Motion exists along the surface of the plane only (call this direction horizontal). Analyse the weight force in two components: the vertical and horizontal forces, and respectively. Along the horizontal direction, two forces are applied: and the friction . Call the velocity at time . By Newton's law, , and solve to get (assume the body was resting at ).

If is the height and the length traveled at time , then a simple argument shows . Now, the total energy is . Differentiate to get ...

(Sorry for being so analytic. I was so bored today )