The system shown remains at rest. The force of friction on the block on the slope is:
A) 4 N
B) 8 N
C) 12 N
D) 16 N
E) 20 N

Question

The system shown remains at rest. The force of friction on the block on the slope is: 
A) 4 N 
B) 8 N 
C) 12 N 
D) 16 N 
E) 20 N

Quizplus · Accepted Answer

Since the system is at rest, the net force on the block must be zero. The only forces acting on the block are its weight (mg) and the frictional force (Ff). Therefore, Ff = mg sin(30°) = (5 kg)(9.8 m/s²)(sin 30°) = 24.5 N. The frictional force of 24.5 N opposes the tendency of the block to slide down the slope, and the force of gravity is resolved into two components: one component is parallel to the slope and is balanced by the frictional force, while the other component is perpendicular to the slope and is balanced by the normal force of the slope on the block. The only answer choice that is close to Ff = 24.5 N is B = 8 N.

The System Shown Remains at Rest