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 the mutual interaction between two bodies. Forces always occur in pairs.
Further, the mutual forces between two bodies are always equal and opposite. This idea was expressed by Newton in the form of the third law of motion . 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 . . 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