Initial conditions have been set up so that a coin of radius r rolls around in a

Question:

Initial conditions have been set up so that a coin of radius r rolls around in a circle, as shown in Fig. The contact point on the ground traces out a circle of radius R, and the coin makes a constant angle ? with the horizontal. The coin rolls without slipping. (Assume that the friction with the ground is as large as needed.) What is the frequency of the circular motion of the contact point on the ground? Show that such motion exists only if R > (5/6)r cos ?.