Third law of motion

What is the origin of the external force on the body ? What agency provides the external force ? The simple answer in Newtonian mechanics is that the external force on a body always arises due to some other body. 
Consider a pair of bodies A and B. B gives rise to an external force on A. A natural question
is: Does A in turn give rise to an external force on B ? In some examples, the answer seems
clear. If you press a coiled spring, the spring is compressed by the force of your hand. The
compressed spring in turn exerts a force on your hand and you can feel it. But what if the bodies
are not in contact ? The earth pulls a stone downwards due to gravity. Does the stone exert
a force on the earth ? The answer is not obvious since we hardly see the effect of the stone on the
earth. The answer according to Newton is: Yes, the stone does exert an equal and opposite force
on the earth. We do not notice it since the earth is very massive and the effect of a small force on
its motion is negligible.

Thus, according to Newtonian mechanics, force never occurs singly in nature. Force is themutual interaction between two bodies.

To every action, there is always an equal and opposite reaction.

Newton’s wording of the third law is so crisp and beautiful that it has become a part of common

language. For the same reason perhaps, misconceptions about the third law abound. Let

us note some important points about the third law, particularly in regard to the usage of the

terms : action and reaction.

1. The terms action and reaction in the third law mean nothing else but ‘force’. Using different

terms for the same physical concept can sometimes be confusing. A simple and clear way of stating the third law is as follows :

Forces always occur in pairs. Force on a body A by B is equal and opposite to the

force on the body B by A.

2. The terms action and reaction in the third law may give a wrong impression that action

comes before reaction i.e action is the cause and reaction the effect. There is no causeeffect

relation implied in the third law. The force on A by B and the force on B by A act

at the same instant. By the same reasoning, any one of them may be called action and the

other reaction.

3. Action and reaction forces act on different bodies, not on the same body. Consider a pair

of bodies A and B. According to the third law,

FAB = – FBA 

(force on A by B) = – (force on B by A)

Thus if we are considering the motion of any one body (A or B), only one of the two forces is
relevant. It is an error to add up the two forces and claim that the net force
is zero.

ALSO SEE =

NEWTONS FIRST LAW OF MOTION

NEWTONS SECOND LAW OF MOTION