# Tricky Collision Problem

1. Dec 13, 2009

### jj2443

1. The problem statement, all variables and given/known data
A small hard block of mass 3m is suspended from a thread of length L. A second block of mass m is located on an incline, originally at rest, a height y above the level of the large mass. When the smaller block is released it slides, without friction, down the ramp, and then collides elastically with the larger block. The large block swings around so that the tension in the string just barely drops to zero at the top of the loop. The small block slides back up the ramp, rising to a maximum vertical height h.

2. Relevant equations
conservation of momentum: pi=pf
p=mv

3. The attempt at a solution

2. Dec 13, 2009

### MaxL

Drawing a picture is always a good way to start.

3. Dec 14, 2009

### jj2443

I've done a picture, and have a basic understanding that I should use conservation of energy to find velocity of the block at bottom of incline. Then I need conservation of momentum in an elastic collision to find velocity of ball on string...(?) How does this help me find the new h and the L?

4. Dec 15, 2009

### MaxL

So you know the KE of both objects before the collision, and you want to know the KE of both objects after the collision. There's two unknowns, so you'll need two equations. You got one, which comes from the conservation of momentum. What's the other equation? (Hint: It comes from the fact that the collision is elastic)