The ‘impossible’ skateboard trick
Physics and skateboarding isn’t the first thing that comes to mind as an real-world example of physics at work. However, skateboarding is an amazing example of physics at work but this essay isn’t about that. This essay is about the ‘impossible’ skateboard trick. But why is it impossible?
First of all, what is this trick? It was originally created by Rodney Mullen and is, quite simply, referred to as the ‘Impossible’. Now, the trick has many variations but let’s focus on the original. In the trick, the board wraps vertically over the back foot in a 360 degree rotation. It doesn’t seem so ‘impossible’ but let’s look at the physics.
Screenshot from Physics Girl’s video on the subject. Link at end.
Let’s start with establishing the three axis on a skateboard. Let’s call them the long axis, the mid axis and perpendicular axis. The long one is, as the name states, the longest and goes across the middle of the board lengthways. The intermediate or mid axis is in the middle of the long axis but goes across the width of the board. The perpendicular is the vertical one in the space where the mid and long axis intersect. This will help us understand the basics of skateboarding. Kickflips, for example, flip on the long axis. All skateboard tricks are some kind of mix of flipping or rotating around said axis. But again, what does this have to do with the trick? As I just mentioned skateboard tricks consist of flipping around those axis but there’s one very important thing called the Intermediate Axis Theorem. What is the Intermediate Axis Theorem?
The Intermediate Axis Theorem is full of very complex math which essentially leads and proves that any object, with those three axis, if spun on the intermediate axis is unstable. That applies to books, tennis rackets, skateboards and even phones. So, if you want, you can check it yourself with your smartphone. So if the Intermediate Axis Theorem says that items spun on that axis are unstable, how does the skateboard not fly off?