This is not dircectly a coursework question, rather I need someone to tell me what is correct and what is not correct in my assumption of Momentum and collisions, esp conservation of linear momentum. For some reason I stand to belive there are two diffirent equations for conservation of linear momentum Pbefore=Pafter. In one case we have m1u1+m2u2 = m1v1m1v2 In the other we have 1/2m1u1^2 + 1/2m2u2^2 = 1/2m1v1^2 + 1/2m2v2^2 For some reson I stand to belive that they are for Perfectly inelastic collisions and Elastic collisions respectivly. Is this the case? Is there some "key word" in a work question that I can look for to say "Ah! I'm going to have use equation x"?