A brick is resting on a rough incline as shown in the figure. The friction force acting on the brick, along the incline, is
A) zero.
B) equal to the weight of the brick.
C) greater than the weight of the brick.
D) less than the weight of the brick.

Question

A brick is resting on a rough incline as shown in the figure. The friction force acting on the brick, along the incline, is 
A) zero. 
B) equal to the weight of the brick. 
C) greater than the weight of the brick. 
D) less than the weight of the brick.

Quizplus · Accepted Answer

The friction force acting on the brick is opposing the motion of the brick along the incline. Since the brick is at rest, the net force acting on it is zero. This means that the friction force must be equal and opposite to the component of the weight of the brick that is parallel to the incline. The weight of the brick can be resolved into two components: one perpendicular to the incline (normal force) and the other parallel to the incline (mg*sinθ). Since the brick is not moving, the friction force must be equal to the parallel component of the weight, which is mg*sinθ. As sinθ is less than 1, the friction force is less than the weight of the brick. Therefore, the correct choice is D.

A Brick Is Resting on a Rough Incline as Shown