# The incredible physique of Simone Biles’ double pike Yurchenko

Gymnastics is a extremely difficult sport, and not just extreme for Olympic athletes like five-time medalist (so far) Simone Biles. Physics is also quite difficult. Consider something seemingly simple, like a flip.

There will be a version of the flip in all four events of women’s gymnastics: floor, bars, vault and beam. This is one of the two types of rotations that a gymnast can perform in the air. In physical terms, a flip is a head-to-toe rotation around an imaginary axis that passes through the gymnast’s hips. For the second type of rotation, a twist, imagine an axis that goes from head to toe.

It might be easier to see them. These two animations were created in Python. (You can see the code here and here.)

A gymnast can actually do both of these types of rotations at the same time, which is what makes the sport so interesting to watch. In physics, we would call this type of movement a “rigid body rotation”. But, clearly, humans aren’t rigid, so the math to describe rotations like this can be quite complicated. For the sake of brevity, let’s limit our discussion to flips.

There are three types of flips. There is an arrangement, in which the gymnast keeps his body in an upright position. There is a pike, in which they bend at an angle of about 90 degrees at the hips. Finally, there is a tuck, with the knees pulled towards the chest.

What is the difference, in terms of physics?

Rotations and moment of inertia

If you want to understand the physics of a rotation, you have to consider the moment of inertia. I know this is a strange sounding term. Let’s start with an example involving boats. (Yes, boats.)

Suppose you are standing on a dock next to a small boat floating there that is not moored. If you put your foot on the boat and push it off, what happens? Yes, the boat is moving away, but it is doing something else. The boat too accelerated as he moves away. This change in speed is acceleration.