Here's a question from my workbook that I can't seem to figure out. A proton (mass = 1.6 x 10^27 kg) moves with a speed of 6 Mm/s. Upon colliding elastically with a stationary particle of unknown mass, the proton rebounds on its own path with a speed of 3.6 Mm/s. Find the mass of the unknown particle. Let's start with what I know. I know that momentum = mass times velocity. or p = mv I also know that since it's elastic, it isn't losing any energy. So I used the equation P1 + P2 = P1 + P2 (1.67 x 10^-27 kg)(6 Mm/s) + 0 = (1.67 x 10^-27 kg)(-3.6 Mm/s) + P2 I said 3.6 Mm/s was negative since it was in the opposite direction. So from there I got: 3.006 x 10^-20 kg(Mm/s) = 1.08216 x 10^-26 kg(Mm/s) + P2 P2 = 1.923 x 10^-26 But, from there I'm not sure where to go. I'm trying to find the mass of this particle, but I need to know it's velocity. Any help or hints is appreciated.