If all motion is relative, then why is the general theory of relativity so much more dense with mathematics? I'm not asking you to explain the math--there is no way I would ever grasp that. What I'm confused on is why non-uniform motion (accelerated or decelerated motion) is so different from uniform motion. There seems to be some serious physical (mechanical) implications at work here that I'm not understanding. Here's another way of looking at my question: What does it really mean, physically speaking, to be in uniform motion? Let's also refer to Einstein's book here. He uses a train and an embankment and a passenger to illustrate this. Conductor hits the brakes and the passenger "jerks" forward. Quote: "The retarded motion is manifested in the mechanical behavior of bodies relative to the person in the carriage." What is he really saying here? Another quote: "...it would appear to be impossible that the same mechanical laws hold relatively to the non-uniformly moving carriage, as hold with reference to the carriage when at rest or in uniform motion." That last quote is what is really confusing me. Obviously, he goes on to prove that the "it would appear" part is wrong, but WHY is it that inuitively we would think that the laws wouldn't apply to this different form of motion? That's the part I'm missing. One of other way of explaining my struggle with this: what is so special about acceleration? What am I missing? Mentz, I like your example. I'm hoping Mr. Donis will show up and clarify it just a little more, or give another example like that one.