The Basketball and the Tennis Ball

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The Basketball and the Tennis Ball

If we were to take a tennis ball and a basketball and drop them to the floor, they would both bounce upwards reaching a height slightly less than from where they had been dropped (figure to the right of the title). We shall call this our "control".

Now we wonder what would happen were we to align the two so that the tennis ball is directly under (and touching) the basketball and then drop the pair. If both continue to fall in perfect vertical alignment, you will see that the tennis ball does not bounce but remains on the floor as the basketball bounces upwards to a point just slightly higher than it did in the control drop.

Ah, but what if we reverse the order and put the basketball directly beneath the tennis ball? Upon being dropped the startling thing happens: the tennis ball bounces off the top of the basketball so high that it may hit the ceiling! Meanwhile the basketball did not bounce nearly as high as it has done previously.

Why is all this? In the case where the basketball was on top, the small momentum of the tennis ball was transferred entirely to the basketball, which added along with its own large bouncing momentum allowed it to bounce just a little higher than it did when dropped alone. However, when the tennis ball was on top, the large momentum of the bouncing basketball was transferred to the small tennis ball and it became so energized that it could bounce very high indeed! So remember to step aside when you do this or else you might get hit in the nose!

This should be not at all surprising because we use this same principle in a number of our sporting games as we hit baseballs with a heavy bat, ping-pong balls with a relatively heavy paddle, and tennis balls of course with a tennis racket. In all cases, the exiting ball is travelling far faster than was the bat, paddle or racket. (You doubt this? Next time you see a baseball batter break a bat when hitting the ball, notice that the ball goes very far while the end of the bat only lands in the in-field. Or, see how far you can bat the baseball, and then pretend you are about to bat a ball but let go of the bat and let it fly out of your hands. It won't go nearly as far as the batted ball.) This are all experiences with the transfer of momentum.

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