NCERT Textbook: Force & Laws of Motion | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


In the previous chapter, we described the
motion of an object along a straight line in
terms of its position, velocity and acceleration.
We saw that such a motion can be uniform
or non-uniform. We have not yet discovered
what causes the motion. Why does the speed
of an object change with time? Do all motions
require a cause? If so, what is the nature of
this cause? In this chapter we shall make an
attempt to quench all such curiosities.
For many centuries, the problem of
motion and its causes had puzzled scientists
and philosophers. A ball on the ground, when
given a small hit, does not move forever. Such
observations suggest that rest is the “natural
state” of an object. This remained the belief
until Galileo Galilei and Isaac Newton
developed an entirely different approach to
understand motion.
In our everyday life we observe that some
effort is required to put a stationary object
into motion or to stop a moving object. We
ordinarily experience this as a muscular effort
and say that we must push or hit or pull on
an object to change its state of motion. The
concept of force is based on this push, hit or
pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted or felt a
force. However, we always see or feel the effect
of a force. It can only be explained by
describing what happens when a force is
applied to an object. Pushing, hitting and
pulling of objects are all ways of bringing
objects in motion (Fig. 9.1). They move
because we make a force act on them.
From your studies in earlier classes, you
are also familiar with the fact that a force can
be used to change the magnitude of velocity
of an object (that is, to make the object move
faster or slower) or to change its direction of
motion. We also know that a force can change
the shape and size of objects (Fig. 9.2).
(a) The trolley moves along the
direction we push it.
(c) The hockey stick hits the ball forward
(b) The drawer is pulled.
Fig. 9.1: Pushing, pulling, or hitting objects change
their state of motion.
(a)
(b)
Fig. 9.2: (a) A spring expands on application of force;
(b) A spherical rubber ball becomes oblong
as we apply force on it.
9 9
9 9 9
F F F F FORCE ORCE ORCE ORCE ORCE     AND AND AND AND AND L L L L LAWS AWS AWS AWS AWS     OF OF OF OF OF M M M M MOTION OTION OTION OTION OTION
Chapter
Page 2


In the previous chapter, we described the
motion of an object along a straight line in
terms of its position, velocity and acceleration.
We saw that such a motion can be uniform
or non-uniform. We have not yet discovered
what causes the motion. Why does the speed
of an object change with time? Do all motions
require a cause? If so, what is the nature of
this cause? In this chapter we shall make an
attempt to quench all such curiosities.
For many centuries, the problem of
motion and its causes had puzzled scientists
and philosophers. A ball on the ground, when
given a small hit, does not move forever. Such
observations suggest that rest is the “natural
state” of an object. This remained the belief
until Galileo Galilei and Isaac Newton
developed an entirely different approach to
understand motion.
In our everyday life we observe that some
effort is required to put a stationary object
into motion or to stop a moving object. We
ordinarily experience this as a muscular effort
and say that we must push or hit or pull on
an object to change its state of motion. The
concept of force is based on this push, hit or
pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted or felt a
force. However, we always see or feel the effect
of a force. It can only be explained by
describing what happens when a force is
applied to an object. Pushing, hitting and
pulling of objects are all ways of bringing
objects in motion (Fig. 9.1). They move
because we make a force act on them.
From your studies in earlier classes, you
are also familiar with the fact that a force can
be used to change the magnitude of velocity
of an object (that is, to make the object move
faster or slower) or to change its direction of
motion. We also know that a force can change
the shape and size of objects (Fig. 9.2).
(a) The trolley moves along the
direction we push it.
(c) The hockey stick hits the ball forward
(b) The drawer is pulled.
Fig. 9.1: Pushing, pulling, or hitting objects change
their state of motion.
(a)
(b)
Fig. 9.2: (a) A spring expands on application of force;
(b) A spherical rubber ball becomes oblong
as we apply force on it.
9 9
9 9 9
F F F F FORCE ORCE ORCE ORCE ORCE     AND AND AND AND AND L L L L LAWS AWS AWS AWS AWS     OF OF OF OF OF M M M M MOTION OTION OTION OTION OTION
Chapter
box with a small force, the box does not move
because of friction acting in a direction
opposite to the push [Fig. 9.4(a)]. This friction
force arises between two surfaces in contact;
in this case, between the bottom of the box
and floor’s rough surface. It balances the
pushing force and therefore the box does not
move. In Fig. 9.4(b), the children push the
box harder but the box still does not move.
This is because the friction force still balances
the pushing force. If the children push the
box harder still, the pushing force becomes
bigger than the friction force [Fig. 9.4(c)].
There is an unbalanced force. So the box
starts moving.
What happens when we ride a bicycle?
When we stop pedalling, the bicycle begins
to slow down. This is again because of the
friction forces acting opposite to the direction
of motion. In order to keep the bicycle moving,
we have to start pedalling again. It thus
appears that an  object maintains its motion
under the continuous application of an
unbalanced force. However, it is quite
incorrect. An object moves with a uniform
velocity when the forces (pushing force and
frictional force) acting on the object are
balanced and there is no net external force
on it. If an unbalanced force is applied on
the object, there will be a change either in its
speed or in the direction of its motion. Thus,
to accelerate the motion of an object, an
unbalanced force is required. And the change
in its speed (or in the direction of motion)
would continue as long as this unbalanced
force is applied. However, if this force is
9.1 Balanced and Unbalanced
Forces
Fig. 9.3 shows a wooden block on a horizontal
table. Two strings X and Y are tied to the two
opposite faces of the block as shown. If we
apply a force by pulling the string X, the block
begins to move to the right. Similarly, if we
pull the string Y, the block moves to the left.
But, if the block is pulled from both the sides
with equal forces, the block will not move.
Such forces are called balanced forces and
do not change the state of rest or of motion of
an object. Now, let us consider a situation in
which two opposite forces of different
magnitudes pull the block. In this case, the
block would begin to move in the direction of
the greater force. Thus, the two forces are
not balanced and the unbalanced force acts
in the direction the block moves. This
suggests that an unbalanced force acting on
an object brings it in motion.
Fig. 9.3: Two forces acting on a wooden block
What happens when some children try to
push a box on a rough floor? If they push the
(a)(b) (c)
Fig. 9.4
FORCE AND LAWS OF MOTION 115
Page 3


In the previous chapter, we described the
motion of an object along a straight line in
terms of its position, velocity and acceleration.
We saw that such a motion can be uniform
or non-uniform. We have not yet discovered
what causes the motion. Why does the speed
of an object change with time? Do all motions
require a cause? If so, what is the nature of
this cause? In this chapter we shall make an
attempt to quench all such curiosities.
For many centuries, the problem of
motion and its causes had puzzled scientists
and philosophers. A ball on the ground, when
given a small hit, does not move forever. Such
observations suggest that rest is the “natural
state” of an object. This remained the belief
until Galileo Galilei and Isaac Newton
developed an entirely different approach to
understand motion.
In our everyday life we observe that some
effort is required to put a stationary object
into motion or to stop a moving object. We
ordinarily experience this as a muscular effort
and say that we must push or hit or pull on
an object to change its state of motion. The
concept of force is based on this push, hit or
pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted or felt a
force. However, we always see or feel the effect
of a force. It can only be explained by
describing what happens when a force is
applied to an object. Pushing, hitting and
pulling of objects are all ways of bringing
objects in motion (Fig. 9.1). They move
because we make a force act on them.
From your studies in earlier classes, you
are also familiar with the fact that a force can
be used to change the magnitude of velocity
of an object (that is, to make the object move
faster or slower) or to change its direction of
motion. We also know that a force can change
the shape and size of objects (Fig. 9.2).
(a) The trolley moves along the
direction we push it.
(c) The hockey stick hits the ball forward
(b) The drawer is pulled.
Fig. 9.1: Pushing, pulling, or hitting objects change
their state of motion.
(a)
(b)
Fig. 9.2: (a) A spring expands on application of force;
(b) A spherical rubber ball becomes oblong
as we apply force on it.
9 9
9 9 9
F F F F FORCE ORCE ORCE ORCE ORCE     AND AND AND AND AND L L L L LAWS AWS AWS AWS AWS     OF OF OF OF OF M M M M MOTION OTION OTION OTION OTION
Chapter
box with a small force, the box does not move
because of friction acting in a direction
opposite to the push [Fig. 9.4(a)]. This friction
force arises between two surfaces in contact;
in this case, between the bottom of the box
and floor’s rough surface. It balances the
pushing force and therefore the box does not
move. In Fig. 9.4(b), the children push the
box harder but the box still does not move.
This is because the friction force still balances
the pushing force. If the children push the
box harder still, the pushing force becomes
bigger than the friction force [Fig. 9.4(c)].
There is an unbalanced force. So the box
starts moving.
What happens when we ride a bicycle?
When we stop pedalling, the bicycle begins
to slow down. This is again because of the
friction forces acting opposite to the direction
of motion. In order to keep the bicycle moving,
we have to start pedalling again. It thus
appears that an  object maintains its motion
under the continuous application of an
unbalanced force. However, it is quite
incorrect. An object moves with a uniform
velocity when the forces (pushing force and
frictional force) acting on the object are
balanced and there is no net external force
on it. If an unbalanced force is applied on
the object, there will be a change either in its
speed or in the direction of its motion. Thus,
to accelerate the motion of an object, an
unbalanced force is required. And the change
in its speed (or in the direction of motion)
would continue as long as this unbalanced
force is applied. However, if this force is
9.1 Balanced and Unbalanced
Forces
Fig. 9.3 shows a wooden block on a horizontal
table. Two strings X and Y are tied to the two
opposite faces of the block as shown. If we
apply a force by pulling the string X, the block
begins to move to the right. Similarly, if we
pull the string Y, the block moves to the left.
But, if the block is pulled from both the sides
with equal forces, the block will not move.
Such forces are called balanced forces and
do not change the state of rest or of motion of
an object. Now, let us consider a situation in
which two opposite forces of different
magnitudes pull the block. In this case, the
block would begin to move in the direction of
the greater force. Thus, the two forces are
not balanced and the unbalanced force acts
in the direction the block moves. This
suggests that an unbalanced force acting on
an object brings it in motion.
Fig. 9.3: Two forces acting on a wooden block
What happens when some children try to
push a box on a rough floor? If they push the
(a)(b) (c)
Fig. 9.4
FORCE AND LAWS OF MOTION 115 SCIENCE 116
removed completely, the object would
continue to move with the velocity it has
acquired till then.
9.2 First Law of Motion
By observing the motion of objects on an
inclined plane Galileo deduced that objects
move with a constant speed when no force
acts on them. He observed that when a marble
rolls down an inclined plane, its velocity
increases [Fig. 9.5(a)]. In the next chapter,
you will learn that the marble falls under the
unbalanced force of gravity as it rolls down
and attains a definite velocity by the time it
reaches the bottom. Its velocity decreases
when it climbs up as shown in Fig. 9.5(b).
Fig. 9.5(c) shows a marble resting on an ideal
frictionless plane inclined on both sides.
Galileo argued that when the marble is
released from left, it would roll down the slope
and go up on the opposite side to the same
height from which it was released. If the
inclinations of the planes on both sides are
equal then the marble will climb the same
distance that it covered while rolling down. If
the angle of inclination of the right-side plane
were gradually decreased, then the marble
would travel further distances till it reaches
the original height. If the right-side plane were
ultimately made horizontal (that is, the slope
is reduced to zero), the marble would continue
to travel forever trying to reach the same
height that it was released from. The
unbalanced forces on the marble in this case
are zero. It thus suggests that an unbalanced
(external) force is required to change the
motion of the marble but no net force is
needed to sustain the uniform motion of the
marble.  In practical situations it is difficult
to achieve a zero unbalanced force. This is
because of the presence of the frictional force
acting opposite to the direction of motion.
Thus, in practice the marble stops after
travelling some distance. The effect of the
frictional force may be minimised by using a
smooth marble and a smooth plane and
providing a lubricant on top of the planes.
Fig. 9.5: (a) the downward motion; (b) the upward
motion of a marble on an inclined plane;
and (c) on a double inclined plane.
Newton further studied Galileo’s ideas on
force and motion and presented three
fundamental laws that govern the motion of
objects. These three laws are known as
Newton’s laws of motion. The first law of
motion is stated as:
An object remains in a state of rest or of
uniform motion in a straight line unless
compelled to change that state by an applied
force.
In other words, all objects resist a change
in their state of motion. In a qualitative way,
the tendency of undisturbed objects to stay
at rest or to keep moving with the same
velocity is called inertia. This is why, the first
law of motion is also known as the law of
inertia.
Certain experiences that we come across
while travelling in a motorcar can be
explained on the basis of the law of inertia.
We tend to remain at rest with respect to the
seat until the drives applies a braking force
to stop the motorcar. With the application of
brakes, the car slows down but our body
tends to continue in the same state of motion
because of its inertia. A sudden application
of brakes may thus cause injury to us by
Page 4


In the previous chapter, we described the
motion of an object along a straight line in
terms of its position, velocity and acceleration.
We saw that such a motion can be uniform
or non-uniform. We have not yet discovered
what causes the motion. Why does the speed
of an object change with time? Do all motions
require a cause? If so, what is the nature of
this cause? In this chapter we shall make an
attempt to quench all such curiosities.
For many centuries, the problem of
motion and its causes had puzzled scientists
and philosophers. A ball on the ground, when
given a small hit, does not move forever. Such
observations suggest that rest is the “natural
state” of an object. This remained the belief
until Galileo Galilei and Isaac Newton
developed an entirely different approach to
understand motion.
In our everyday life we observe that some
effort is required to put a stationary object
into motion or to stop a moving object. We
ordinarily experience this as a muscular effort
and say that we must push or hit or pull on
an object to change its state of motion. The
concept of force is based on this push, hit or
pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted or felt a
force. However, we always see or feel the effect
of a force. It can only be explained by
describing what happens when a force is
applied to an object. Pushing, hitting and
pulling of objects are all ways of bringing
objects in motion (Fig. 9.1). They move
because we make a force act on them.
From your studies in earlier classes, you
are also familiar with the fact that a force can
be used to change the magnitude of velocity
of an object (that is, to make the object move
faster or slower) or to change its direction of
motion. We also know that a force can change
the shape and size of objects (Fig. 9.2).
(a) The trolley moves along the
direction we push it.
(c) The hockey stick hits the ball forward
(b) The drawer is pulled.
Fig. 9.1: Pushing, pulling, or hitting objects change
their state of motion.
(a)
(b)
Fig. 9.2: (a) A spring expands on application of force;
(b) A spherical rubber ball becomes oblong
as we apply force on it.
9 9
9 9 9
F F F F FORCE ORCE ORCE ORCE ORCE     AND AND AND AND AND L L L L LAWS AWS AWS AWS AWS     OF OF OF OF OF M M M M MOTION OTION OTION OTION OTION
Chapter
box with a small force, the box does not move
because of friction acting in a direction
opposite to the push [Fig. 9.4(a)]. This friction
force arises between two surfaces in contact;
in this case, between the bottom of the box
and floor’s rough surface. It balances the
pushing force and therefore the box does not
move. In Fig. 9.4(b), the children push the
box harder but the box still does not move.
This is because the friction force still balances
the pushing force. If the children push the
box harder still, the pushing force becomes
bigger than the friction force [Fig. 9.4(c)].
There is an unbalanced force. So the box
starts moving.
What happens when we ride a bicycle?
When we stop pedalling, the bicycle begins
to slow down. This is again because of the
friction forces acting opposite to the direction
of motion. In order to keep the bicycle moving,
we have to start pedalling again. It thus
appears that an  object maintains its motion
under the continuous application of an
unbalanced force. However, it is quite
incorrect. An object moves with a uniform
velocity when the forces (pushing force and
frictional force) acting on the object are
balanced and there is no net external force
on it. If an unbalanced force is applied on
the object, there will be a change either in its
speed or in the direction of its motion. Thus,
to accelerate the motion of an object, an
unbalanced force is required. And the change
in its speed (or in the direction of motion)
would continue as long as this unbalanced
force is applied. However, if this force is
9.1 Balanced and Unbalanced
Forces
Fig. 9.3 shows a wooden block on a horizontal
table. Two strings X and Y are tied to the two
opposite faces of the block as shown. If we
apply a force by pulling the string X, the block
begins to move to the right. Similarly, if we
pull the string Y, the block moves to the left.
But, if the block is pulled from both the sides
with equal forces, the block will not move.
Such forces are called balanced forces and
do not change the state of rest or of motion of
an object. Now, let us consider a situation in
which two opposite forces of different
magnitudes pull the block. In this case, the
block would begin to move in the direction of
the greater force. Thus, the two forces are
not balanced and the unbalanced force acts
in the direction the block moves. This
suggests that an unbalanced force acting on
an object brings it in motion.
Fig. 9.3: Two forces acting on a wooden block
What happens when some children try to
push a box on a rough floor? If they push the
(a)(b) (c)
Fig. 9.4
FORCE AND LAWS OF MOTION 115 SCIENCE 116
removed completely, the object would
continue to move with the velocity it has
acquired till then.
9.2 First Law of Motion
By observing the motion of objects on an
inclined plane Galileo deduced that objects
move with a constant speed when no force
acts on them. He observed that when a marble
rolls down an inclined plane, its velocity
increases [Fig. 9.5(a)]. In the next chapter,
you will learn that the marble falls under the
unbalanced force of gravity as it rolls down
and attains a definite velocity by the time it
reaches the bottom. Its velocity decreases
when it climbs up as shown in Fig. 9.5(b).
Fig. 9.5(c) shows a marble resting on an ideal
frictionless plane inclined on both sides.
Galileo argued that when the marble is
released from left, it would roll down the slope
and go up on the opposite side to the same
height from which it was released. If the
inclinations of the planes on both sides are
equal then the marble will climb the same
distance that it covered while rolling down. If
the angle of inclination of the right-side plane
were gradually decreased, then the marble
would travel further distances till it reaches
the original height. If the right-side plane were
ultimately made horizontal (that is, the slope
is reduced to zero), the marble would continue
to travel forever trying to reach the same
height that it was released from. The
unbalanced forces on the marble in this case
are zero. It thus suggests that an unbalanced
(external) force is required to change the
motion of the marble but no net force is
needed to sustain the uniform motion of the
marble.  In practical situations it is difficult
to achieve a zero unbalanced force. This is
because of the presence of the frictional force
acting opposite to the direction of motion.
Thus, in practice the marble stops after
travelling some distance. The effect of the
frictional force may be minimised by using a
smooth marble and a smooth plane and
providing a lubricant on top of the planes.
Fig. 9.5: (a) the downward motion; (b) the upward
motion of a marble on an inclined plane;
and (c) on a double inclined plane.
Newton further studied Galileo’s ideas on
force and motion and presented three
fundamental laws that govern the motion of
objects. These three laws are known as
Newton’s laws of motion. The first law of
motion is stated as:
An object remains in a state of rest or of
uniform motion in a straight line unless
compelled to change that state by an applied
force.
In other words, all objects resist a change
in their state of motion. In a qualitative way,
the tendency of undisturbed objects to stay
at rest or to keep moving with the same
velocity is called inertia. This is why, the first
law of motion is also known as the law of
inertia.
Certain experiences that we come across
while travelling in a motorcar can be
explained on the basis of the law of inertia.
We tend to remain at rest with respect to the
seat until the drives applies a braking force
to stop the motorcar. With the application of
brakes, the car slows down but our body
tends to continue in the same state of motion
because of its inertia. A sudden application
of brakes may thus cause injury to us by
FORCE AND LAWS OF MOTION 117
impact or collision with the panels in front.
Safety belts are worn to prevent such
accidents. Safety belts exert a force on our
body to make the forward motion slower. An
opposite experience is encountered when we
are standing in a bus and the bus begins to
move suddenly. Now we tend to fall
backwards. This is because the sudden start
of the bus brings motion to the bus as well
as to our feet in contact with the floor of the
bus. But the rest of our body opposes this
motion because of its inertia.
When a motorcar makes a sharp turn at
a high speed, we tend to get thrown to one
side. This can again be explained on the basis
of the law of inertia. We tend to continue in
our straight-line motion. When an
unbalanced force is applied by the engine to
change the direction of motion of the
motorcar, we slip to one side of the seat due
to the inertia of our body.
The fact that a body will remain at rest
unless acted upon by an unbalanced force
can be illustrated through the following
activities:
Activity ______________9.1
• Make a pile of similar carom coins on
a table, as shown in Fig. 9.6.
• Attempt  a sharp horizontal hit at the
bottom of the pile using another carom
coin or the striker. If the hit is strong
enough, the bottom coin moves out
quickly. Once the lowest coin is
removed, the inertia of the other coins
makes them ‘fall’ vertically on the table.
Galileo Galilei was born
on 15 February 1564 in
Pisa, Italy. Galileo, right
from his childhood, had
interest in mathematics
and natural philosophy.
But his father
Vincenzo Galilei wanted
him to become a medical
doctor. Accordingly,
Galileo enrolled himself
for a medical degree at the
University of Pisa in 1581 which he never
completed because of his real interest in
mathematics. In 1586, he wrote his first
scientific book ‘The Little Balance [La
Balancitta]’, in which he described
Archimedes’ method of finding the relative
densities (or specific gravities) of substances
using a balance. In 1589, in his series of
essays – De Motu, he presented his theories
about falling objects using an inclined plane
to slow down the rate of descent.
In 1592, he was appointed professor of
mathematics at the University of Padua in
the Republic of Venice. Here he continued his
observations on the theory of motion and
through his study of inclined planes and the
pendulum, formulated the correct law for
uniformly accelerated objects that the
distance the object moves is proportional to
the square of the time taken.
Galileo was also a remarkable craftsman.
He developed a series of telescopes whose
optical performance was much better than
that of other telescopes available during those
days. Around 1640, he designed the first
pendulum clock. In his book ‘Starry
Messenger’ on his astronomical discoveries,
Galileo claimed to have seen mountains on
the moon, the milky way made up of tiny
stars, and four small bodies orbiting Jupiter.
In his books ‘Discourse on Floating Bodies’
and ‘Letters on the Sunspots’, he disclosed
his observations of sunspots.
Using his own telescopes and through his
observations on Saturn and Venus, Galileo
argued that all the planets must orbit the Sun
and not the earth, contrary to what was
believed at that time.
Galileo Galilei
(1564 – 1642)
Fig. 9.6: Only the carom coin at the bottom of a
pile is removed when a fast moving carom
coin (or striker) hits it.
Page 5


In the previous chapter, we described the
motion of an object along a straight line in
terms of its position, velocity and acceleration.
We saw that such a motion can be uniform
or non-uniform. We have not yet discovered
what causes the motion. Why does the speed
of an object change with time? Do all motions
require a cause? If so, what is the nature of
this cause? In this chapter we shall make an
attempt to quench all such curiosities.
For many centuries, the problem of
motion and its causes had puzzled scientists
and philosophers. A ball on the ground, when
given a small hit, does not move forever. Such
observations suggest that rest is the “natural
state” of an object. This remained the belief
until Galileo Galilei and Isaac Newton
developed an entirely different approach to
understand motion.
In our everyday life we observe that some
effort is required to put a stationary object
into motion or to stop a moving object. We
ordinarily experience this as a muscular effort
and say that we must push or hit or pull on
an object to change its state of motion. The
concept of force is based on this push, hit or
pull. Let us now ponder about a ‘force’. What
is it? In fact, no one has seen, tasted or felt a
force. However, we always see or feel the effect
of a force. It can only be explained by
describing what happens when a force is
applied to an object. Pushing, hitting and
pulling of objects are all ways of bringing
objects in motion (Fig. 9.1). They move
because we make a force act on them.
From your studies in earlier classes, you
are also familiar with the fact that a force can
be used to change the magnitude of velocity
of an object (that is, to make the object move
faster or slower) or to change its direction of
motion. We also know that a force can change
the shape and size of objects (Fig. 9.2).
(a) The trolley moves along the
direction we push it.
(c) The hockey stick hits the ball forward
(b) The drawer is pulled.
Fig. 9.1: Pushing, pulling, or hitting objects change
their state of motion.
(a)
(b)
Fig. 9.2: (a) A spring expands on application of force;
(b) A spherical rubber ball becomes oblong
as we apply force on it.
9 9
9 9 9
F F F F FORCE ORCE ORCE ORCE ORCE     AND AND AND AND AND L L L L LAWS AWS AWS AWS AWS     OF OF OF OF OF M M M M MOTION OTION OTION OTION OTION
Chapter
box with a small force, the box does not move
because of friction acting in a direction
opposite to the push [Fig. 9.4(a)]. This friction
force arises between two surfaces in contact;
in this case, between the bottom of the box
and floor’s rough surface. It balances the
pushing force and therefore the box does not
move. In Fig. 9.4(b), the children push the
box harder but the box still does not move.
This is because the friction force still balances
the pushing force. If the children push the
box harder still, the pushing force becomes
bigger than the friction force [Fig. 9.4(c)].
There is an unbalanced force. So the box
starts moving.
What happens when we ride a bicycle?
When we stop pedalling, the bicycle begins
to slow down. This is again because of the
friction forces acting opposite to the direction
of motion. In order to keep the bicycle moving,
we have to start pedalling again. It thus
appears that an  object maintains its motion
under the continuous application of an
unbalanced force. However, it is quite
incorrect. An object moves with a uniform
velocity when the forces (pushing force and
frictional force) acting on the object are
balanced and there is no net external force
on it. If an unbalanced force is applied on
the object, there will be a change either in its
speed or in the direction of its motion. Thus,
to accelerate the motion of an object, an
unbalanced force is required. And the change
in its speed (or in the direction of motion)
would continue as long as this unbalanced
force is applied. However, if this force is
9.1 Balanced and Unbalanced
Forces
Fig. 9.3 shows a wooden block on a horizontal
table. Two strings X and Y are tied to the two
opposite faces of the block as shown. If we
apply a force by pulling the string X, the block
begins to move to the right. Similarly, if we
pull the string Y, the block moves to the left.
But, if the block is pulled from both the sides
with equal forces, the block will not move.
Such forces are called balanced forces and
do not change the state of rest or of motion of
an object. Now, let us consider a situation in
which two opposite forces of different
magnitudes pull the block. In this case, the
block would begin to move in the direction of
the greater force. Thus, the two forces are
not balanced and the unbalanced force acts
in the direction the block moves. This
suggests that an unbalanced force acting on
an object brings it in motion.
Fig. 9.3: Two forces acting on a wooden block
What happens when some children try to
push a box on a rough floor? If they push the
(a)(b) (c)
Fig. 9.4
FORCE AND LAWS OF MOTION 115 SCIENCE 116
removed completely, the object would
continue to move with the velocity it has
acquired till then.
9.2 First Law of Motion
By observing the motion of objects on an
inclined plane Galileo deduced that objects
move with a constant speed when no force
acts on them. He observed that when a marble
rolls down an inclined plane, its velocity
increases [Fig. 9.5(a)]. In the next chapter,
you will learn that the marble falls under the
unbalanced force of gravity as it rolls down
and attains a definite velocity by the time it
reaches the bottom. Its velocity decreases
when it climbs up as shown in Fig. 9.5(b).
Fig. 9.5(c) shows a marble resting on an ideal
frictionless plane inclined on both sides.
Galileo argued that when the marble is
released from left, it would roll down the slope
and go up on the opposite side to the same
height from which it was released. If the
inclinations of the planes on both sides are
equal then the marble will climb the same
distance that it covered while rolling down. If
the angle of inclination of the right-side plane
were gradually decreased, then the marble
would travel further distances till it reaches
the original height. If the right-side plane were
ultimately made horizontal (that is, the slope
is reduced to zero), the marble would continue
to travel forever trying to reach the same
height that it was released from. The
unbalanced forces on the marble in this case
are zero. It thus suggests that an unbalanced
(external) force is required to change the
motion of the marble but no net force is
needed to sustain the uniform motion of the
marble.  In practical situations it is difficult
to achieve a zero unbalanced force. This is
because of the presence of the frictional force
acting opposite to the direction of motion.
Thus, in practice the marble stops after
travelling some distance. The effect of the
frictional force may be minimised by using a
smooth marble and a smooth plane and
providing a lubricant on top of the planes.
Fig. 9.5: (a) the downward motion; (b) the upward
motion of a marble on an inclined plane;
and (c) on a double inclined plane.
Newton further studied Galileo’s ideas on
force and motion and presented three
fundamental laws that govern the motion of
objects. These three laws are known as
Newton’s laws of motion. The first law of
motion is stated as:
An object remains in a state of rest or of
uniform motion in a straight line unless
compelled to change that state by an applied
force.
In other words, all objects resist a change
in their state of motion. In a qualitative way,
the tendency of undisturbed objects to stay
at rest or to keep moving with the same
velocity is called inertia. This is why, the first
law of motion is also known as the law of
inertia.
Certain experiences that we come across
while travelling in a motorcar can be
explained on the basis of the law of inertia.
We tend to remain at rest with respect to the
seat until the drives applies a braking force
to stop the motorcar. With the application of
brakes, the car slows down but our body
tends to continue in the same state of motion
because of its inertia. A sudden application
of brakes may thus cause injury to us by
FORCE AND LAWS OF MOTION 117
impact or collision with the panels in front.
Safety belts are worn to prevent such
accidents. Safety belts exert a force on our
body to make the forward motion slower. An
opposite experience is encountered when we
are standing in a bus and the bus begins to
move suddenly. Now we tend to fall
backwards. This is because the sudden start
of the bus brings motion to the bus as well
as to our feet in contact with the floor of the
bus. But the rest of our body opposes this
motion because of its inertia.
When a motorcar makes a sharp turn at
a high speed, we tend to get thrown to one
side. This can again be explained on the basis
of the law of inertia. We tend to continue in
our straight-line motion. When an
unbalanced force is applied by the engine to
change the direction of motion of the
motorcar, we slip to one side of the seat due
to the inertia of our body.
The fact that a body will remain at rest
unless acted upon by an unbalanced force
can be illustrated through the following
activities:
Activity ______________9.1
• Make a pile of similar carom coins on
a table, as shown in Fig. 9.6.
• Attempt  a sharp horizontal hit at the
bottom of the pile using another carom
coin or the striker. If the hit is strong
enough, the bottom coin moves out
quickly. Once the lowest coin is
removed, the inertia of the other coins
makes them ‘fall’ vertically on the table.
Galileo Galilei was born
on 15 February 1564 in
Pisa, Italy. Galileo, right
from his childhood, had
interest in mathematics
and natural philosophy.
But his father
Vincenzo Galilei wanted
him to become a medical
doctor. Accordingly,
Galileo enrolled himself
for a medical degree at the
University of Pisa in 1581 which he never
completed because of his real interest in
mathematics. In 1586, he wrote his first
scientific book ‘The Little Balance [La
Balancitta]’, in which he described
Archimedes’ method of finding the relative
densities (or specific gravities) of substances
using a balance. In 1589, in his series of
essays – De Motu, he presented his theories
about falling objects using an inclined plane
to slow down the rate of descent.
In 1592, he was appointed professor of
mathematics at the University of Padua in
the Republic of Venice. Here he continued his
observations on the theory of motion and
through his study of inclined planes and the
pendulum, formulated the correct law for
uniformly accelerated objects that the
distance the object moves is proportional to
the square of the time taken.
Galileo was also a remarkable craftsman.
He developed a series of telescopes whose
optical performance was much better than
that of other telescopes available during those
days. Around 1640, he designed the first
pendulum clock. In his book ‘Starry
Messenger’ on his astronomical discoveries,
Galileo claimed to have seen mountains on
the moon, the milky way made up of tiny
stars, and four small bodies orbiting Jupiter.
In his books ‘Discourse on Floating Bodies’
and ‘Letters on the Sunspots’, he disclosed
his observations of sunspots.
Using his own telescopes and through his
observations on Saturn and Venus, Galileo
argued that all the planets must orbit the Sun
and not the earth, contrary to what was
believed at that time.
Galileo Galilei
(1564 – 1642)
Fig. 9.6: Only the carom coin at the bottom of a
pile is removed when a fast moving carom
coin (or striker) hits it.
SCIENCE 118
five-rupees coin if we use a one-rupee coin, we
find that a lesser force is required to perform
the activity. A force that is just enough to
cause a small cart to pick up a large velocity
will produce a negligible change in the motion
of a train. This is because, in comparison to
the cart the train has a much lesser tendency
to change its state of motion. Accordingly, we
say that the train has more inertia than the
cart. Clearly, heavier or more massive objects
offer larger inertia. Quantitatively, the inertia
of an object is measured by its mass. We may
thus relate inertia and mass as follows:
Inertia is the natural tendency of an object to
resist a change in its state of motion or of
rest. The mass of an object is a measure of
its inertia.
uestions
1. Which of the following has more
inertia: (a) a rubber ball and a
stone of the same size? (b) a
bicycle and a train? (c) a five-
rupees coin and a one-rupee coin?
2. In the following example, try to
identify the number of times the
velocity of the ball changes:
“A football player kicks a football
to another player of his team who
kicks the football towards the
goal. The goalkeeper of the
opposite team collects the football
and kicks it towards a player of
his own team”.
Also identify the agent supplying
the force in each case.
3. Explain why some of the leaves
may get detached from a tree if
we vigorously shake its branch.
4. Why do you fall in the forward
direction when a moving bus
brakes to a stop and fall
backwards when it accelerates
from rest?
9.4 Second Law of Motion
The first law of motion indicates that when
an unbalanced external force acts on an
Activity ______________9.2
• Set a five-rupee coin on a stiff playing
card covering an empty glass tumbler
standing on a table as shown in
Fig. 9.7.
• Give the card a sharp horizontal flick
with a finger. If we do it fast then the
card shoots away, allowing the coin to
fall vertically into the glass tumbler due
to its inertia.
• The inertia of the coin tries to maintain
its state of rest even when the card
flows off.
Fig. 9.7: When the playing card is flicked with the
finger the coin placed over it falls in the
tumbler.
Activity ______________9.3
• Place a water-filled tumbler on a tray.
• Hold the tray and turn around as fast
as you can.
• We observe that the water spills. Why?
Observe that a groove is provided in a
saucer for placing the tea cup. It prevents
the cup from toppling over in case of sudden
jerks.
9.3 Inertia and Mass
All the examples and activities given so far
illustrate that there is a resistance offered by
an object to change its state of motion. If it is
at rest it tends to remain at rest; if it is moving
it tends to keep moving. This property of an
object is called its inertia. Do all bodies have
the same inertia? We know that it is easier to
push an empty box than a box full of books.
Similarly, if we kick a football it flies away.
But if we kick a stone of the same size with
equal force, it hardly moves. We may, in fact,
get an injury in our foot while doing so!
Similarly, in activity 9.2, instead of a
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