This isn't an actual homework problem, but it feels like it could be. 1. The problem statement, all variables and given/known data Two masses, m1 and m2 are involved in an elastic collision. The initial velocities v1_0 and v2_0 are 1 and -2, respectively. The final velocities v1_1 and v2_1 are -3 and 0, respectively. Solve for m1 and m2. 2. Relevant equations Given the collision is elastic, here are the two relevant equations: 1. m1*v1_0 + m2*v2_0 = m1*v1_1 + m2*v2_1 2. 1/2 (m1*v1_0^2 + m2*v2_0^2) = 1/2 (m1*v1_1^2 + m2*v2_1^2) 3. The attempt at a solution I'm not sure what I'm doing wrong, but when I try to solve these 2 equations with 2 unknowns, I end up getting m1 = m2 = 0. For example, using equation 1 & 2 from above: 1. m1 - 2 * m2 = -3 * m1 → 4 * m1 = 2 * m2 → 2 * m1 = m2 2. m1 + 4 * m2 = 9 * m1 → 4 * m2 = 8 * m1 → 2 * m1 = m2 At this point, you only really have one equation so you can't solve for two unknowns, but if you do substitute 2 * m1 for m2 in the other equation you get 2 * m1 = 2 * m1 --> 0 = 0. Any help would be appreciated, thanks.