Simple question: Rest Frame: In their respective rest frames, the following objects have the following rest masses: Object A Mass = 1 kg ; Object B Mass = 1 kg. Total Rest Mass of system: 2 kg. A & B accelerate and reach some constant Velocity. At this velocity, their relative masses are determined to be 1.5 kg each. They head straight at each other and collide. Their collision is perfectly inelestic. Let's call the resulting object, Object C. It is my understanding that the rest mass of this object is 3 kg. Total rest mass of system = 3 kg. So rest mass of A & B seperate = 2 kg. Rest mass of A+B = 3 kg. Is this correct? A simple Duh, Yes of Course will suffice.