A student strikes a block at the bottom of a ramp giving it an initial speed v0

When we move the block on the ramp, the friction force is the force which opposes its motion.

Explanation:

When we apply force in the forward direction, the block moves horizontally on the floor or ramp. The force which oppose the motion of the block is known as force of friction. When the block is at rest, static friction act upon it which prevent the block from movement but when the block start motion, this static friction replace by the kinetic friction which opposes the moving body and reduce the speed.