Reactive centrifugal force

In classical mechanics, a reactive centrifugal force forms part of an action–reaction pair with a centripetal force.

In accordance with Newton's first law of motion, an object moves in a straight line in the absence of any external forces acting on the object. A curved path may however ensue when a physical acts on it; this force is often called a centripetal force, as it is directed toward the center of curvature of the path. Then in accordance with Newton's third law of motion, there will also be an equal and opposite force exerted by the object on some other object, such as a constraint that forces the path to be curved, and this reaction force, the subject of this article, is sometimes called a reactive centrifugal force, as it is directed in the opposite direction of the centripetal force.

Unlike the inertial force or fictitious force known as centrifugal force, which always exists in addition to the reactive force in the rotating frame of reference, the reactive force is a real Newtonian force that is observed in any reference frame. The two forces will only have the same magnitude in the special cases where circular motion arises and where the axis of rotation is the origin of the rotating frame of reference. It is the reactive force that is the subject of this article.

Any force directed away from a center can be called "centrifugal". Centrifugal simply means "directed outward from the center". Similarly, centripetal means "directed toward the center". The "reactive centrifugal force" discussed in this article is not the same thing as the centrifugal pseudoforce, which is usually what's meant by the term "centrifugal force".

The figure at right shows a ball in uniform circular motion held to its path by a massless string tied to an immovable post. The figure is an example of a centrifugally-directed real force. In this system a centripetal force upon the ball provided by the string maintains the circular motion, and the reaction to it, usually called the reactive centrifugal force acts upon the string. In this model, the string is assumed massless and the rotational motion frictionless, so no propelling force is needed to keep the ball in circular motion.

...
Wikipedia