NEWTON'S THIRD LAW OF MOTION
From
Newton's first law, we learnt what force is. The second law gave us a measure
of force. It established that force acting on a body is the product of mass of
the body and acceleration of the body. Newton's third law of motion tells us
about the nature of the force.
According
to this law, when an object A exerts a force on another object B, the second
object B exerts a force back on the first object A, at the same instant. These
two forces are always equal in magnitude, but opposite in direction. The two
opposing forces are also known as the forces of action and reaction. The forces
of action and reaction act on two different objects A and B. They never act on
the same object. Thus, the third law of motion tells us that when two objects
interact with each other, there occurs a pair of equal and opposite forces, and
not just one force. Hence,
Newton's third law of motion states that to every action,
there is always an equal and opposite reaction, i.e., the forces of action and
reaction are always equal and opposite. These forces act on two different
objects and never cancel each other. Each force produces its own effect.
To prove
this law, we can perform a simple experiment as detailed here :
Fig. 2.6
shows two spring balances A and B connected together. The fixed end of spring
balance A is attached to a rigid support like a wall. When a force is applied
by pulling the fixed end of spring balance B to the right, both the spring
balances show the same readings on their scales. It means force exerted by
spring balance B on A is equal in magnitude, but opposite in direction to the
force exerted by spring balance A on B.
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If force exerted by spring balance B on spring balance A is
called Action, then force exerted by spring balance A on B is called
Reaction. As the readings on the scales of two spring balances are equal,
therefore, the forces of action and reaction are always equal. The force of
action on spring balance A is to the right, and the force of reaction on
spring balance B is to the left. Thus, the forces of action and reaction are
opposing forces. Obviously, these forces of action and reaction are acting on
different objects, if action is on A, the reaction is on . Further, although the action and reaction forces are always
equal in magnitude, yet these forces may not produce accelerations of equal
magnitude. This is because each force acts on a different object which may
have different mass. The force acting on lighter body will produce greater
acceleration and vice versa. SOME EXAMPLES OF
NEWTON'S THIRD LAW OF MOTION Some of our day-to-day observations can be explained in
terms of Newton's third law of motion as follows: 1. Walking To walk on the ground, we push the ground backwards with our
foot, along OA. As a reaction, the ground pushes our foot forward with the
same force, along OB. It is this forward reaction force of the ground that
enables us to walk forward. Walking becomes difficult when the ground is slippery or it
is covered with snow or sand. This is because we can exert much smaller force
in the form of backward action on the ground. The forward reaction of the
ground will reduce accordingly. 2. Swimming While trying to swim, a swimmer pushes the water backwards
with her hands and feet. This is the force of action. The water pushes the
swimmer forward with the same force (of reaction). 3. Recoiling of gun When a bullet is fired from a gun, the gun recoils, i.e.,
the gun moves backwards through a small distance, giving jerk to the shoulder
of the gunman. This is because on firing, the gun exerts some force on the
bullet (i.e., action) in forward direction. In turn, the bullet exerts an
equal force on the gun (i.e,, reaction) in the backward direction. The
distance moved by the gun is smaller because gun is much heavier than the
bullet, 4. Man and boat Suppose a sailor is in a boat at rest near a river bank. As
the sailor jumps from the boat to the river bank, the boat is pushed away
from the bank. This happens due to equal forces of action and reaction. 5. The flying of rockets and jet planes In a rocket, the fuel burnt appears in the form of hot and
highly compressed gases. These are made to escape through a nozzle in the
downward direction. As a reaction, the rocket moves upwards with the same
force. The forces of action and reaction. In a jet plane, the burnt gases rush out of a jet at the rear end (back
end) of the aircraft. As a reaction, the jet plane moves forward with an
equal force. 6. The case of hose pipe To put out fire, the firemen direct a powerful stream of water on fire,
from a hose pipe. While doing so, the hose pipe is to be held strongly
because of its tendency to move backwards. As the water rushes out at a great
speed from the hose pipe in the forward direction (action), the hose pipe
tends to move backwards, due to an equal force of reaction.
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