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.
