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# Simone Biles’ incredible Yurchenko double pike physique

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

There are three types of somersaults: a horizontal one, in which the gymnast keeps the body in a straight position; a pike position, in which the hips are bent at an angle of approximately 90 degrees; and a tucked position, with the knees raised towards the chest.

What is the difference, in terms of physics?

Rotations and moment of inertia

If you want to understand the physics of rotation, you have to take into account the moment of inertia. I know it’s a strange-sounding term. Let’s start with an example involving ships. (Yes, ships.)

Suppose you’re standing on a dock next to a small boat that’s just floating there and isn’t tied down. If you put your foot on the boat and push it, what happens? Yes, the boat moves away, but it does something else. The boat also speed up As it moves away, this change in velocity is an acceleration.

Now imagine that you are moving along the dock and you pick up a much larger boat, such as a yacht. If you put your foot on it and push it, using the same force for the same amount of time as you did for the smaller boat, does it move? Yes, it does. However, it does not increase its speed as much as the smaller boat because it has a greater mass.

The key property in this example is the mass of the ship. The greater the mass, the harder it is to change the motion of an object. We sometimes call this property of objects the inertia (not to be confused with The moment of inertia—We’ll get to that soon.)

When we push the boat, we can describe this force-motion interaction with a form of Newton’s Second Law. It looks like this: