
on a crowded street or a person is jogging
in a park. These are some instances of
non-uniform motion.
Activity ______________ 8.5
• The data regarding the motion of two
different objects A and B are given in
Table 8.1.
• Examine them carefully and state
whether the motion of the objects is
(b)
uniform or non-uniform.
Fig. 8.2
100 SCIENCE
2020-21
