Consider a rod in the absence of gravity. If we were to give the left end of the rod a perpendicular nudge in the upwards direction, it spins about its centre of mass.
If we were to analyse what is happening on a molecular scale, we find that the molecules in the vicinity of the nudge had moved upwards, pulling the molecules down the rod in the same direction. This continues down the rod, resulting in the rod spinning in clockwise.
What I do not understand is why the upward motion of the molecules ceases when we reach the centre of gravity, and the molecules on the right side of the centre of gravity in turn move downwards. In order for the molecules to move downwards, mustn't there be a downward force exerted on the right side of the rod?
In this situation, shouldn't the effect of the force be transferred down the rod, resulting in the rod spinning about its right end? Why does the effect stop at the center of mass of the rod, and then reverse directions? I have tried looking for an answer but has yet to find one.