Someone told me I'd be more likely to get help here. Say you have these two balls moving in opposite directions. The balls float in the air and thus by themselves have negligible friction, but each is carrying a (detachable) bar across the ground, which has friction. On the very instant the balls collide with each other, they let go of their respective bar and get attached to the other ball's bar (see pic). Would this collision be frictionless, and thus elastic, or not? I really could see it going either way.