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Q & A: why do things bounce?

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Most recent answer: 01/25/2013
Why do things bounce?
- matt (age 8)
mt. bethel, ga
Hi Matt,
Great question! When you throw a ball against a wall, or drop it onto the floor, you notice that it bounces back up again. If you were to look very carefully when it was just hitting the wall (or the floor) you would see that the ball was squished a bit. When you pick up a ball and squeeze it with your hand, you notice that it wants to pop back out and make itself round again. In fact, you feel the ball pushing back out against your hand when you squeeze it. In the same way, when the ball hits the floor and gets squished, it pushes back against the floor to try to make itself round again. This pushes the ball back up into the air, and is how a bounce happens.


(published on 10/22/2007)

Follow-Up #1: bouncing molecules

But why does it pop back when you squeeze it? Speak on the molecular level.
- Anonymous (age 68)
Springville,Iowa USA
Ok, to speak on the molecular level I'll specialize to solid rubber balls. When the ball is just sitting around, the long twisty rubber molecules relax into a configuration with low energy. Actually, to be more precise, it's a configuration with low free energy, which I can define if you want a follow up. That's like a spring sitting in the relaxed position with nothing stretching or compressing it. When the ball hits something, it gets distorted out of shape, meaning that the molecules are forced into some other configuration with higher energy. There's a force pushing back toward the low energy configuration, just like the way a spring pulls or pushes back toward its relaxed length, or like a gravitational force pulls things down toward the lowest point they can reach. That's what makes the ball pop back.

You may be concerned that I didn't describe the lowest energy configuration in detail. Actually, for rubber it's a pretty random-looking tangle, with the details depending on accidents as the rubber is prepared. The point is that whatever that configuration is, that's how the ball chooses to be before it gets distorted, and that's what the restoring force points toward.

Mike W.

(published on 10/22/2007)

Follow-Up #2: silly putty fast and slow

We found your explanation today after I unexpectedly hurt my eye bouncing silly putty. It hurt! I asked my mom why stuff bounces. Your explanation with the ball example made a lot of sense to me ( which kind of surprised my mom). But, we noticed that silly putty is different... When you squish it it stays in the shape you squished it into ( not trying to return to it's previous shape). But, if you throw it hard onto the hardwood floor it bounces back really hard. Is this related to how you can pull it into thin strands if you pull slowly but it breaks if you pull it really quickly/ suddenly?
- George Wear (age 6)
Santa Cruz, CA USA

Yes, that's a brilliant observation. It takes a while for the silly putty molecules to slide past each other and form a whole new shape. If you squash or stretch it faster, they just bend like rubber molecules and pretty much bounce back before they have a chance to slide.

Please keep us in mind in a few years when you go to college on your way to becoming a physicist.

Mike W.

(published on 01/25/2013)

Follow-up on this answer.