Angular momentum is a concept that most people don’t know too much about save for those well-versed in JC and O Levels physics. However, this shouldn’t be the case for figure skaters who leverage it all the time in their careers, right? Well, not exactly. Whether they understand it or not doesn’t really matter as long as they consistently execute one of the all-time classic skating moves in their programmes or routines: the spin.
The most basic variation of this move involves a skater starting in the standing position and then spinning about in their vertical axis. After several rotations, they pull both of their arms close to their bodies which increases their speed. Doing the inverse results in the opposite effect, leading to all sorts of graceful spin variations.
So, how exactly do ice skaters turn angular momentum into astonishing spins? Read on to learn more, but first, you may want to test this out for yourself.
You don’t need to visit the ice rink to give it a try; any spinning chair or stool will suffice. Simply seat yourself, stretch your arms out once you start rotating, and then tuck your arms inward.
Angular Momentum: Definition and Examples
That said, what exactly is angular momentum? If you’re taking physics tuition, your first thought may be that it is the angular equivalent of linear momentum. Specifically, it is defined as the product of angular velocity and the moment of inertia. Additionally, it is also something that can be conserved.
To get a better idea of that last part, let’s look at a good example of another conserved quantity like mass. Now, let’s recall what happens when we add vinegar to baking soda. Doing so gives rise to some gas and foams. The cool part about this reaction is that if you measure the mass of the two initial ingredients and their resulting mixture, you’ll end up with the same amount. In other words, mass is conserved (in most cases, that is) since it remains the same before and after.
Going back on track, angular momentum is a measurable quantity for rotating objects. As mentioned previously, it’s a product of how fast the object spins and the moment of inertia. Between the two, the latter is a bit more complicated and requires further explanation.
The moment of inertia is a property of an entity or object that depends on the mass distribution in relation to its rotation axis. The greater the amount of and the further the mass from this axis, the larger the moment of inertia. A quick way to see this in action is by attaching two everyday objects that have some weight to it on a stick.
Tape them at both ends of the stick first and then try rotating the entire contraption back and forth from its middle section. Afterwards, relocate the weights closer to the stick’s centreline and try rotating it again. You’ll see that the first version is noticeably more difficult to manipulate.
Tying Everything Together
Let’s review everything we’ve covered so far. First, angular momentum is contingent on angular velocity and an object’s mass distribution. One can change this momentum by exerting a twisting force or torque. However, without this force, the angular momentum remains in a conserved state.
In the context of a figure skater’s movements, there is minimal torque exerted on their body when it’s in the vertical spinning position due to the slippery ice and close proximity of the skates to the axis of rotation. This means that the skater’s angular momentum should stay constant. But as we’ve explained earlier, making any changes, such as bringing your arms inward, will lead to a decreased moment of inertia. Since the angular momentum must remain static, the angular velocity will then have to increase to conserve the momentum.
Conclusion
If you’re interested, you could also easily measure the angular velocity before and after you tuck your arms in and then use the data to calculate the change in the moment of inertia. But beyond taking these measurements, it’s best to leave the actual movements of this classic figure skating manoeuvre to the professionals since all that spinning takes a long time to get used to.
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