I was recently having an IM conversation with my friend about the principle of relativity - I'd been reading up on special and general relativity all day. Special relativity is based upon the postulate that all uniform motion is relative, and general relativity extends this to non-uniform motion, and non-inertial reference frames. So how does this tie in with the role of energy in classical mechanics? Me: Okay... You know how when you accelerate one way, you can say the whole universe accelerates the other way? Relative motion? So how does this work for energy? Doesn't it take energy to accelerate something? Force = mass x acceleration, and work is done when a force moves something by a distance, and energy is expended when work is done so wouldn't there be a different amount of energy expended to accelerate a small mass (you) as opposed to the mass of the whole universe except you? Friend: That's a little like the twin paradox.. The resolution of which is the fact that it's you that's being forced to move, not the universe. Me: So what's all this about all reference frames being equivalent? The Principle of Relativity... all reference frames are equally valid ways of looking at a situation. To say you accelerate one way is equivalent to saying the universe accelerates the other way. Wouldn't saying you're being forced to move and not the Universe invalidate this? My friend didn't know the answer to this, and we changed subject. I'm still 17 and though I'm interested in physics my knowledge of relativity still comes from popular science books, so I haven't had any formal teaching in it yet. I'm hoping there's an obvious flaw in my logic. Can someone help me and point it out? Thanks.