Newton’s Third Law of Motion states: ‘To every action there is an equal and opposite reaction’. The Third Law of Motion indicates that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first object. These two forces are always equal in magnitude, but opposite in direction. For example, when we placed a wooden block on the ground, this block exerts a force equal to its weight, W = mg acting downwards to the ground. This is the action force. The ground exerts an equal and opposite force W’ = mg on the block in the upward direction. This is the reaction force.
This video explains how to study the Third Law of Motion using two spring balances.