## Monday, February 10, 2014

### Science of the Olympics: Jumps, Spins, and Other Awesomeness

Continuing the fun of my Science of the Olympics blog series, it's time to take a look at how skiers, snowboarders, and figure skaters do all those totally awesome tricks they do.

To understand these moves, we're going to dive into a bit of a physics lesson. Let's address the most basic idea in physics first: forces.

The way I explain forces to elementary school kids is that forces are pushes and pulls. Forces themselves are invisible, so you can't actually see a force, but you can see the effect it causes. Gravity is an easy example of a force, and especially great for thinking about the invisibility factor.

Other examples include the force of friction, the electrical force, and forces generated by your own muscles.
 The force of friction is less on ice, so skaters can "stay in motion" longer.

Do you remember Newton's laws?

An overview:

1. Objects in motion stay in motion (and objects at rest stay at rest), unless a force acts upon them.

2. Force is equal to mass x acceleration.

3. Forces come in pairs. For every force observed, there is an opposite and equal force.

Now that we've gotten that out of the way, on to the flips and jumps and stuff!

The Science of Jumps

Let's go basic first. Jumping is at the core of not just figure skating, but many other winter sports, such as snowboarding and skiing.

By pushing off the ground, an athlete creates a vertical velocity--that is, an upward motion at a particular speed. The bigger the push, the higher they go. But what the push is actually doing is showing off Newton's Third Law (see above). The athlete pushes into the snow and/or ice, and then the snow and/or ice pushes back. That's what allows for lift to occur.

There is another way athletes get into the air in the Olympic Games, and that is to fling themselves off an edge. Eep. Think ski jumpers, or snowboarders on the halfpipe. In these cases, what propels the athlete upwards isn't just their legs pushing down, but rather the momentum of the athlete prior to reaching that edge they're going to fly off of.

Momentum is mass x velocity. A bigger mass will create more momentum, as would a larger velocity (which, by the way, is speed with an assigned direction). Skiers, for example, gain velocity by traveling downhill rapidly before doing the flinging-themselves-off-an-edge thing.

But what about once an athlete's airborn? At that point, they can go onto do loads of other awesome stuff.

The Science of Flips and Twists

So once an athlete has gotten into the air, sometimes they do amazing things like flip themselves around. They'll accomplish this by shifting their mass and letting gravity assist. Gravity is a pulling force that conveniently always pulls in the same direction here on Earth: down.

With the help of gravity, athletes can tap into a rotational energy and start utilizing angular momentum. What that means, is that they use gravity to tip them and start a spin, and then pull their body inwards to keep that spin going. This causes them to flip around in the air, in whichever direction they shifted their mass towards. And the more angular momentum the get, the faster they will twirl through their flip or twist.

Now, to make this possible, they'll need a pivot point--a place in space to rotate around. And this leads us nicely into my final section for this post...

The Science of Spin

In this diagram, I've made two different arrows to help make sense of angular momentum.

Imagine going around this green pivot point on the black oval I've drawn. The trajectory out by the blue arrow is farther from the pivot point, and the trajectory near the red is closer. Both have the same momentum, but since the red arrow section is closer, it moves past the pivot point much faster than the blue arrow's section does.

Moral: Closer in = faster spin. This is the Law of Conservation of Angular Momentum. Angular momentum stays the same at every point of orbit, and is calculated by multiplying a few elements, such as velocity and radius. To make sure this law is heeded, velocity (speed in a certain direction) increases when the radius (distance out from the center) decreases. Simple as that!

A smaller radius for a bigger velocity. It's a trade-off that all these Olympic trick-doers rely on, but perhaps none so beautifully and dramatically as the figure skaters.

Figure skaters use the law of conservation of angular momentum to increase the speed of their spin as they go. They start spread out (big radius), and pull in their body to a tight form (small radius) as the spin goes on. This results in a spin that appears to magically speed up as it goes.

So as much as scientists and athletes stereotypically don't have much to do with one another, the pinnacle of athletes people--aka Olympians--actually rely on science a great deal to make them look as awesome as physically possible. Heck yeah! Gold medal for Physics!