February 23, 2010 | 4
Figure skating is one of the most popular sports in the winter Olympics. In this exclusive Scientific American video, contributing editor Christie Nicholson takes you inside the sport, to explore the physics behind a figure skater’s spectacular moves. Along the way, she discovers her inner Kristi Yamaguchi.
Presented by Christie Nicholson
Filmed and edited by Eric R. Olson
Have you ever wondered how these Olympic figure skaters float so fluidly over the ice or spin so fast through the air?
Well, Olympic figure skaters endure years of physical training but there’s something else that’s crucial for their success and that’s the law of physics. And like all physical systems a figure skater’s body depends on three things and that’s energy, motion and mass.
But first of all, let’s take a look at ice. We wouldn’t have ice skating at all if it wasn’t for a very unusual property that makes ice slippery. For instance, if we tried to skate over another type of solid like glass, we’d quickly learn why glass skating never caught on. There’s simply too much friction between the metal of the skate’s blade and the surface of the glass.
But ice is different. Even at temperatures well below freezing, its great for skating. And that’s because water molecules on the surface of solid ice form very loose chains, as opposed to water molecules in an ice crystal at the center. And what these loose chains form is a frictionless, almost liquid-like surface.
But even on a nearly frictionless surface like ice, it takes a force–Newton’s second law–to get a skater’s mass moving. And in the human body that force starts out as something we can’t see, as potential energy trapped in muscle cells. And these cells contain something called ATP, a kind of molecular spring that stores the energy from food.
When a nerve impulse from the skater’s brain signals her leg muscles into action, millions of ATP molecules pass their energy onto protein fibers in her legs. And these fibers act in concert to extend her leg, pushing against the ice and turning potential chemical energy into kinetic energy–the energy of motion.
So to move forward on the ice what you do is dig your blade in and you push back like this to move forward. But if I wanted to move up, as in a jump, what I do is I push down, the ice generates an upward force and its that force that propels me in the air.
But what happens when a skater starts spinning and then get’s faster and faster and faster, long after she has pushed off the ice. Well here its physics again and the idea is that momentum must always be conserved.
It works like this. As a skater spins she is carried along by a rotational momentum and this momentum depends on two factors–how far the skater extends out from her own central axis and her speed of rotation. Once she is spinning the only factor the skater can control is the distance from her own center, so she pulls her limbs inward, and because her momentum must be conserved the spin gets proportionally faster.
The same principle applies to spinning jumps, the skater starts with her arms outstretched and pulls them in tighter to get the most spins in the air. Of course, you need a lot more than physics to be a good skater and from the looks of me, I need a lot more practice.
For Scientific American, I’m Christie Nicholson waiting to get back on the ice.