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If there is a disc rotating about its centre, let the surface be frictionless and if a coin is placed anywhere on the disc (not at the center) why doesn't it fly off even though there is a centrifugal force acting on it with respect to the disc's frame?

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marked as duplicate by jinawee, Kyle Kanos, Brandon Enright, Chris White, John Rennie Jan 10 '14 at 10:24

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If it is friction-less then there will be no tangential forces acting upon the coin. Do a sum of forces on the coin to figure out which way it is going to move. –  ja72 Jan 9 '14 at 17:02

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There is no such thing as "centrifugal force." It's a "pseudoforce" which simplifies some problems but clouds the reality of the situation. In your setup, the coin starts out with zero momentum. Since the disc is frictionless, there is nothing (no force) acting on the coin -- where of course I'm ignoring the vertical force due to gravity and the equal-and-opposing vertical force applied by the disk to keep the coin at a fixed altitude.

Now, it might be interesting if you started out with a spinning disk that was not frictionless and had a coin sitting on its surface, then at some point scientificomagically turned off the friction. Calculate the coin's momentum at that instant and see what happens.

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