Hey, it's me, Destin. Welcome back to Smarter Every Day. So you've probably observed that cats almost always land on their feet. Today's question is why. Like most simple questions, there's a very complex answer. For instance, so let me re-word this question. How does a cat go from feet up to feet down in a falling reference frame without violating the conservation of angular momentum?
嘿，是我，Destin。歡迎回到Smarter Every Day。所以你可能已經發現貓咪幾乎總是用腳落地。今天的問題是為什麼？就像是大部分的簡單問題一樣，這有個很複雜的答案。舉例來說，所以讓我改寫一下這問題。一隻貓如何在墜落過程的參考系統中從四腳朝天轉變成腳朝下，而不違反角動量守恆定律？
Now I've studied free falling bodies, my own in fact, in several different environments, and once I get my angular rotation started in one direction, I can't stop it. Today, we're gonna use a high speed camera, we're not gonna use Alley, because this is my daughter's cat. I don't want to hurt it. We're gonna use a stunt cat. Let me introduce you to Gigi the stunt cat.
I'll just flip the video vertical, and then motion track the cat. It's just gonna take a lot more effort in post. We're gonna try to do it in a way that doesn't make anybody mad. That's pretty hard to do. I'm gonna drop a cat. Ready, Gigi? Good. Checking out the high speed data there, Gigi?
OK, the first thing a cat does when it's falling is try to figure out which way is up. It does this either with a gyro in the ear, or with its eyes. Three...two...
Ready to talk cat physics? Alright, so check out this footage I captured with the Phantom Miro, while Gigi goes to get a drink of water. So here's what's interesting about this to me. If you'll notice at the beginning of the drop the cat is not rotating. Half way through the drop the cat is rotating, and then at the very end Gigi somehow stops rotating. Newton's first law says that an object at rest will stay at rest unless acted on by an external force. I see no external forces on this cat. So what's happening here? It's not making sense to me. OK, so in order to really get the right data, we're gonna have to drop her ninety degrees out of phase. Ready, girl? This time watch her tail. Three...two...one...
OK, so you think you've figured it out? Check this out. You probably noticed that when the cat was falling, her tail was rotating in the direction opposite of where her body was rotating. What's interesting about that is that that's not how it works. In fact, even Bobtail cats can do this. It's called the cat righting reflex. I'll prove it to you. I came across some video from the 60s when the air force was researching micro gravity for future astronauts. Turns out they took some cats up on parabolic flights. He tries to rotate his tail to flip over, but it doesn't work. He just ends up nutating wildly. Then he does something interesting. He takes his back and he bends it, and when he bends his back and then creates motion, something interesting happens.
Aah. Now we're getting somewhere. So let me show you one more cat flip with the Miro, and we'll figure this out. OK, the arched back ends up being pretty important. What he does is he divides his body up into two separate rotational axes that are tilted from one another.
When he's released, he pulls his front paws in and does the ice skater trick. He decreases his moment of inertia in the front so he can spin fast up there, but in the back he pushes his legs away from him, increasing his moment of inertia, so a really large twist in the front equals a really small twist in the back in the opposite direction and the torques equal out.
So as soon as he gets his front paws in under him, all he has to do is extend those legs back out to increase that moment of inertia and stop the front twist, and extend his back legs along that rear axis. That allows him to twist those around really fast, and then all he has to do is pull them back in under his body and then extend all four legs and brace for impact.
So thank you for your attention. I hope you learned something pretty cool about cats. If you don't mind... Ooh! OK, ok, I'm done! If you would, go check out your other cat videos after trying to catch Gigi. Wooh! A little too rowdy. If you'd like to, click Gigi the cat to subscribe, we'd appreciate it. I hope you had a good one. And aah... Get the ball! You gotta catch her first. I got it, you want it? Look... Very cool cat. She let us drop her hundreds of times. Or, you know, maybe just five. Want your ball? Go get it!
Click her if you want to subscribe, but right now we're playing fetch.
Ready? Go get it!
I'm Destin. You're getting Smarter Every Day. Have a good one.
There you go.
There you go. You earned it.
Let me explain why cats are wizards at physics. First of all, they have physiology working for them. They have no working clavicle like I do, and secondly, they have very flexible back bones.
The talent needs a break.
Thirty centimeters or higher. Anything below that is unsafe.
Not that we condone dropping cats. Come here! Come here! There you go. Go get in the catapult.
3...2...1... Oh, man.