In section 8-2 of Sears and Brehme's "Introduction to the Theory of Relativity", the authors derive the formula for the "relativistic mass" of a particle in motion by analyzing a completely inelastic collision between identical objects from more than one frame of reference. I'm having a little trouble following it because their analysis relies heavily on a Brehme space-time diagram, which is a kind I'm not used to. My question however is, is this even possible to do? In an inelastic collision the rest masses of the objects change, and nowhere in their analysis do I see this mentioned or factored into the equations. Also, when I try to analyze the collision myself - without using their diagram but with the same overall approach - I get a different (wrong) answer. I don't know if I'm making a mistake or if authors are somehow cheating. I won't go into any of the mathematical details here in this first entry just in case someone out there already knows the answer to this question based on the information I've given, or already has the text. Thanks.