College Physics (4th Edition)

$\theta = 16.7^{\circ}$
Let $\theta$ be the angle between the ramp and the ground when the box starts to slide. At that angle, the force of friction exerted on the box is equal in magnitude to the component of the box's weight directed down the ramp. $mg~sin~\theta = F_N~\mu_s$ $mg~sin~\theta = mg~cos~\theta~\mu_s$ $tan~\theta = \mu_s$ $\theta = tan^{-1}~\mu_s$ $\theta = tan^{-1}~0.30$ $\theta = 16.7^{\circ}$