Reactive centrifugal force
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In classical mechanics, reactive centrifugal force is a reaction force paired with a centripetal force. A centripetal force is one that causes a body to follow a curved path (rotation). The reactive force is supplied by the rotating body, and acts on the body that supplies the centripetal force to the rotating body. A mass undergoing rotational motion accelerates toward the axis of rotation. This centripetal acceleration is caused by a centripetal force exerted on the mass by some other object. In accordance with Newton's third law of motion, the rotating mass exerts an equal and opposite force on that object. This reactive force is directed from that object toward the rotating mass. Unlike the pseudo force or fictitious force known as centrifugal force, the reaction force is a real Newtonian force that is observed in an inertial (non-accelerating) system. It is this reactive force that is the subject of this article.
The term "reactive centrifugal force", in relation to the reaction force to a centripetal force, is seldom referred to in modern physics and mechanics. The term "centrifugal force" usually refers to the fictitious force in a rotating reference frame that appears due to the rotation of the coordinate system. The reactive centrifugal force that is paired with a centripetal force is not always directed away from the axis of rotation, from the point of view of the body that it acts on. In a two-body rotation, such as a planet and moon rotating about their common center of mass or barycentre, the forces on both bodies are centripetal. In that case, the reaction to the centripetal force of the planet on the moon is the centripetal force of the moon on the planet. The direction of the reaction force is sometimes referred as centrifugal  as it is in the direction from the planet toward the moon. Due to the symmetry of a two-body rotation, one could refer to both forces as centrifugal reaction forces.
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The figure at right shows a ball in uniform circular motion held to its path by a massless string tied to an immovable post. 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.
Newton's first law requires that any body not moving in a straight line is subject to a force, and the free body diagram shows the force upon the ball (center panel) exerted by the string to maintain the ball in its circular motion.
Newton's third law of action and reaction states that if the string exerts an inward centripetal force on the ball, the ball will exert an equal but outward reaction upon the string, shown in the free body diagram of the string (lower panel) as the reactive centrifugal force.
The string transmits the reactive centrifugal force from the ball to the fixed post, pulling upon the post. Again according to Newton's third law, the post exerts a reaction upon the string, labeled the post reaction, pulling upon the string. The two forces upon the string are equal and opposite, exerting no net force upon the string (assuming that the string is massless), but placing the string under tension.
It should be noted, however, that the reason the post appears to be "immovable" is because it is fixed to the earth. If the rotating ball was tethered to the mast of a boat, for example, the boat mast and ball would both experience rotation about a central point.
Even though the reactive centrifugal is rarely used in analyses in the physics literature, the concept is applied within some mechanical engineering concepts. An example of this kind of engineering concept is an analysis of the stresses within a rapidly rotating turbine blade. The blade can be treated as a stack of layers going from the axis out to the edge of the blade. Each layer exerts an outward (centrifugal) force on the immediately adjacent, radially inward layer and an inward (centripetal) force on the immediately adjacent, radially outward layer. At the same time the inner layer exerts an elastic centripetal force on the middle layer, while and the outer layer exerts an elastic centrifugal force, which results in an internal stress. It is the stresses in the blade and their causes that mainly interest mechanical engineers in this situation.
Another example of a rotating device in which a reactive centrifugal force can be identified used to describe the system behavior is the centrifugal clutch. A centrifugal clutch is used in small engine-powered devices such as chain saws, go-karts and model helicopters. It allows the engine to start and idle without driving the device, but automatically and smoothly engages the drive as the engine speed rises. A spring is used to constrain the spinning clutch shoes. At low speeds, the spring provides the centripetal force to the shoes, which move to larger radius as the speed increases and the spring stretches under tension. At higher speeds, when the shoes can't move any further out to increase the spring tension, due to the outer drum, the drum provides some of the centripetal force that keeps the shoes moving in a circular path. The force of tension applied to the spring, and the outward force applied to the drum by the spinning shoes are the corresponding reactive centrifugal forces. The mutual force between the drum and the shoes provides the friction needed to engage the output drive shaft that is connected to the drum. Thus the centrifugal clutch illustrates both the fictitious centrifugal force and the reactive centrifugal force.
Reactive centrifugal force, being one-half of the reaction pair together with centripetal force, is a concept which applies in any reference frame. This distinguishes it from the inertial or fictitious centrifugal force, which appears only in rotating frames.
|Reactive centrifugal force||Inertial centrifugal force|
|Any||Only rotating frames|
|Bodies moving in
|Acts as if emanating from the rotation axis,
it is a so-called fictitious force or d'Alembert force
|The object(s) causing
the curved motion,
not upon the body in
|All bodies, moving or not;
if moving, coriolis force is present as well
|Direction||Opposite to the
causing curved path
|Away from rotation axis,
regardless of path of body
|Kinetic analysis||Reaction to
the centripetal force
|Included as a fictitious force in
Newton's second law
according to D'Alembert's principle
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