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You botched your forward double somersault and biffed it hard on the water. Why does it have to hurt so bad?

Hosted by: Michael Aranda
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Sources:

https://ocw.mit.edu/courses/physics/8-01x-physics-i-classical-mechanics-with-an-experimental-focus-fall-2002/study-materials/jumping.pdf
http://www.cnn.com/2013/07/23/health/belly-flop-risks/index.html
http://www.nbcnews.com/health/body-odd/why-do-belly-flops-hurt-so-much-ask-professor-splash-f6C10257575
[Intro]

You can hear a belly flop from across the pool. Instead of a nice little splash from someone diving smoothly into the water, there's a loud smack. And you can usually spot the culprit right away because they're floating on top of the water, probably with a bright red, painful belly.

Thankfully, belly flops almost never cause serious injuries; but, they do hurt way more than when you actually manage to dive properly. And it's all Isaac Newton's fault—well, okay, really it's the fault of physics, but Newton came up with the laws of motion that explain what's going on.

Moving a body-sized amount of water in an instant puts a whole lot of force on our bellies. When you dive into the pool, all that water has to move—or be displaced—to make room for your belly.

And, because of Newton's laws, we know that the force your body puts on the water to move it depends on two things: the mass of the water you're moving, and its acceleration.

The acceleration is mostly controlled by the height of the diving board. When you jump from a higher board, you'll hit the water faster, which means the water speeds up more quickly to get out of your way. Since it's all about height, that's not what makes the difference between a perfect dive and a belly flop.

It's the mass of the displaced water that really matters. When you hit the surface in a perfect swan dive, you're only displacing the water right at your fingertips. It's not very much water, so it doesn't take much force to move it.

And even when the rest of your body follows, you're still only moving the water a little bit at a time. Compare that to a belly flop, where the entire front of your body hits the surface at once.

Even if you're going the same speed as in a swan dive, about ten to twenty times as much water has to move in the same amount of time—which takes ten to twenty times as much force. And if your body exerts a force on the water, the water exerts an equal force back right back on your body. That's another thing we learned from Newton.

Which means that in a belly flop, you're hit by up to twenty times as much force as in a swan dive. And when that enormous force is suddenly put on something soft and squishy—like your torso—it can be downright painful.

So the next time you flub a perfect dive or accept that dare, remember to blame good old Isaac Newton.

Thanks for asking, and thanks especially to all of our patrons on Patreon, who keep these answers coming. If you'd like to submit questions to be answered, you can go to patreon.com/scishow. And don't forget to go to youtube.com/scishow and subscribe.

[Outro]