Question

A block of mass 2 kg is released from the top of a rough incline of a height of 2 m. If the velocity of the block on reaching the bottom of the incline is 4 m/s, what is the magnitude of work done by the frictional force?

Answer 1

Just apply work-energy theorem on the block. Total work done by all the forces on the block should be equal to change in its kinetic energy. There are three forces acting on the block:

Gravity, mg (applied by earth)
Normal reaction (applied by the surface of the inclined plane)
Frictional force (applied by the surface of the inclined plane)
Work done by friction comes out to be negative here which is as expected, as the friction always acted here opposite to the instantaneous velocity of the block. Friction is a non-conservative force and it causes loss of mechanical energy (from the earth-block system) in the form of heat, sound, light etc
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