Page 106 - Understanding NCERT Science 09

The force exerted on the ball F is,
F = ma = (20/1000) kg × (– 0.02 m s )
-2

= – 0.0004 N.
The negative sign implies that the
frictional force exerted by the table is
Fig. 8.10: Action and reaction forces are equal and
opposite to the direction of motion of
opposite.
the ball.
Suppose you are standing at rest and
intend to start walking on a road. You must
8.5 Third Law of Motion
accelerate, and this requires a force in
accordance with the second law of motion.
The first two laws of motion tell us how an
Which is this force? Is it the muscular effort
applied force changes the motion and provide
you exert on the road? Is it in the direction
us with a method of determining the force.
we intend to move? No, you push the road
The third law of motion states that when one
below backwards. The road exerts an equal
object exerts a force on another object, the
and opposite force on your feet to make you
second object instantaneously exerts a force
move forward.
back on the first. These two forces are always
It is important to note that even though
equal in magnitude but opposite in direction.
the action and reaction forces are always
These forces act on different objects and never
equal in magnitude, these forces may not
on the same object. In the game of football
produce accelerations of equal magnitudes.
sometimes we, while looking at the football
This is because each force acts on a different
and trying to kick it with a greater force,
object that may have a different mass.
collide with a player of the opposite team.
When a gun is fired, it exerts a forward
Both feel hurt because each applies a force
force on the bullet. The bullet exerts an equal
to the other. In other words, there is a pair of
and opposite force on the gun. This results in
forces and not just one force. The two
the recoil of the gun (Fig. 8.11). Since the gun
opposing forces are also known as action and
has a much greater mass than the bullet, the
reaction forces.
acceleration of the gun is much less than the
Let us consider two spring balances
acceleration of the bullet. The third law of
connected together as shown in Fig. 8.10. The
motion can also be illustrated when a sailor
fixed end of balance B is attached with a rigid
jumps out of a rowing boat. As the sailor
support, like a wall. When a force is applied
jumps forward, the force on the boat moves it
through the free end of spring balance A, it is
backwards (Fig. 8.12).
observed that both the spring balances show
the same readings on their scales. It means
that the force exerted by spring balance A on
balance B is equal but opposite in direction
to the force exerted by the balance B on
balance A. Any of these two forces can be called
as action and the other as reaction. This gives
us an alternative statement of the third law of
motion i.e., to every action there is an equal
and opposite reaction. However, it must be
remembered that the action and reaction
always act on two different objects, Fig. 8.11: A forward force on the bullet and recoil
of the gun.
simultaneously.

FORCE AND LAWS OF MOTION 95

Rationalised 2023-24