1. The problem statement, all variables and given/known data A particle (of mass m velocity v) makes a perfect elastic collision with a stationary particle. After the collision both particles travel 30 degrees from original path. Use conservation of momentum/energy to obtain 3 equations relating the masses/velocities. 2. Relevant equations 3. The attempt at a solution I can get two equations, one from using initial momentum = final momentum, and the other using initial kinetic energy = final kinetic energy, but I am unsure of where the third equation comes from. Any help would be appreciated.