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.