Thread by @rjallain

Oh, everyone is going crazy over this Simone Biles tumbling pass. You need some #physics physics to go with it? I got you covered.

https://twitter.com/bubbaprog/status/1160760528987394049

1/ Here is the physics of a flip with a twist, it's about angular momentum.

wired.com/2016/08/physic…

2/ Once Simone is in the air, there is no external torque on her so her angular momentum vector must be constant. However, angular momentum and angular velocity DO NOT have to be in the same direction. I know, that's crazy.

3/ In that gif, the yellow arrow represents the angular momentum vector and the red is angular velocity.

You can also show this twisting with a chalk eraser. What happens when you toss it in the air?

4/ Notice that the eraser tumbles and flips? That's because the angular momentum and angular velocity are different. The main cause is that the moment of inertia (the rotational mass) is more complicated than just a simple number. Actually, it's a tensor.

5/ So, there are two ways to "twist" in the air. The first is a torque twist. This is where you use your feet in contact with the ground to apply a torque. It's pretty easy - even I can do it.

6/ The other twist is an angular momentum twist. This is where a gymnast changes body position while in the air. This change in position changes the moment of inertia and causes angular velocity to be different than angular momentum. Here is an example.

7/ Here is an even BETTER example. What happens when you have as much time in the air as you want? This is aboard SkyLab.

Notice he doesn't start twisting until he already started flipping?

8/ Need EVEN MORE #physics? How about this paper from @AAPTHQ

"Do springboard divers violate angular momentum conservation?" (1979) Cliff Frohlich

aapt.scitation.org/doi/abs/10.111…

9/ Or "An insight into the Biomechanics of Twisting" (1997) Hardy Fink

usagym.org/pages/home/pub…

10/ "Twisting Somersault" (2015) H. Dullin, W. Tong

arxiv.org/abs/1510.08046

11/ What about the force needed to get in the air? Here is an analysis of Aaron Cook doing a standing double back - that's a pretty tough thing to do.

wired.com/story/the-mind…

12/ Here is the video of the standing double back (flip).

13/ The key is to get in the air long enough to complete the flip. Otherwise, you land on your head or back (that's bad).

Once in the air, the center of mass of the human acts just like a ball - higher starting velocity means a higher jump and a longer time in the air.

14/ One way to make a back flip easier is to start rotating BEFORE you jump.

Here is an example where you start in a handstand. Notice how she rotates before jumping. Yes, that's my daughter @EllieAllain

@EllieAllain 15/ Finally - here is another blog post version of this gymnastics tumbling.

wired.com/story/the-twis…