Setup: A block is pushed up an rough incline plane by a force parallel to the incline plane.

Observations:

• The force of kinetic friction depends upon the force pressing the block into the incline plane, i.e. the component of the force of gravity normal to the plane, FN = m g cos(ø).
• Kinetic friction opposes the direction of motion of the blocks motion.
• If the block came to rest and stayed at rest, then the direction of static friction would depend upon which is bigger, the applied force F_app or the component of gravity down the incline plane m g sin(ø). When F_app > m g sin(ø), then the force of static friction is down the incline plane, opposing the motion that would ensue if their were no friction. Static friction could (and would) be up the incline plane if F_app < m g sin(ø) because now the block would slide down the incline plane if there were no friction.