A very basic (or maybe not?) question about relativity This is something I have been doubting about for a rather long time, but now I want to try and ask other people to explain it to me. I know it might sound rather weird, but please try and think about my problem, and about where my thinking is going wrong. We all know, movement is relative. I.e., from my viewpoint, the train is moving, but from the train's viewpoint, I move. It seems to me that this does not completely apply for acceleration. After all, I don't feel any forces when the train next to me accelerates. I know this sounds rather confused, but why is this the case?