Monday, May 13, 2013

Centrifugal vs. Centripetal

I went to my daughter’s open house at school last Friday and afterwards we played a bit of tether ball before heading home. And while I’m not that great at tether ball, I can explain the forces involved in the game.

   Centrifugal force is what is often used to describe what happens to the ball as it rotates around the pole—it’s being pushed as far away from the pole as possible. But in actuality, centrifugal force is a fictitious force. The only force being applied to the ball, pulling it toward the center of rotation, is a centripetal or center-seeking force. There is nothing actually pulling the ball away from the string, what you have is just inertia as described by Newton in his First Law of Motion: An object at rest remains at rest unless acted upon by a force and an object in motion remains in motion—at a constant velocity—unless acted upon by a force. 
   Newton based his first law on the work of Galileo, who described what he called the Law of Inertia: “A body at rest remains at rest and a body in motion continues to move at constant velocity along a straight line unless acted upon by an external force.” Until Galileo, it was thought that one must exert a force in order to keep an object in motion. Galileo recognized that the reason moving bodies eventually come to rest is because of resistance forces such as friction. Without friction, bodies would continue to move at constant velocity. But I digress…
   So if you were to cut the rope as the ball is rotating what would happen? Some might think that the ball would fly away from the pole, but that’s not correct. The ball would actually move perpendicular to the pole, due to inertia. The centripetal force of the rope works against inertia by keeping the ball from travelling in a straight path. It is this constant struggle against inertia that makes it seem that the ball is trying to move away from the pole. What we call a centrifugal force is actually just the effect of inertia working against the centripetal force. Your welcome.

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