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A ciência por trás do "quicar"

Transcrição de vídeo
(Max) Yo. So I'm here to play "squash" for the first time. I don't really know much about it. I hear there's a racquet, probably a ball; it's a lot like tennis, which I'm pretty good at. (Bjorn) Hey man, ready to go? (Max) Let's do this. (Bjorn) Great. Which ball do you want to use? (Max) If it's all the same, let's go with the blue one. (Bjorn) Oh, but they're NOT the same. (Max) I told you we should've used the blue one, the black one's clearly broken. (Bjorn) It's not broken, they're designed to bounce differently. (Max) What do you mean, 'designed differently'?! They're just different colors. (Bjorn) It's not the color. (Max) Then... (Bjorn) They're a different material. (Max) They're both made out of rubber... (Bjorn) Sounds like you need an education in THE SCIENCE OF BOUNCING. When you hold a ball up high, it has a certain amount of what we call 'potential energy'. When you let go and it starts to fall, that potential energy gets converted into 'kinetic energy', and the ball speeds up. And when the ball gets down to the ground, all of the potential energy has been converted into kinetic energy. So when the ball deforms, it stores kinetic energy. And 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. And since it doesn't return to its original shape, there's no way to store the kinetic energy. And since there's no way to store the kinetic energy... (Max) It would make a terrible material for squash balls? (Bjorn) Exactly. There's hardly any bounce. (Max) Okay. So I get why you wouldn't make a squash ball out of modeling clay. That would be lame. But, you still haven't told me why those two rubber balls bounce differently from each other. And, because of that fact, I'm stuck here learning instead of beating you at squash. (Bjorn) So while these balls look similar, they're actually made out of different rubbers. Let's take a look at what happens at the material level. All rubber is made up of these long molecules called 'polymers' tangled together. When the ball deforms, these polymers stretch out, but they quickly return to their original shape. All this stretching causes the polymers to rub up against each other. This transforms some of the kinetic energy into heat. The difference between the two balls is how tightly the polymers are tangled. The looser the polymers, the more they rub against one another, and the more heat they produce. And the more heat, the less kinetic energy they have. (Max) So, the less kinetic energy, the less bounce? (Bjorn) Right on. (Max) So, the bouncy ball is made out of tight polymers, and the other one is made out of loose polymers? (Bjorn) Exactly. (Max) Sweet! Let's play. (Bjorn) You know, the temperature of ball also affects its bounce. (Max) There's more? (Bjorn) Definitely! The warmer a ball is, the stretchier the polymers. And the stretchier the polymers, the more bounce. (Max) So, is there, like, an oven around here where we can heat these up a bit? (Bjorn) We don't need an oven. Remember what I told you earlier? Every time a ball bounces, a little bit of kinetic energy is turned into heat. So if we bounce the balls a lot... (Max) There'll be a lot of heat... (Bjohn) Looks like the ball's not the only thing getting heated up... So I've agreed to stop "cheating" in the future and Max is going to help me finish our experiment here. (Max) Begrudingly, I might add. (Bjorn) So, here's the ball we've been hitting around and here's an identical one, except it hasn't been bounced, so it's colder. So when we bounce these- (Max) Wait wait wait, I got this. The warmer one's gonna bounce highest. (Bjorn) Let's see if you're right. (Max) 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, so a stretchier and warmer ball will bounce higher. (Bjorn) You got it. (Max) So let's see if you can do more than just talk about squash. (Bjorn) Oh, you're on. Friend. *Incomprehensible yelling and beeps* (Max) I love your headband! (Bjorn) Your shirt's pretty cool itself. (Max) I love your sweatshirt! (Bjorn) You have a nice wig, who bought it for you? (Max) You bought it for me!