How to tear a crazy skateboard loop with physics

Why study physics, ask yourself? Scroll down, my friend. This video of a guy rolling in a 360-degree vertical loop in a skatepark – cool like ice, without a helmet – is a sufficient answer.

Clearly, his understanding of the acceleration of a circularly moving object allowed him to accurately calculate the required speed for a pipe of this diameter to keep the wheels on the concrete all around. Do you want to impress children in your local park? Let's see how it goes!

How does he fall?

That's the secret here – it does not fall because it's somehow is fall. It's deep. To see what I mean, let's break it down in terms of strengths. In fact, once our skating physicist is gone and rolls, only two forces act: the downward force of gravity and the strength of the concrete. Pretty routine up to now.

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

The strength of resistance is called the normal force. (This is "normal" in the geometric sense, as perpendicular to the surface on which you are standing.) This is what keeps you from falling on the sidewalk under the influence of gravity on a daily basis. Its direction is generally ascending, as opposed to gravity, and its magnitude is all that is needed to prevent such strange things.

But that's when things start to become less normal: when you close the loop, this second force does not push up anymore, it's more towards the center of the circle to prevent you from flying through the wall of the cylinder . When you are at the top of the circle, for example, it grows down on you instead of.

Here is a diagram showing the two forces on the skater. The marked arrow mg is the gravitational attraction, and F_NOT is the normal force.