An object having mass of 90 kg and traveling at a speed of 0.850c collides with a stationary object having mass 1400 kg. The two objects stick together. (8pts) a) Find the speed of the composite object. b) Find the mass of the composite object Hello, I would appreciate any insight into the above problem, I have the solution, yet I do not understand their approach which ultimately finds the relativistic momentum and divides the relativistic Energy equations. Prior, to looking at the solution, I know this to be an inelastic collision, so I only applied the momentum equation which was incorrect. What is troubling me, is why this approach and also why I cannot consider the "final" Mass to be the 900 + 1400 if I am only considering delta P. Thank you. EDIT: So I've read in another textbook, why M is simply not the sum, yay, and then follows why I should consider both equations. So I guess, my question now, simply is: since M is not the sum I should consider both. Is this correct?