字幕列表 影片播放 列印英文字幕 What's the top request for a video topic that I get? Do the physics of insert random sport. And I always hesitate because science is interesting when you have a question driving your curiosity. And if that question is, what's the physics in skateboarding? Then it's only going to be interesting to you if you're already into physics and skateboarding. It's like making a montage of tractors mowing. It has limited appeal. So why did I decide to do this video? Well, let's be honest. The possibility of working with Rodney Mullen came up. I can't believe I'm even saying that. And so I googled some videos of Rodney Mullen. And then I watched video, after video, after video. And I realized I have so many questions. How does he do that? Honestly, like from a physics standpoint. Let's just start with, how do you get the skateboard off the ground? Which initially sounds like a simple question. So through this unrelenting inquisitive brain, I became so interested in skateboarding. What? And fortunately, Rodney Mullen is the kind of guy who also loves to think about science and physics. And he agreed to meet for this video and to let me just direct him on whatever tricks I wanted to analyze. And I brought along a couple of friends who happen to really know how to use high-speed cameras. It's past my bed time. By the way, I'm Dianna. And you're watching \"Physics Girl.\" And this video is about why skateboarding is an incredibly rich combination of fundamental physics with really difficult mechanics. And it is a beautiful example of physics in action. OK. So despite the fact that I surf and I snowboard, I do not skate. So let's head back to the studio where we can look at what we filmed with Rodney. Well, it went really straight forward. We did a bunch of 360s. And so that's cultivation of angular momentum. So you're coming out wide. And what happens on that, because it's a nose wheelie, that one is one where you can't pull in your arms too fast because you spin right out of control. Can confirm. It's amazing to me how much of Rodney's use of physics is so inherent in his comfort with the skateboard. So you know how that works, right? As long as you keep the bigger radius, then your velocity will stay kind of mellow until friction will dissipate the energy. So you can gradually pull them in and keep your velocity kind of sort of constant. But if you yank them in, then your velocity increases like crazy. And you'll be unstable. And you'll throw yourself out. And I would have ended up in the lights. I don't know about you, but it seems to me like I could have just allowed Rodney to keep teaching us the physics of skateboarding. But I had too many burning questions. So here are the things that brought out my deepest curiosity. When I first started looking at skate tricks, I noticed that most of the tricks are some combination of the skateboard flipping or rotating about its three major axes. Oh, first of all, I think it's going to be really useful for us to talk about the skateboard as having three different axes. Bear with me. I promise I won't call them x, y, and z. Let's call them the long axis, the mid axis, and the perpendicular axis. So once I realized that, I realized the skateboard is shaped a lot like something that I play with every day. Try this with me. I made Rodney do it. If you try flipping your phone about the long axis. OK, kickflip-style. We just did impossibles. I love how you said it in skateboarding terms. Yes, spin it kickflip-style. Or the perpendicular axis. It's whatever. But if you try flipping it about the mid axis-- well, try it. That seems trickier. I think just hold it. Oops. It did a gainer. No. That's really hard. It gets messy. The reason it's tricky to flip about the mid axis is not just a hard trick. It's a thing. It's a mathematical thing known as the intermediate axis theorem. Get this. It's the same exact reason that this T-handle spinning in the space station spontaneously flips around over and over. The intermediate axis theorem will affect a tennis racket, a book, anything where the object has three different obvious axes and the moment of inertia is different for all three. What I mean by that is that the oomph that you need to spin it about each of the individual axes is different for all three of them. The axis with the middle level of oomph needed to get it to spin in the case of the phone is that mid axis, known more generally as the intermediate axis. The reason why the mid axis is so hard to spin involves a lot of complicated math that all works out to define the intermediate axis theorem, which states that inherently. Spin about the intermediate axis in an object like this is always unstable. So there it is. That's why flipping it is so hard. It's really hard. Yeah. One might say-- Impossible. OK. Skateboarders everywhere are starting to go ooh. Do you see where I'm going with this? I think I do. The rest of us are like huh? So during my research on skateboarding-- said no one cool, ever-- I asked if there was a trick where the skateboard spins about the intermediate axis. And I was told that there was. And it's called the impossible. Watch Rodney's ollie impossible. His foot actually guides the board to make sure that it keeps spinning about just that axis. When I was asking him, is there a trick like that? And he was like yeah, there is. But you follow it with your foot. And it was, like, interesting. And if he lets it go, well, physics says that it will probably become unstable. In fact, he did another trick where it starts out spinning like an impossible. But look what happens as soon as he lets it go. Almost immediately, it started spinning with much more complicated motion because it became unstable. Because to me, this one without the foot seems like it would be impossible because of the intermediate axis theorem. Well done. I learned something. That's really cool. In fact, that's huge in skating. A lot of tricks are about that, where some movements are easier, but they become more unpredictable. And so it's a wisdom to know what to aim at. The reason that skateboarders have to keep their foot on the boards to guide an impossible is partially to overcome the intermediate axis theorem. It's the same reason that the T-thing in space starts spontaneously flipping. That connection is so cool to me. OK. But now, skateboarders might not have had Newtonian mechanics in mind when they named the impossible. So here's Rodney with a little bit of fun history on the name. Kickflips had been done. Shove-its had been done. But impossible had not been done. It was called impossible because it would take too long to hang out. And you'd never really get it down. So what happened was I got hurt. I'm credited with creating the trick or whatever. So-- Wait. The impossible? Yeah. And so I got hurt. Or whatever. I didn't name it. I sat in front of the TV. And I only had really one working leg. And I just kept stomping on the tail. And I learned how to follow it. So camera-wise. That's the way the trick works. You learn what all the skaters called scoop. So scoop it. So it's a scoop-type trick. But the way I like the nollie, when it had a more straight and true nollie impossible, is because it goes with the grain. It sort of pole vaulted. And then you track it that way, which is a little harder. That's why so-- it's so rare to see people do it. Yeah. Most of those words made sense to me. OK, sorry. Well, most of your words make sense to me. Fair enough. The tricks and the physics that Rodney likes to talk about are super advanced. But I'm still over here, like, how do you even get the board off the ground? So that's the last question that I've got here. Rodney alluded a little bit to being able to kind of drag the board up with your foot. But that's once the board is already in the air. Skateboards aren't pogo sticks. What I mean by that is they don't have springs in them. Trust me. I asked the experts during my deepest moments of ignorance. But the board is every seesaw you've ever ridden. It's got lever action all over the place. Look closely for clues at how Rodney gets off the ground. It's there. His foot is pushing down the board past the wheel, which seesaws the other side up. Then it hits the ground hard. And pushes the board up in the air. Consequently, the earth was pushed down because of Newton's third law of equal forces. But the earth forgave Rodney for that. It's just beautiful physics. And then the art comes in, you know, controlling the board. A big thing in skating is so you stood up. And whatever you do, you try not to pat down your board because a lot of us do this. Snapping up and then too early, they're pushing. Where some skinny little kid, he'll just know how to snap, move with it so everything is there. And he rolls off. And the board is still on its way up. It's a drag and a roll. So a lot of what you see is an efficiency of movement. All right.