Mechanical energy is conserved

...even kids know

Sun Apr 15 2018

Well.. I should say “almost conserved”, to be precise.

Anyways, one day I was in a sport store with a friend. We were playing around with all kinds of balls, as it usually happens when I go in a sport store. While thinking about the next game to play, I tried to replicate one of the most common experiments related to the conservation of mechanical energy.

So I put a tennis ball on top of a kids-size football and let them bounce on the ground. Clearly, the tennis ball bounced higher than the dropping point. During the process, both balls damped out some energy because of their viscoelastic behaviour. However, the remaining elastic energy related to the strain in the football transferred to the tennis ball, providing this energy bump that let the latter bounce very high.


My friend and me kept doing this over and over, just because it’s fun to how some fundamental laws work with real life examples. We also added a basketball to the game, which made the tennis ball go even higher. After a while a kid saw us… it was clear he wanted to play with us, and we clearly let him join us.

This is a short video that shows us playing. The kid counts “Uno, Due, Tre” (in Italian) to let me drop the balls. Just look at his eyes…


Alberto Scorrano

Roberto Gentile Researcher in seismic resilience engineering at University College of London

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