incline plane~

You need to distinguish between the stage when the block starts up and the stage when the is going up the inclined plane.

The Law of Conservation of Mechanical Energy applies to the second stage, i.e. when the block is moving up the plane, under which the gain in potential energy is in expense of the loss of kinetic energy.

However, the Law doesn't apply when the block starts up. In this process, an external force needs to act on the block (whithout the pushing of any force, the block would never move) to give it an acceleration and hence an initial velocity. The gain in kinetic energy of the block is a result of the work done by such external pushing force.

In fact, I don't exactly understand what is your question. But i try to answer you.

either the inclined plane is smooth or rough(i.e.with friction)
KE decreases by conservation of energy.

mind you, there is an assumption of using the law of conservation of energy in this case, which is "there is no external force acting on the block".



If the block starts from rest,

the block will slide down,
PE decreases, and KE increases. (by the law of conservation of energy)