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  • 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.