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How Ice Skaters Turn Physics Into Astonishing Spins

How Ice Skaters Turn Physics Into Astonishing Spins
From Wired - February 12, 2018

Many people do not know too much about angular momentumand that's fine. But what about figure skaters? Whether they understand the concept of angular momentum does not matter but they use it in one of the all time classic skating moves. You have seen it before. The skater starts off in a standing position and spins about the vertical axis. After a few rotations, the skater pulls both arm in closer to the body and spins faster.In physics, we call this conservation of angular momentum.

Just as an example, here is this same maneuver performed on a rotating platform instead of on ice.

Really, you can try something like this on your own. Sit on a nice spinning chair or stool. Start with your arms stretched out as you spin and then bring your arms in. Do not barf.

But what exactly is angular momentum? In short, it is something that we can calculate that can be conserved. That's a tough definition, so let me give an example of a conserved quantitylike mass (which only mostly conserved). Suppose you take add some baking soda to vinegar. If you have ever done this, you will see that the resulting mixture foams and produces some gas. But here's the cool part. If you measure the mass of the stuff you start with (vinegar and baking soda) it's the same as the mass of the stuff you end up with (carbon dioxide and water and sodium acetate). Boom, mass is conserved. It's the same before and after.

OK, I have to point out that mass is not always conserved.n a nuclear reaction, the mass of the stuff before does not have to be equal to the mass of the stuff after. But if you look at energy (and include mass in the energy), then energy is conserved.

Now for angular momentum. The angular momentum is a quantity that we can calculate for rotating object. It's the product of the angular velocity (how fast it spinsrepresented with the symbol ) and the moment of inertia (using the symbol I). I think most people are OK with the idea of the angular velocitybut the moment of inertia thing is a bit more complicated. Basically, the moment of inertia is a property of an object that depends on the distribution of the mass about the rotation axis. If you have more mass further away from the axis of rotation, the moment of inertia is larger than if that was was close to the axis.

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