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The Science of Bouncing

6 minutes

[disco music playing]

Yo. So I'm here to play "squash" for the first time. I don't know much about it. I hear there's a racquet, a ball. It's like tennis, which I'm good at. Hey, man, ready to go? Let's do this. Great. Which ball do you want? It's all the same. Blue. But they're not the same. I said to use the blue. The black's broken. It's not broken. They're designed to bounce differently. "Designed differently"? They're just different colors. It's not the color. They're a different material. They're both rubber. Sounds like you need an education in the Science of Bouncing. When you hold a ball up high, it has what's called potential energy. When you let go and it starts to fall, potential energy gets converted into kinetic energy and the ball speeds up. When it reaches the ground, all of the potential energy has been converted into kinetic energy. So when the ball deforms, it stores kinetic energy. When the ball returns to its original shape, it releases that energy, making it bounce back up. We call balls that deform and bounce elastic, but not all balls are like this. Some, like modeling clay, are what we call inelastic. When we drop the clay, it deforms permanently. Since it doesn't return to its original shape, there's no way to store the kinetic energy, and since it can't store kinetic energy... It would make a terrible material for squash balls? Exactly. There's hardly any bounce. Okay, so I get why you wouldn't make a squash ball out of modeling clay. But you haven't told me why those two rubber balls bounce differently from each other. Because of that fact, I'm stuck here learning instead of beating you at squash. So while these balls look similar, they're actually made out of different rubbers. Let's look at what happens at the material level.

Rubber is made up of long molecules called polymers tangled together. When the ball deforms, these polymers stretch out, then quickly return to their original shape. All this stretching causes the polymers to rub against each other, transforming some of the kinetic energy into heat. The difference between the balls is how tightly the polymers are tangled. Loose polymers rub against on one another more, producing more heat but retaining less kinetic energy. The less kinetic energy, the less bounce? Right on. So, the bouncy ball is made out of tight polymers and the other out of loose polymers? Exactly. Sweet. Let's play. You know, the temperature of a ball also affects its bounce. There's more? Definitely! The warmer a ball is, the "stretchier" the polymers, and the "stretchier" the polymers, the more bounce. So, is there an oven around here where we can heat these up? We don't need one. Remember what I said? Every time a ball bounces, some kinetic energy is turned into heat. If we bounce the balls a lot... There'll be a lot of heat. Looks like the ball's not the only thing heating up. I've agreed to stop "cheating" in the future, and Max will help finish our experiment. Begrudgingly, I might add. Here's the ball we've been hitting, and here's an identical one that hasn't been bounced, so it's colder. When we bounce these-- I got this. The warmer one will bounce higher. Let's see. Booyah! Since you're determined to teach me science today, let me make sure I got this straight. A ball's bounce is determined by both its material and temperature. "Stretchier" and warmer balls bounce higher. You got it. Let's see you do more than talk about squash. Oh, you're on, friend. [shouting with words bleeped out] Accessibility provided by the U.S. Department of Education. I love your headband! Your shirt's cool. I love your sweatshirt! Nice wig. Who bought it? You bought it!

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Think all squash balls bounce the same? Think again! Two MIT scientists look into what makes things bounce better than others. Ready for some physics? Part of the "Science Out Loud" series.

Media Details

Runtime: 6 minutes

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