Question

A large crate filled with physics laboratory equipment must be moved up an incline onto a truck.

The crate is at rest on the incline. What can you say about the force of friction acting on the crate?

A. the frictional force points up the incline

B. the frictional force points down the incline

C. the frictional force is zero

A physicist attempts to push the crate up the incline. The physicist senses that if he applies slightly more force the crate will move up the incline but cannot muster enough strength to get the motion started. What can you say now about the force of friction acting on the crate?

A. the frictional force points up the incline

B. the frictional force points down the incline

C. the frictional force is zero

The first physicist gets a second physicist to help. They both push on the crate, parallel to the surface of the incline, and it moves at constant speed up the incline. How does the force exerted by the two physicists on the crate compare with the force of friction on the crate?

A. force of 2 physicists < force of friction

B. force of 2 physicists = force of friction

C. force of 2 physicists > force of friction

Answer 1

Concepts and reason

Use property of frictional force that it always acts opposite to the relative motion of object.

Fundamentals

Frictional force always acts opposite to the relative motion of object and surface.

If there would not be any frictional force then box will move downward due to gravity. So, frictional force must act in upward direction of the incline.

Since the frictional force must act in upward direction of the incline so, the frictional force cannot point down the incline and it cannot be zero.

If there were no frictional force than the body will move in upward direction because when force is applied in the upward direction along the incline, the force must balance the component of gravitational force acting downward along the incline and for the current situation applied force must be greater than the component of gravitational force so that applied force must overcome the gravitational force and frictional force. So, frictional force is acting downward along the incline.

The force applied by the two physicists must be greater than the sum of frictional force and the component of gravitational force acting along the direction of the incline. So, that resultant motion will be in upward direction. Hence, it is clear that the force applied by the two physicists must be greater than the frictional force.

Ans:

The frictional force points up the incline.