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 remains at rest, the forces must be balanced. The forces acting on the block on the slope are its weight (mg) acting vertically downwards, the normal force (N) acting perpendicularly upwards, and the force of friction (Ff) acting parallel to the slope and opposite to the direction of motion. Since the block is not moving, the force of friction must be equal in magnitude to the component of the weight of the block acting parallel to the slope. This component is given by mg*sin(30°) = (1/2)mg. Therefore, the force of friction is (1/2)mg = (1/2)(2 kg)(9.8 m/s^2)(sin 30°) = 4.9 N. The closest answer choice is B, 8 N, which is the correct choice.

The System Shown Remains at Rest