# How does gymnastics work? How are gymnasts able to manipulate their bodies while in the air? It seems unreal.

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How does gymnastics work? How are gymnasts able to manipulate their bodies while in the air? It seems unreal.

In: Physics

My physics teacher lectured me about this one! Although this may not be eli5-ish so bear with me.

So there’s a lot of factors when it comes to a person doing tricks in the air. Let’s keep it simple and start with a backflip.

Mechanically, to backflip, you jump in the air, “knee your chin”, tuck in, and then land. Now let’s unpack that.

For jumping, you’re putting force on your legs/foot against the ground to push yourself up against gravitational force.

Next, “knee the chin” and tuck in. I didn’t mean that quite literally, but when I was taught on how to do a backflip, you bring your knees towards your chin. But you’re not exactly trying to knee upwards, no. **This is the interesting part here**. When you’re in the air, or you’re floating, or there’s no ground or anything, you can still move your body part. But when you do, you are changing your distribution of mass. So when you tuck in or knee your chin, you changed your mass to be more towards the center. Whenever you change the distribution of mass, there’s always the momentum force that is following. Like when you throw a punch, your entire body is feeling like it wants to go towards the direction you threw your punch. The same with doing a back flip and tucking in. And that’s basically what’s happening during a backflip.

So whats with the backflipping explanation having to do with gymnastics? Well let’s keep in mind that I’m not a gymnast, but my physics teacher loved gymnastics and made her exams have word problems having to do with gymnastics and also half of her lectures are based on gymnastics. In other words, I’m regurgitating.

I’d like to imagine that what I explained in the interesting bit about kneeing the chin and tucking is what’s happening in gymnastics. It’s essentially the changing of mass following with the momentum force coming from yourself. HOWEVER, with different tricks comes with keeping aerodynamics in mind because air friction is a thing.

Honestly, I have a opposite expectation. They haven’t perform things that are weird enough. We live in a world where there are things like [torque-free gyroscopic precession](https://en.wikipedia.org/wiki/Precession), [Dzhanibekov effect](https://en.wikipedia.org/wiki/Tennis_racket_theorem), [boomerang](https://en.wikipedia.org/wiki/Boomerang), and [curveball](https://en.wikipedia.org/wiki/Magnus_effect) that exist, so what the gymnasts do doesn’t seem all that unreal.

When you see people do all sorts of weird tricks in the air, the biggest factor is the interaction between angular momentum and moment of inertia. Angular momentum can be difficult to change in the air (impossible without air drag), but what can change is moment of inertia by changing mass distribution. The most basic example of this is increasing rotational speed by retracting your hand.

Smaller effects are air drag and optical illusion. Air drag are generally small (compared to human moment of inertia), so this effects is negligible for indoor gym, but is much more significant to things like snowboarding. Optical illusions are hard to quantify, but human eyes are not actually as accurate at judging movement as you might think.