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The Science of Bouncing

6 minutes

(Describer) Science Out Loud. On a squash court, a man wearing a curly wig with a headband stretches.

[disco music playing]

Yo. So I'm here to play "squash" for the first time.

(Describer) Air quotes.

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?

(Describer) A man with with a long wig and headband.

Let's do this. Great. Which ball do you want?

(Describer) Blue or black.

It's all the same. Blue. But they're not the same.

(Describer) He drops them and the blue one bounces more.

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.

(Describer) Behind the title “Let’s Bounce”, the players jump and high-five in slow-motion. The long-haired one holds out the blue ball.

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.

(Describer) On the floor, part of it flattens.

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.

(Describer) ...shown as a drawing.

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.

(Describer) The curly-haired player turns back from walking away.

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.

(Describer) Behind the title “Bounce Time”, they play squash, hitting a ball with rackets against the walls.

(Describer) The curly-haired player throws down his wig.

Looks like the ball's not the only thing heating up.

(Describer) He smacks a wall and puts the wig back on.

I've agreed to stop "cheating" in the future,

(Describer) Air quotes.

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.

(Describer) They drop the balls, and the warmer one bounces higher.

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.

(Describer) When he pushes Max’s chest a little, Max throws down a towel.

[shouting with words bleeped out]

(Describer) Title: Made with love at MIT. The last scene is run again without the beeps.

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!

(Describer) Funding to purchase and make this educational program accessible was provided by the U.S. Department of Education. Contact the Department of Education by telephone at 1-800-USA-LEARN, or online at www.ed.gov.

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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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