It is my understanding that the faster an object moves, the more energy is required to accelerate it. As an object approaches the speed of light, an infinite amount of energy is required to further accelerate it, which is why no object can travel faster than the speed of light. But movement is relative to a frame of reference, which is where I get confused. Say, for example, the only two objects in the universe are two spaceships, A and B. A stays stationary (relative to B), while B accelerates away from from A. As B's acceleration increases, it needs more and more energy to increase it's acceleration. Let's say that A then starts moving and catches up to B. Now they are both accelerating through space, but they are neck and neck with each other and have no other objects to reference in order to gauge their speed. In fact, if they stopped accelerating and feeling whatever g forces they were, they could both claim that they are stationary. They then start accelerating again. Would they require more energy than they previously did to accelerate further, or less because they were technically stationary before they resumed accelerating? I hope that clear. Obviously I'm a bit confounded by this. Thanks for your help.