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How Ice Skaters Turn Physics Into Astonishing Spins

Many folks do not know an excessive amount of about angular momentum—and that is advantageous. But what about determine skaters? Whether they perceive the idea of angular momentum would not matter however they use it in one of many all time traditional skating strikes. You’ve seen it earlier than. The skater begins off in a standing place and spins in regards to the vertical axis. After a couple of rotations, the skater pulls each arm in nearer to the physique and spins quicker. In physics, we name this conservation of angular momentum.

Just for instance, right here is that this identical maneuver carried out on a rotating platform as an alternative of on ice.

Gif: Rhett Allain

Really, you may strive one thing like this by yourself. Sit on a pleasant spinning chair or stool. Start together with your arms stretched out as you spin after which deliver your arms in. Don’t barf.

But what precisely is angular momentum? In quick, it’s one thing that we will calculate that may be conserved. That’s a troublesome definition, so let me give an instance of a conserved amount—like mass (which solely largely conserved). Suppose you’re taking add some baking soda to vinegar. If you’ve got ever carried out this, you will notice that the ensuing combination foams and produces some fuel. But this is the cool half. If you measure the mass of the stuff you begin with (vinegar and baking soda) it is the identical because the mass of the stuff you find yourself with (carbon dioxide and water and sodium acetate). Boom, mass is conserved. It’s the identical earlier than and after.

OK, I’ve to level out that mass is not all the time conserved. n a nuclear response, the mass of the stuff earlier than would not should be equal to the mass of the stuff after. But for those who take a look at power (and embody mass within the power), then power is conserved.

Now for angular momentum. The angular momentum is a amount that we will calculate for rotating object. It’s the product of the angular velocity (how briskly it spins—represented with the image ω) and the second of inertia (utilizing the image I). I believe most individuals are OK with the concept of the angular velocity—however the second of inertia factor is a little more difficult. Basically, the second of inertia is a property of an object that is dependent upon the distribution of the mass in regards to the rotation axis. If you’ve extra mass additional away from the axis of rotation, the second of inertia is bigger than if that was was near the axis.

Here is an excellent fast demo—and you’ll do this at dwelling. I’ve two sticks with juice containers taped to them such that each sticks (plus juice) have the identical mass. However, there’s a distinction. One stick has the juice containers on the ends of the stick (excessive second of inertia) and one stick has them taped to the center of the stick (low second of inertia). Now take a look at what occurs whenever you attempt to rotate these sticks forwards and backwards (bear in mind—they’re the identical mass). Oh, to make issues extra enjoyable I gave the upper second of inertia persist with the stronger lady. Also, here’s a longer video model of this demo.

So let’s evaluate. The angular momentum is dependent upon each the angular velocity and the mass distribution of the article. You can change this angular momentum by exerting a torque (a twisting drive)—however with no exterior torque, the angular momentum is conserved.

Now getting again to the ice skater. In the vertical spinning place, there may be little or no torque exerted on the system (since ice is slippery and the skates are near the axis of rotation). This implies that the angular momentum ought to keep at a continuing worth. But what occurs for those who change one thing—like bringing your arms nearer to your physique? This would lower the second of inertia. Since the angular momentum has to remain fixed, the angular velocity should enhance. It’s the one strategy to preserve angular momentum.

Here is one other view (from the highest) of this identical transfer—only for enjoyable.

Gif: Rhett Allain

Really, you might simply take some measurements from this. It would not be too troublesome to measure the angular velocity each earlier than and after the arms being pulled in. From that, you might calculate the change within the second of inertia. But nonetheless, I believe this transfer is greatest left to professionals—the spinning would make me sick.

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