Physics Of The Winter Olympics

If you’re like me, you enjoyed watching the Winter Olympics. Wasn’t it
exciting to see these exceptional athletes perform at such a high level of
competence and grace?
Today we’ll examine a few physics principles behind some of the
competitions we saw on TV.
First of all, let’s see what we can learn about figure skating. Whenever I
see a figure skater go into a spin, I’m tempted to tell my wife, “Of course you
know this can be explained by conservation of angular momentum, right?”
Her response might be to give me the look, which means “Why can’t you
just watch and enjoy this like everyone else?” So I don’t usually say anything.
But since I’m not married to you, I’ll proceed.
Angular momentum is defined as something called the moment of inertia
times angular velocity. Angular velocity is related to how many complete turns
the skater makes every second. We will call this her rate of rotation. The moment
of inertia of skaters is larger when they have their arms spread out, and is smaller
when their arms are close to their body.
You know you’re talking to a physicist when he or she says, “Let’s
approximate you by a cylinder.” Or, even worse, “by a sphere.” At which point,
you might say, “So you think I have the shape of a ball or a can of soup? How
insulting!”
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Don’t take it personally, okay? All physicists know is cylinders and
spheres.
The following comes from the web site “The Physics of Everyday Stuff”:
“A crude approximation of the skater’s shape, good enough for the purpose here,
says that she is a solid cylinder made up of most of her mass plus three rods
representing her arms and a leg.”
Thus we have below our representation of a lovely figure skater with her
arms and one leg extended. (Notice the grace and beauty.)
Next we see our figure skater with her arms and legs pulled into her body.
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Now, as we said before, according to the principle of conservation of
angular momentum, the moment of inertia times the rate of rotation is a constant.
Don’t panic! This isn’t that hard. We have the product of two numbers
equals a constant. It’s just like: 16 x 1 = 8 x 2 = 4 x 4 = 2 x 8 = 1 x 16
When the skater has her arms and one leg extended, she has a large
moment of inertia and, therefore a small rate of rotation. But when she pulls in
her arms and one leg in, she has a small moment of inertia, so she will have a
greater rate of rotation.
The web site “The Physics of Everyday Stuff,” estimates that the moment
of inertia when a skater has her arms and one leg extended is about 12 times
greater than when she has her arms and legs tucked in. What this means is that
if a skater is rotating at 2 revolutions per second with her arms and leg extended
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But when her arms and legs are pulled in, she will rotate up to 24 revolutions per
second! This is only an approximation because, of course, skaters are not
cylinders. (I do understand that, okay?)
Conservation of angular momentum also explains why high divers, when
they want to spin fast, tuck into a ball. But when they want to spin slow, they
extend their body straight out.
The Popular Mechanics web site gives additional insights into figure
skating. The following is quoted from “The Science Behind 7 Winter Olympic
Events”:
“A 45-degree jump gives skaters 0.55 seconds of time–enough to complete
all but the devilish triple axel, which requires 0.65 to 0.75 seconds and a spin rate
of 420 rpm, the engine idling speed of some cars.”
The most baffling Winter Olympic sport to me has got to be curling. I spent
hours watching it, not because I cared who won, but because I had no idea what
was going on.
The same Popular Mechanics web site explains something about curling:
“In curling, teams slide a 42-pound granite stone down an ice sheet toward
a target…A liquid layer (on the ice) reduces front-ward friction, and the stone
spins and slides in the same direction. This is where the sweepers get involved.
Two players use brooms to scrub the ice ahead of the stone, enhancing the liquid
film in order to adjust curl (how much the stone veers to either side) and the
length. The U.S. squad’s tests have shown that sweepers can “drag” a stone up
5
to 16 extra feet.”
It makes sense that rubbing the brooms just ahead of the stone could
cause some of the ice on the surface to turn to water, which reduces the friction
between the stone and the ice. The same thing happens when a driver on an icy
road gets less traction by floor-boarding it. The spinning tires cause some of the
ice on the surface to turn to water, which makes it even more slippery, so the
driver has little chance to move forward. It’s the same idea except with a sliding
stone.
Next Olympics why not come over and we’ll watch figure skating together?
Please, my wife is pleading with you!