A block is at rest on a rough incline as shown. The frictional force acting on the block,along the incline,is
A)zero.
B)equal to the weight of the block.
C)greater than the weight of the block.
D)less than the weight of the block.

Question

A block is at rest on a rough incline as shown.  The frictional force acting on the block,along the incline,is
A)zero. 
B)equal to the weight of the block. 
C)greater than the weight of the block. 
D)less than the weight of the block.

Quizplus · Accepted Answer

Since the block is at rest, the net force on it must be zero. The weight of the block acts straight downwards, and it can be resolved into components parallel and perpendicular to the incline. The perpendicular component is balanced by the normal force of the incline, which acts perpendicular to the surface. The parallel component of weight acts downwards along the incline, and the frictional force acts upwards along the incline to balance it. Since the block is at rest, the frictional force must be equal and opposite to the parallel component of weight. Therefore, the frictional force is less than the weight of the block.

A Block Is at Rest on a Rough Incline as Shown