Centrifugal Force - The False Force

An evil word has worked its way into our daily vocabulary, and with it, an incorrect understanding of the way physics works.  "Centrifugal Force" ( Latin for "center fleeing") is often used to describe why mud gets spun off a spinning tire, or water gets pushed out of the clothes during the spin dry cycle of your washer.  It is also used to describe why we tend to slide to the outer side of a car going around a curve.  It is a common explanation...the only problem is all of it is absolutely wrong!!!  Centrifugal force does not exist...there is no such thing...it is a ghost we tend to blame odd behavior on.

Take for example this common situation.  You are riding in a car going around a curve.  Sitting on your dashboard is a cassette tape.  As you go around the curve, the tape moves to outside edge of the car.  Because you don't want to blame it on ghosts, you say "centrifugal force pushed the tape across the dashboard."--wwrroonngg!!  When we view this situation from above the car, we get a better view of what is really happening.  The animation below shows both views at the same time.  The top window shows you the bird's eye view of the car and the tape, while the bottom window shows you the familiar view from the passenger.

casette.gif (19844 bytes)

The car tires on the road have a enough static friction to act as centripetal force which forces the car to go around the curve.  The tape on the slippery dashboard does not have enough friction to act as a centripetal force, so in the absence of a centripetal force the tape follows straight line motion.  The car literally turns out from underneath the tape, but from the passenger's point of view it looks as though something (a ghost force?) pushed the tape across the dashboard.  If the car you are riding in has the windows rolled down, then the tape will leave the car (or does the car leave the tape?) as it follows its straight line path.  If the windows are rolled up, then the window will deliver a centripetal force to the tape and keep it in a circular path.

Any time the word Centrifugal Force is used, what is really being described is a Lack-of-Centripetal Force.


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2000 Science Joy Wagon