posted by Jon Routh .
Two blocks are initially held at rest on 30° frictionless ramps . The 4 kg block is 1.25 m from the base of the ramp and the 3 kg block is 5 m above the base of the ramp. The two blocks are then released and slide down the ramps and onto a frictionless horizontal surface. The two blocks collide elastically with each other and then move backward up the ramps again. The 4 kg block comes to a stop 3.09 m up its ramp. How far up the ramp does the 3 kg move before coming to a stop?
Since every process is elastic or frictionless, the total potential energy must be the same when the colliding and sliding are over, even though the heights may change for each block. Use that fact to compute the final height of the 3kg block, above the horizontal base. That and the ramp angle will tell you how far it slid.