Years ago I read about this but can't find any reference to it now. The underlying reason, as I remember it, is to do with the electro-magnetic force, which I think is responsible for the majority of physical interactions. But anyway: if your hand pushes on a bicycle wheel for e.g., the electrons in your hand repel the electrons on the rim of the wheel, thereby "impelling" those atoms forwards. The remainder of the atoms follow in a chain reaction. In other words, one part of the wheel has to move first before the remainder of the wheel. If all of the wheel moved at exactly the same time, it wouldn't actually get anywhere; it would just stay on the spot, spinning, and creating a rut in the ground. I'm not sure this is correct; it's just what I remembered reading years ago. Whatever the underlying process actually is, it would describe movement generally. I'm not really asking for any maths on this, such as formulas to work out force or anything. I'm interesting solely in the underlying reasons for how things can move forward in space. Is anyone familiar with this? And is there a proper name for this theory/paradox? Any links to a site that explains this better would be much appreciated. Thanks.