Maybe a silly question, but I nevertheless interested in the answer. A steel rod oriented north south sits on a table. Consider two molecules, N and S, at the the north and sound end of rod respectively. A force in the northerly direction is applied on point S of the rod and the rod moves 1 inch north. My question is basically, does point S move before point N or does point N move before point S, or do they actually move simultaneously to the limit of our observational powers? What I'm wondering is, at the molecular level, if you imagine the steel molecules to be incompressible spheres (like glass marbles in column) in a lattice, each sphere tangent to its neighbor, then in order for a sphere at the at the south end to move in the northerly direction, its neighbor to the north must move before it can move into the space the neighbor formerly occupied. So that suggests the idea that the force is transmitted down the rod to the end of rod at the north end, and the first movement occurs at point N moves an infinitessmal distance northward (pushing air out of the way) which then makes room for the sphere at Point a's neighbor to the south to move into the space it formerly occupied and so on. So on the molecular level, while the force is applied at point S and travels North, the movement starts at the north end and "travels" to the south end. Alternatively I guess, the idea that molecules are better pictures as "compressible elastic" sphere, which deform but exert pressure when deformed (like a ballon), so the steel rod moves in the same way a column of balloons in a tube might move. The mechanism I can't quite understand is the one in which the two points N and S move at precisely the same moment, and how to understand how that could happen? How does the the molecule at point N "know" to move when a pressure is a applied at point "S"?