 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.
There's so much more physics
I'd like to talk about.
But I got to start thinking
about other questions
of the universe.
So if you like
this video and you
want to explore more random
or unusual phenomena,
consider subscribing to
"Physics Girl" and come back.
And I have to tell you
about the high-speed camera
guys have YouTube channels,
BeyondSlowMotion and Kuma
Films.
I will link to both of
those in the description
and at the end of this video.
If you like beautiful videos,
definitely worth checking out.
I owe a lot of
thanks to those guys.
I got to thank Rodney for
sharing his knowledge with us.
It was amazing.
Thank you to you guys.
Thank you so much for watching.
And whether or
not you come back,
have a great day being curious
about your world and happy
physicsing.
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