Centripetal force is the central force felt by an object moving in a circle. It is the force that is necessary to keep the object moving in this circle, otherwise, the object would follow a straight line path, in accordance with Newton's Laws. Assuming this circle is of radius, r, and the object moves with constant velocity, v, then the centripetal acceleration is:

The centripetal force, Fc, which is directed towards the center of the circle, is then:

Centrifugal force is not a true force at all; it is an effect seen when the observer is assuming a rotating reference frame is an inertial frame. This is done quite often with systems such as the earth, since it is only a factor when the rotation is high in comparison to the size of the system.

To see how this force behaves, consider...