How to Rip a Crazy Skateboard Loop With Physics

Why study physics, you ask? Scroll down, friend. This video of a guy rolling through a 360-degree vertical loop at a skatepark—cool as ice, with no helmet—is answer enough.

Clearly, his understanding of acceleration for an object in circular motion enabled him to calculate the precise speed required, for a pipe of this diameter, to keep wheels on concrete all the way around. Want to wow the kids at your local park? Let’s see how it’s done!

Credit: Evan Mock

How does he not fall?

That’s the secret here—he doesn’t fall because he sort of is falling. This is deep. To see what I mean, let’s break it down in terms of forces. Really, once our skater-physicist has pushed off and is rolling along, there are only two forces acting on him: the downward force of gravity and the resisting force of the concrete. Pretty routine so far.

The gravitational interaction is a constant force pointing toward the center of the planet, with a magnitude equal to your mass times the gravitational field (which is 9.8 newtons per kilogram on Earth).

The resisting force is called the normal force. (That’s “normal” in the geometric sense, as in perpendicular to the surface you’re standing on.) This is what, in everyday life, keeps you from falling through the sidewalk under the influence of gravity. Its direction is usually upward, in opposition to gravity, and its magnitude is whatever’s needed to stop weird things like that from happening.

But here’s where things start to get, well, less normal: When you loop the loop, that second force is no longer pushing upward—it’s pushing toward the center of the circle to keep you from flying through the wall of the cylinder. When you’re at the top of the circle, for instance, it actually pushes down on you instead of up.

Here is a diagram showing both forces on the skater. The vector labeled mg is the gravitational pull, and FN is the normal force.

Illustration: Rhett Allain

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