The Science of Hockey

I understand from usually reliable sources that there are some rather important hockey games being played this week. This works out well for me, because hockey is chock-full of interesting scientific phenomena.

I am not the first to notice this. The Exploratorium in San Francisco, the granddaddy of all science centres, has devoted an extensive portion of its Web site to The Science of Hockey. I’m just going to mention some highlights.

Let’s begin with the ice. Scientists have long thought (and I have dutifully written in previous columns) that either pressure or friction melts the ice, creating a thin layer of water that allows skates (and pucks, and curling rocks, etc.) to slide. This meant that warmer ice was slipperier than colder ice, because below a certain temperature–around -20 C–the ice was supposedly too cold for pressure or friction to create this thin layer of water. But new calculations show that skates and pucks don’t generate enough pressure to instantly liquefy ice. Instead, Professor Gabor Somorjai of the Lawrence Berkeley National Laboratory has found, ice is slippery because of a “quasi-fluid” layer that coats the surface–even when the ice is as cold as -129 C.

This “quasi-fluid” layer consists of molecules that are vibrating rapidly, as they would in water, but only in one direction–up and down. (If they also moved side-to-side, the water-like layer would become true water–which is what happens above freezing.) The warmer the ice, the thicker this quasi-fluid layer is. That may explain the perceived differences between “slow” and “fast” ice in hockey games–the more of the quasi-fluid layer a skater or a puck has to push through, the greater the drag.

Hockey may be slightly faster on “fast” ice, but even on “slow” ice, it’s pretty fast. Some NHL players can skate faster than 20 miles per hour. (I’ll be using non-metric measurements for the rest of this column because that’s what The Exploratorium–and the NHL–uses.) This is possible because there is so little friction between the slippery ice and the very small portion of the skate blade that is in contact with it. Skates are sharpened along both edges (and hollowed down the middle) both to minimize friction and to make it possible for the edges to dig into the ice, providing leverage when a player wants to accelerate and friction when he wants to stop.

All that speed makes for memorable collisions. The Exploratorium’s Science of Hockey site offers a handy calculator that allows you to pick two players, assign them skating speeds, and then find out the amount of energy that would be released by a head-on collision between them.

For example, according to the calculator, Eric Lindros (230 pounds) and Jeff Friesen (200 pounds), each traveling at 20 miles per hour, would collide with a stopping force of 777 pounds (ouch!), producing 7745 joules of energy–enough to shoot a puck more than 14 1/2 miles at an initial speed of 476 miles per hour or light a 60-watt lightbulb for two minutes and nine seconds.

Speed is also part of shooting, especially the slap shot, which can hurl a puck at the net at over 100 miles per hour. Part of that energy comes from the transfer of the player’s weight from legs to stick; part of it comes from the stick itself, which hits the ice a foot or more behind the puck and bows as a result. When the blade hits the puck, the bow is released, imparting the energy stored in the bent stick to the puck. There’s also a slight snap of the wrists, which causes the puck to spin; that makes it more stable in flight (because spinning objects resist being tipped).

Between the hurtling puck and the net stands the goalie, who has only 0.456 seconds to react to a 90-mile-per-hour slapshot from the blue line, 60 feet away–and only 0.152 seconds to react to one from 20 feet away. That makes reaction time the goalie’s most important ability. Although quick reaction times may be genetic, goalies can improve them with practice, as well as their ability to judge where a shot is headed as it flies toward them.

I’ve only scraped (or maybe Zambonied) the topmost layer of the Exploratorium’s extensive “Science of Hockey” site in this column. Replete with multimedia as well as text, it’s well worth checking out.

I’m sure you can squeeze in a few minutes between games.

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