Mechanics / Electrodynamics
February 29, 2016
In Newton’s Principia the Third Law of Motion states: ‘to every action there is always
opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.’ The Law only refers to the ‘bodies’, not other factors. The Law does not take into account the inherent characteristics, nature, compositions, flexibility, rigidity, magnitude, distinctiveness of interacting bodies etc. The bodies may be of steel, wood, rubber, cloth, wool, sponge, spring, typical plastic, porous material, specifically fabricated material etc. For all such bodies if the action is same, then the reaction must be the same. In the qualitative explanation given after the definition to the Law, Newton expressed Action and Reaction in terms of push or pull (force) and velocity. It can be mathematically understood that in elastic collisions (coefficient of restitution, e = 1) the Action and Reaction are equal only under certain conditions. In numerous cases we find that action and reaction are not equal as the Principia’s third law of motion as it does not take in account vital factors like inherent nature and characteristics of interacting bodies.