Hi Guys, I'm looking through one of the practice GRE tests, and I think that they have answered their own question incorrectly. I thought I'd get another opinion. Here it is: 36. A lump of clay whose rest mass is 4 kilograms is traveling at three-fifths the speed of light when it collides head-on with an identical lump going the opposite direction at the same speed. If the two lumps stick together and no energy is radiated away, what is the mass of the composite lump? (A) 4 kg (B) 6.4 kg (C) 8 kg (D) 10 kg (E) 13.3 kg I think it should be (C) since the lumps are presumably at rest after the collision, but the practice test says that the correct answer is (D). Doing the math for relativistic mass, a 4 kg lump would indeed have a mass of 5 kg when traveling 3/5c, but they're not moving after the collision. Am I missing something? [Sorry, I'm new here. I just read the sticky, and see that this might not be the appropriate forum for my question. Rather than double post it, I'll wait to see if it gets moved -- apologies!] Thanks!