Question

A 6.0 kg block initially at rest is pushed against a wall by a 100 N force as shown. The coefficient of kinetic friction is 0.30 while the coefficient of static friction is 0.50. What is true of the friction acting on the block after a time of 1 second? 

(A) Static friction acts upward on the block. 

(B) Kinetic friction acts upward on the block 

(C) No friction acts on the block 

(D) Kinetic friction acts downward on the block. 

(E) Static friction acts downward on the block.

Answer 1

Correct option (D) Kinetic friction acts downward on the block.

The normal force comes from the perpendicular component of the applied force which is Fcosθ = 50N. The maximum value of static friction is then µF_N = 25N. The upward component of the applied force is Fsinθ = 87N. ΣF_y = F_up – mg = 87N– 60N > 25N. Since the net force on the block is great than static friction can hold, the block will begin moving up the wall. Since it is in motion, kinetic friction is acting opposite the direction of the block’s motion