## College Physics (4th Edition)

(b) $\mu_s \geq 0.5$
The correct answer is: (b) $\mu_s \geq 0.5$ Since the heater doesn't budge when Tim pushes with a force of 400 N, the force of static friction must be 400 N. However, a force of 400 N is not necessarily the maximum possible force of static friction. Therefore: $F_N~\mu_s \geq 400~N$ $(800~N)~\mu_s \geq 400~N$ $\mu_s \geq 0.5$