In classical mechanics, centrifugal force is the experience of the inertia of an object moving in circular motion, causing it to move away from the center. It is often designated a "fictitious force".

We know from Newton's first law of motion that a body will maintain a constant velocity unless a net force (the sum of all the forces acting) acts upon it. Thus, for a body to be moving in a circle, a net force must exist to stop it from traveling in a straight line. This force is the centripetal force, the only action force necessary for a circular motion. By Newton's third law of motion, centripetal force is accompanied by an opposite reaction force – on the object providing the centripetal force. This is centrifugal force.

From an inertial (non-accelerating) reference frame, outside the rotation, the centrifugal "force" can simply be described as the inertia of the object, which must be continually overcome by the centripetal force to maintain a circular path. If the centripetal force was to disappear, the centrifugal force would disappear too, and the object would move in a straight line, tangent to the circle.

However, when observing from within a frame of reference rotating along with the object, it is the centrifugal force that appears to be acting on otherwise stationary objects, and the reacting centripetal force that is keeping them stationary. When working within a rotating frame of reference, calculations can be simplified by treating centrifugal force as if it was an action force. This is important in weather forecasting, or when designing spinning objects like centrifuges and propellors.

If W is the weight of the revolving body in pounds, R is the radius of the circular path in feet, and N is the rate of rotation in rpm, then the centrifugal force F in pounds is given by:

F = .00034084 WRN²

[The Encyclopedia Americana, 1940]

Like all Newtonian physics, the above assumes that there is some universal frame of reference from which it can be determined whether or not an object is moving. The theory of relativity dispenses with this, and views "inertial forces" like the centrifugal and Coriolis effects as fully "real", while it is gravity that becomes "fictitious".