I have a question with regards to the increase in mass of an an accelerated object. I actually posted this question yesterday but couldn't find it, so maybe it didn't get through... Anyway, consider 2 large masses very far away from each other. This system has a certain gravitational potential energy. They will accelerate towards each other due to gravity. Say they reach about 60% light speed by the time they hit each other. According to relativity, their masses will be increased (due to their high velocity), along with the attractive force between them. They will collide and convert their energy into heat. If the masses are now pushed away from each other, to their original positions, at a slower velocity (say 1000 m/s), won't this take less energy than they had when they crashed together since they will now be back to their normal masses, as their masses won't be increased due to their slow speed. The attractive force between them will thus also be less. This example makes it seem that energy has been created from nothing due to the mass increase of the objects. Am I missing out something obvious?