Instead of visiting today’s Physics lecture, I went to ALDI. (Which is a supermarket.) While waiting in the queue, I had the opportunity to watch two cashiers practicing.
As most supermarkets ALDI provides triangular bars to be used as stop signs at the checkout’s conveyor belt. (Almost like “Toblerone” except you can’t eat ’em.) This bars are supported at a runner parallel to the conveyor. The problem is that people always take them from the back end of the runner while the cashier puts them on at the opposite end. If there are only a few bars left, the cashier might wish to push them back in order to make things easier for his customers. It was quite exciting for me to watch two ways of doing this.
Cashier A knocked his bar firmly against the ones already lying at the runner. The bars are made from aluminum sheets. Neglecting the (rather thin) terminal plastic capes, they show almost ideal elastic behavior. Hence the bar hitting the first obstacle of equal mass preforms a complete transfer of its momentum. (As illustrated in this animation.) This is repeated with every bar. In the end only one (namely the last) bar is pushed to the end of the conveyor while all others stay in place.
Next time he knocked the bar harder — making things even worse. Now the last bar collided with the stopper at the end of the runner, was reflected and the reaction occurred a second time the other way round, leading to an unchanged arrangement of bars except he got back the one he just propelled forward.
Cashier B, however, was more concerned about the physics behind his working place. He took the bar and accelerated the other bars gently by pushing it against them while still holding it in his hand. When he could no longer stretch his arm, he let the bar go and was satisfied by the intended movement of all the bars to the runner’s end.
So you see, it’s quite crucial to pay attention to your physics lectures. What was the first sentence again?