What is the coefficient of static friction between a coin and a tilted book?

[idkgirl]

Homework Statement

A coin of mass m is at rest on a book that has been tilted at an angle θ with the horizontal. By experimenting, you find that when θ is increased to 13o, the coin is on the verge of sliding down the book, which means that even a slight increase beyond 13o produces sliding. What is the coefficient of static friction μs between the coin and the book?

Homework Equations

F=ma, f(static friction) = (μ)*normal force

The Attempt at a Solution

I drew out the problem, but from here I am sort of lost.

 

[Nugatory]

The weight of the coin is a force downwards, right? Rewrite that force as the sum of a force perpendicular (normal) to the surface of the book and parallel to the surface of the book. The parallel force is trying to make the coin slide; it's opposed by the force of friction, which is related to the normal force.