1. The problem statement, all variables and given/known data We could change to inertial reference frame in a collision to follow a mass. But what would the down side be if we did? 2. Relevant equations Vf1=(M1-M2)/(M1+M2)*Vi1 +2M2/(M1+M2)*Vi2 Vf2=2M1/(M1+M2)*Vi1 +(M1-M2)/(M1+M2)*Vi2 3. The attempt at a solution This question appears too simple and my teacher doesn't want to even tell me I'm partially right. But it seems that if you followed one mass the final velocity would be higher it actually is, it is higher by the speed of the object that we follow. If you play around with the equation you see that this is true..... i think. So am I right or wrong. And, if I'm right, what am I missing?