Re: pinhole Centripetal vs. centrifugal force

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Nov 20 2002 - 15:29:21 PST 

Message-Id: <l03110766ba01c89c0bf3@[192.174.3.125]>
Date: Wed, 20 Nov 2002 15:29:21 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Centripetal vs. centrifugal force

I Suggest sticking with the centripetal force.

With no net force an object travels in a straight line at constant velocity.

An object travelling in a circle therefore needs a net force directed
toward the center, the centripetal force, to deflect it from straight line
motion.

I put a weight like a tennnis ball in a knee high stocking.
Notice that the stretch of the stocking shows the force on the ball.
Put in more balls and the stocking stretches more. Swing the ball overhead
faster and faster and the stocking stretches more.

The centrifugal force occurs if you are in a rotating reference frame, for
example you are in a room which is moving in a circular path. To force you
to move in a circle, the wall of the room pushes on you. (This is a
centripetal force.) You notice that the wall is pushing on you and yet you
seem to be at rest in a room (you don't know the room is moving in a
circle.) So, since you think you are at rest, the net force on you must be
zero. You can feel the wall pushing on you so you make up another force
that must be acting on you, a force that cancels the force from the wall,
this is the centrifugal force.

Paul Doherty

>I am introducing centripetal force to my physics classes and I really want
>some demonstrations that will stress the "center-seeking" centripetal force.
> Some demonstrations that I have found so far seem to stress the
>"centrifugal force." What can I do to clarify these two ideas. Also, I put
>together the clear plastic tubing (used at the convection workshop) and spun
>around in a circle. Am I correct in stating that this demonstration also
>portrays the outward motion of the water? Any ideas would be appreciated.
>Thanks,
>Shannon McManus
>
>
>
>
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