I was thinking about a rod colliding with an external wall, if you will, and began thinking what would happen if the collision was elastic (not sure if that is correct in this context, but simply the energy of the rod is the same before and after the collision.) I assume if a particle was normal to a wall that the change in its momentum after the collision would be the same in magnitude, but opposite in direction - energy would be conserved. With a rod however (see my diagram), the collision brings about an external torque (from wall) and thus a change in angular momentum. This would mean that the center of mass would then have to rebound at a lower velocity to conserve energy (there is now rotational energy), correct? But maybe that is getting ahead, would the center of mass rebound in all cases that energy is conserved, or could there be cases where the center of mass still continues in the same direction with less magnitude (while angular momentum is changing.) There may be many variables here. Nonetheless, I am trying to formulate/relate the rod's momentum/angular-momentum before and after the collision. I may simply be missing to many variables, but if anyone could shed some light on what would happen in a collision like this, I would appreciate it.