A gymnast can do both types of rotations at the same time—that’s what makes the sport so intriguing to watch. In physics, we’d call this type of movement “rigid-body rotation.” But humans are clearly not stiff, so the math describing such rotations can be quite complicated. For the sake of brevity, let’s limit our discussion to flips.
There are three types of flips. There is the routine, in which the gymnast keeps her body in a straight position. There is the pike, in which she bends at about 90 degrees at the hips. Finally, there is the tuck, with the knees drawn up to the chest.
What is the difference from a physics perspective?
Rotation and moment of inertia
If you want to understand the physics of rotation, you have to consider the moment of inertia. I know it sounds like a strange term. Let’s start with an example involving boats. (Yes, boats.)
Imagine you are standing on a dock next to a petite boat that is just floating there and is not tied down. If you put your foot on the boat and push it, what will happen? Yes, the boat will move away, but it will do something else. The boat will also accelerates as you move away. This change in speed is acceleration.
Now imagine you’re moving along a dock and you pick up a much larger boat, like a yacht. If you put your foot on it and push it with the same force for the same amount of time as you did on the smaller boat, does it move? Yes, it does. However, it doesn’t gain as much speed as the smaller boat because it has more mass.
The key property in this example is the mass of the boat. With a larger mass, it is harder to change the motion of the object. We sometimes call this property of objects inertia (which should not be confused with moment of inertia—we’ll get to that shortly).
When you push a boat, we can describe this interaction of force and motion using a form of Newton’s Second Law. It looks like this:
Illustration: Rhett Allain