Howcome when we’re spinning around on an office chair with our legs sticking straight out, we can speed up by simply pulling our legs in?
In: Physics
While the explanations involving conservation of angular momentum are perfectly accurate, I feel like a better eli5 might possibly go like this.
When you spin with legs sticking out your legs actually want to get away from you – you might feel it more if you tie or put some extra weight on your feet and try to spin up pretty fast. So when you pull your legs in while spinning you are actually doing work and expending energy. This energy has to go somewhere, so you speed up, i.e. your kinetic energy increases.
[Conservation of Momentum](https://en.wikipedia.org/wiki/Momentum). Objects interact based on their mass and speed (when they collide). If they don’t collide and are not affected by any outside forces, objects will continue to move (inertially) as they were before. That’s why things just drift through space without stopping.
There’s also [conservation of angular momentum](https://en.wikipedia.org/wiki/Angular_momentum), same deal, objects that are spinning will continue to spin unless some force or object acts on them to change that. Angular momentum (the spinning) depends on the dimensions of the object, not just its mass and spin rate.
So when you’re in a chair, because of the ball bearings in the chair, as you spin, the forces of friction acting on you are quite small. You will spin for a while. Your angular momentum will be conserved. However, when you pull in your legs, your “dimensions” change and because you’re a smaller object you spin faster.
This is used in [figure skating](https://www.youtube.com/watch?v=0RVyhd3E9hY) and other sports, and also happens to stars when they collapse due to gravity – neutron stars [spin very fast](https://en.wikipedia.org/wiki/Pulsar).
Conservation of angular momentum. With your arms or legs outstretched they have a larger angular momentum when compared to having them tucked. If you are spinning at a certain rate with legs outstretched and you bring them in you *must* increase your rotation speed in order to keep angular momentum the same.
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