While balancing rotating mass we consider the inertia force (centrifugal force) is equal and opposite to centripetal force which causes the rotation. if both force(applied external force on rotating mass) which causes the motion and force which resist motion (inertia force) are equal and opposite, won't they cancel each other and produce nil effect? if so how is the ball rotating with constant angular velocity In case of reciprocating mass again we have inertia force which is the mass times the acceleration of reciprocating mass and opposite in direction. if the inertia force and motion causing applied force are equal and opposite, isn't the net force on the body is zero?if not why it is so? and why we consider: 1. inertial force equal and opposite to the external force applied 2. what is the real effect of inertia force on a body?