1. The problem statement, all variables and given/known data Generic collision question. For example, lets say you are asked the following question: Object 1 is 10kg and is traveling at 10 m/s Object 2 is 5kg and is traveling at 2 m/s Find their final collisions after impact 2. Relevant equations m1v1 + m2v2 = m1vf1 + m2vf2 Ek1i + Ek2i = Ek1f + Ek2f Ek = (1/2)mv^2 3. The attempt at a solution Now, instead of doing massive algebra and combining the two equations, can I include an object 3 into the scenario, and say it is going at 2 m/s ? According to this new reference, object 1 will be going at 8 m/s and object 2 will be going at 0m/s and thereby be stationary. Now, when I plugin values to the two equations, it gets a WHOLE lot easier because I can instantly cancel some values out and derive two new equations. So, then I can say according to object 3, object 1 goes X m/s after collision, and object 2 goes Y m/s after collision. Now, if i SUBTRACT the 2 m/s from object 1 and object 2, relative to the first reference I will get the final answers X+2 and Y+2 --> m/s So, is this correct? Can I do this? Newtons laws state I should be able to, but my teacher said this is impossible and it doesn't work like that because I can't just add and subtract velocities. I said that all I'm doing is adding a reference point and removing it. I firmly think I can do this. If I cant do this, can someone explain why?