1. The problem statement, all variables and given/known data An object with mass of 900 kg Is moving at a speed of 0.85c when it collides with an object at rest with mass of 1400 kgs. Find the speed and mass of the composite object. 2. Relevant equations mc^2/gamma +mc^2/ gamma=Mc^2 3. The attempt at a solution My issue is that I am not sure which equation I really want to work with. I think the one I listed is for an inelastic collision where the combined mass doesn't move after the collision, and so it would be inappropriate here. I think I need to use momentum and energy equations, but like I said, what equation describes the relativistic mass-energy system for an inelastic collision where the objects keep moving?