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Name:  Sheri D.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Can you give us a simple explanation, definition and examples of centripetal force for us that our elementary students will be able to understand? We are totally confused with the information that we have read already.


Replies:
Centripetal force is the name given to the force which makes an object move in a circular path; a force towards the center. It should not be confused with centrifugal force, a fictitious force from the center.

Centrifugal force is a fictitious force because it does not exist, though sometimes you feel sure it does exist. It exists, or rather seems to exist, only in a non-inertial coordinate system (one which is accelerating or rotating) where Newton's laws are not valid.

A good example is when you are a passenger in a car going around a sharp curve to the left (for example) at high speed. You feel a strong force pushing you to the right. A person in a tall building looking down on you (who is in an inertial frame) would see that you are being accelerated to the left by the car pushing on you, but you in the car just feel a strong force to the right which causes you to push to the right on the car. By Newton's third law, the car then, of course, pushes back on you to the left.

A clearer example might be when a grooup of skaters "crack the whip". A line of skaters join hands, the one at the left stays at the same spot and the others, trying to keep in a straight line, rotate around him. The one on the right, going the fastest, needs the strongest force to keep him going in a circle. This force is provided by the person next to him who finds it harder and harder to keep his grip on the last person as they go faster and faster. Finally, when he lets go, the last person continues at constant speed in the direction he was going when he lost his grip on his friend.

Notice that he does NOT go away from the center of the circle when let go.

An easy demo is to tie a ball, perhaps a tennis ball, to the end of a fairly long string. If you then swing this in a circular horizontal path while holding the string over your head, you will feel the centripetal force you are exerting on the ball to make it go in a circle. Notice that when you let go of the string, the ball continues going in the direction it was going when you let go.

Be careful! Perhaps you should do this outdoors. It might be clearer if you use a rope and a soccer ball.

I hope this is a help. Dick Plano...



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