1. The problem statement, all variables and given/known data A billiard ball ( mass = 10kg, initial velocity is 5 m/s) is launched along x-axis at a stationary billiard ball ( mass = 5kg). After collision, the first ball goes off at 30 degree angle above x-axis and 2nd ball goes off at 45 degree angle below x-axis. Calculate the final velocities of both billiard balls. 2. Relevant equations M1V1 + M2V2 = M1V1final + M2V2final (conservation of momentum) 1/2M1V1^2 + 1/2M2V2^2 = 1/2M1V1final + 1/2M2V2^2 3. The attempt at a solution Typically in most billiard ball problems you are not given both angle measurements, which means you need 3 equations: momentum in x direction, momentum in y direction and Kinetic energy. But, in this case I only have 2 unknowns, not 3. So my question is whether I need to even use the third kinetic energy equation? I don't understand how I can solve for 2 unknowns by using 3 equations. My gut tells me just to use the 2 momentum equations, which is what I have been doing, but whenever I read about elastic collision problems I see that KE equation is also used. Some guidance on this topic would be really appreciated. Thanks.