A gymnast can actually do both of these spins at the same time—that’s what makes the sport so fun to watch. In physics, we call this type of movement “rigid rotation.” But humans obviously aren’t rigid, so the math to describe spins like this can get pretty complicated. For brevity’s sake, let’s limit our discussion to flips.
There are three types of flips. There is a style, where the gymnast keeps their body in a straight position. There is a pike style, where they bend at about a 90-degree angle at the hips. Finally, there is a tuck style, where the knees are pulled up towards the chest.
What is the physical difference?
Rotation and moment of inertia
If you want to understand the physics of rotational motion, you need to consider moment of inertia. I know it’s a strange-sounding term. Let’s start with an example involving a boat. (Yes, a boat.)
Suppose you are standing on a dock next to a small boat that is floating there and is not 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 acceleration as it moves away. This change in speed is an acceleration.
Now imagine you are walking along a dock and you pick up a much larger boat, like a yacht. If you put your foot on it and push on it, using the same force for the same amount of time as you did with the smaller boat, will it move? Yes, it will move. However, it will not accelerate as much as the smaller boat because it has more mass.
The key property in this example is the mass of the boat. With more mass, it is harder to change the motion of the object. We sometimes refer to this property of an object as inertia (not to be confused with moment of inertia—we’ll get to that soon).
When you push a boat, we can describe this force-motion interaction using a form of Newton’s Second Law. It looks like this:
Illustration: Rhett Allain
