Fictitious force, peculiar to circular motion, that is equal but opposite to the centripetal force that keeps a particle on a circular path (see centripetal acceleration ).
For example, a stone attached to a string and whirling in a horizontal circular path is accelerated toward the centre of its path by the tension in the string, the only force acting on the string. However, in a reference frame at rest with the stone, another force
the centrifugal force
must be introduced for Newton's laws of motion to apply. Centrifugal force is a useful concept in analyzing behaviour in rotating systems.