## College Physics (4th Edition)

The correct answer is: (c) The two coins start to slide at the same time Let $\mu_s$ be the coefficient of static friction. Let $m$ be the mass of an object on the book cover. For any angle $\theta$, the maximum possible force of static friction is $mg~cos(\theta)~\mu_s$. The force directed down the slope is $mg~sin(\theta)$. We can find the angle when the object starts to slide: $mg~sin(\theta) = mg~cos(\theta)~\mu_s$ $tan(\theta) = \mu_s$ $\theta = arctan(\mu_s)$ The angle $\theta$ does not depend on the mass. Therefore, the two coins would start to slide at the same time.