What. The. Heck. Did you see that? Simone Biles appears to defy the laws of physics with this epic tumbling pass from the 2019 US Gymnastics Championships. It’s called a triple-double. That means she rotates around an axis going through her hips twice while at the same time rotating about an axis going from head to toe THREE times. Yes, it’s difficult—but it doesn’t defy physics, it uses physics.
Let’s go over the three key parts of this move. You can follow the physics, but I don’t recommend trying this triple-double in your backyard.
If you want to do any kind of flip, you pretty much need to be off the ground for some amount of time. Otherwise, you are just a human rolling around on the floor. That might be fun, but it’s not really tumbling.
Once a human leaves the floor, there is essentially only one force acting on him or her—the gravitational force. This is a downward-pulling force that depends on the local gravitational field (g = 9.8 Newtons per kilogram) and the mass of the human (or whatever object). This constant downward force causes a person to accelerate downward. But because both the force and the acceleration depend on mass, the mass cancels out. All free-falling objects on the surface of the Earth have the same acceleration of -9.8 m/s2.
Rhett Allain, an Associate Professor of Physics at Southeastern Louisiana University, writes about physics for WIRED.
The other great thing about the gravitational force is that it only acts in the vertical direction. This means that there are no net forces in the horizontal direction. With no net force, there is no CHANGE in velocity. Once she’s in the air, Simone’s center of mass will move along with a constant speed—with the same horizontal velocity at which she was running before the jump.
But in the vertical direction, she launches upward with some vertical velocity. This velocity decreases as she travels up until it reaches zero at the highest point of the jump. At that point, she starts moving down and increasing in speed until she returns to the floor.
Since this motion has a constant acceleration, we can model it with what’s called a “kinematic equation.” It is a relationship between position, velocity, and time and it looks like this.