Mechanics / Electrodynamics
February 17, 2017
In Newton’s Principia the Third Law of Motion states: ‘to every action there is always opposed an equal reaction.’ The law only refers to the ‘bodies’, not to any other factors. The law is universally applicable to all elastic or non-elastic or all types bodies and surfaces without any constraints at all. But practically the law does not take into account the significant factors e.g. inherent characteristics, nature, compositions, flexibility, rigidity, magnitude, size, elasticity, shape , distinctiveness of interacting bodies, mode of interactions, point of impact etc. The bodies may be of steel, wood, rubber, cloth, wool, sponge, spring, typical plastic, porous material, mud or kneaded flour or chewing gum specifically fabricated material etc. The interacting bodies may be solid, liquid, gas or mixture of all. For all such bodies if the action is same, then the reaction must be the same. In the Principia Newton had given just three qualitative examples to illustrate the law. Thus, to take elusive and effective factors into account, the Third Law of Motion is generalized as: ‘To every action there may be reaction, but may or may not be always equal and opposite, depending upon the inherent characteristics of the interacting bodies.