4.3 Centrifugal Force

Centrifugal force is often incorrectly identified as a force that pulls an object outward when it is moving in a circular path. There is, in fact, no such force in Newtonian reference frames (non-rotating reference frames). Take the example of a boy swinging ball, tied to a string, around in a circle. If the boy releases the string, the ball will continue in a path that is tangent to its radius at the point of release (see Figure 4.2). It will not fly away from the circle.

What, then, is centrifugal force? Centrifugal, from the Latin words "centri", center, and "fugar", to flee, does describe a force that acts away from the center of a circular path of motion. However, centrifugal forces cannot be observed in Newtonian reference frames, which are all that we have discussed thus far. The idea of a centrifugal force is sometimes useful in discussing motion relative to non-Newtonian, or rotating, reference frames.

Take, for example, a bucket containing water that is attached to a rope and swinging in a circular path. Even though, at times, the bucket is upside-down, the water remains in the bucket. This can be explained using either the concept of centripetal force or that of centrifugal force. If we take the ground as our reference point, then we decide the bucket must have centripetal acceleration. When this acceleration is greater than that of gravity, the water remains in the bucket. On the other hand, if we take the bucket as our frame of reference, it seems that the water is pushing outward, or experiencing a centrifugal force. The water remains in the bucket because this centrifugal force is greater than the force of gravity.

Since centrifugal force is the opposite of centripetal force, we can use Equations 4.1 and 4.2 to calculate centrifugal force and acceleration, except that we must reverse the directions.