Answer
(c) The two coins start to slide at the same time
Work Step by Step
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.
