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

There are three types of somersaults. There is a set-up, where the gymnast holds his body in a straight position. There is a pike, where he bends his hips at an angle of about 90 degrees. Finally, there is a tuck, where the knees are pulled towards the chest.

What is the difference, from a physics point of view?

Rotations and the moment of inertia

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

Imagine you are standing on a dock next to a small boat that is just floating there and not tied down. If you put your foot on the boat and push it, what happens? Yes, the boat moves away, but something else happens. The boat *accelerates* as it moves away. This change in velocity is an acceleration.

Now imagine that you are moving along the dock and you choose a much larger boat, such as a yacht. If you put your foot on it and push it, with the same force and the same time as the smaller boat, does it move? Yes, it does. However, it does not increase in speed as much as the smaller boat, because it has a greater mass.

The most important property in this example is the mass of the boat. With more mass it is harder to change the motion of an object. Sometimes we call this property of objects the *laziness* (which should not be confused with *the moment of inertia*(we’ll come back to that later).

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