• You need a = v^2/r (or a = r omega^2 where omega is in radians per second) to work out the acceleration of the coin. F = ma gives you the static friction force acting on the coin. F = mg gives you the downward force of gravity. If the ratio of the sideways force divided by the downward force is greater than u(s), then the coin will start sliding out. But as it slides out, the acceleration needed to keep it circling goes down. When the new ratio is u(k) then the motion will stop. So solve for the new r by running the same method in reverse.

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