You find a coin resting on your physics book cover and notice that when you open the cover, the coin slips exactly when the cover reaches 25 degrees above the horizontal. What is the coefficient of static friction between the coin and book cover?
A) 2.1
B) 0.91
C) 0.37
D) 0.47
E) The question cannot be answered without knowing the coin's mass.

Question

Quizplus · Accepted Answer

The angle at which the coin slips is the angle at which the gravitational component parallel to the book's surface (mg sin θ) exceeds the static friction force (μs mg cos θ). Therefore, we have:
mg sin θ = μs mg cos θ
Therefore, the coefficient of static friction is:
μs = tan θ = tan 25° = 0.47

You Find a Coin Resting on Your Physics Book Cover