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Centrifugal and Centripetal Force

To the physicist, when dealing with motion around a curved path, it is more analytically correct to identify an “applied force” as the Centripetal Force (“center-seeking”) and the equal and opposite reaction force as the Centrifugal Force (“center-fleeing”).   In ordinary language terms, however, one may only hear the word “centrifugal” to describe the force which is orthogonal (“perpendicular to”) the object in motion.   This is considered perfectly okay since it plainly identifies the essential effect in critical situations such as when a train has derailed from its tracks or car overturned while veering a sharp turn. 

A simple example is whirling a ball on a string.  You will experience that you are constantly pulling the string to maintain the ball in a circular path and therefore the force is toward the center.  The ball however is experiencing an outward tendency which is equal and opposite this centripetal force and rightfully knows it is being forced outward.  (Newton’s Third Law).  Each are the same magnitude and opposite each other.  Centripetal is the applied force and centrifugal is the reaction force.  Each are real and considered the same for practical purposes.  Some try to tag the word “ficticious“ to the centrifugal force and claim that centripetal is the only real force.   However, this is incorrect.  They are only expressing an academic pet-peeve regarding a point-of-view.  It has been shown that the “real damage” (from a train derailment for instance) is from the outward centrifugal force pushing the train off the tracks away from the center.   I’m sure any train operator will agree that the centrifugal force is quite real. ¹

To further satisfy the physicist’s treatise of curvilinear motion, it should also be noted that if the string breaks in the above example of the whirling ball, the ball will not travel outward nor inward, but in a straight line tangential to where the string broke.   At the point where the string breaks, both the centripetal force and centrifugal force have “let go” and no longer exist.  The ball will only continue to move in a straight line.  Likewise, at the point where a derailed train has tipped over and left the tracks, it will continue in a straight line.

A further example is the aerobatic plane exiting a loop maneuver.   Although high downward forces are experienced nearing the exit of the loop, as soon as the plane has steered into a level flying position (by way of elevator control) the centrifugal force will have disappeared.  This can take place almost instantly, and the plane may feel a "reassuring upward bump” as it has left the “danger zone” of the loop exit and establishes a straight-and-level flying position. 

                                                   

1.  In cases of vehicles travelling about a curved path - where no string is attached as in the whirling ball - the applied centripetal force is through the action of steering the vehicle.  For an automobile, the centripetal force will be felt through the tires attempting to grip the road while the vehicle is being turned.  A train is steered by the track (with the train) and the reacting centrifugal force felt through the center of gravity of the train cars (which may tip when reaching their overturning limit – called tipping moment).  For the aerobatic plane, steering is applied through the elevator, and like the train the centrifugal force will react through the center of gravity, but instead produce a downward tipping or downward pitch of the plane.