Physics: A rolling penny

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• Oct 15th 2008, 01:50 PM
davesface
Physics: A rolling penny
If you start a penny rolling on a table with care, you can make it roll in a circle. The coin leans inward with its axis tilted. The radius of the coin is r, the radius of the circular path it follows is R, and velocity is v. Assuming that the coin does not slip, find the angle theta that the axis makes with the horizontal

The answer is given as
tan(theta)=(3v^2)/(2gR)

I always have problems figuring out how to start the problems, but the answer usually becomes clear after I get past the first step. The only equation that I can think of that would help is I=.5mb2, but since mass isn't mentioned in the problem it seems useless.

So far what I have done is draw a free body diagram of the penny with the force of the penny's weight acting straight down, the normal force acting straight up, and frictional force directing it toward the center of the circular path it follows.

Where would be a good place to start?