Friction and Inclines

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?

Ftwophysicists is greater than Ffriction.

Which of the following is the most general explanation for why the box accelerates down the board after it begins to slide (rather than sliding with constant speed)?

The coefficient of kinetic friction is less than the coefficient of static friction.

In general, how does the coefficient of static friction compare to the coefficient of kinetic friction for the same two materials?

The coefficient of static friction is greater than the coefficient of kinetic friction.

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?

The frictional force points down the incline

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

The frictional force points up the incline.

A packing crate is sitting at rest on an inclined loading ramp. How does the magnitude of the force of static friction compare to the other forces acting on the crate?

The magnitude of the force of static friction is equal to the magnitude of the component of the weight of the crate parallel to the inclined ramp.

When two objects slide against one another, which of the following statements about the force of friction between them is true?

The magnitude of the frictional force is always equal to μkn.

When two objects are in contact with no relative motion, which of the following statements about the frictional force between them is true?

The magnitude of the frictional force may be either equal to or less than μsn.