Crosschecking the physics of hockey

Jun 8 2010 - 7:14pm

Justin Burton, a University of Chicago physicist, is no hockey fan. He comes from Cincinnati, and therefore roots for the Reds (baseball) and Bengals (football). But he has been giving a series of lectures this spring called "The Physics of Stuff." As the Blackhawks and Flyers face off in the Stanley Cup finals, he's been thinking about the game's dynamics, and answered a few questions for the Chicago Tribune:

Q: How violent are the collisions between opposing hockey players as they race across the ice?

A: Hockey is interesting because players are traveling faster than in almost any other sport. At maximum speed, they are skating up to 24 miles an hour, and a collision will generate thousands of pounds of force for a split second, which is why hockey injuries are commonplace. A 200-pound hockey player skating 24 mph carries as much kinetic energy as a 350-pound football player running 18 miles an hour.

Q: What principles of physics are at play with the slap shot?

A: Hockey players are taught to swing the blade so it hits the ice about one foot behind the puck. As the hockey stick slides across the ice, it can bend to almost a 30-degree angle. When the blade strikes the puck, it launches the puck at more than 100 miles an hour because of all the energy in the bent stick. If the player cleanly strokes the puck without striking the ice, the maximum speed would be much lower. You really have to get the stick bent.

Q: Why is ice slippery, allowing both skates and puck to glide across it?

A: That question has a long, interesting history that goes back to the 1860s, when (renowned physicist ) Michael Faraday theorized there is a very thin layer of water on ice, demonstrating it by holding two ice cubes together, resulting in them freezing together.

Later there was a theory that skaters could glide across the surface of ice because the pressure of the skater's weight on the blade causes the ice surface to melt. That proved not to be true.

Faraday was right, there is a thin layer of water on the ice surface even at 328 degrees below zero Fahrenheit, but we still don't know why. Because it is so ubiquitous, it's hard to think of water as a weird organic chemical with a lot of strange properties. Ice floats because water in its solid state is less dense than in its liquid state -- for example, the opposite of alcohol.

Q: How fast does a puck have to be flying to knock out seven teeth in a player's mouth?

A: It is probably not so much a matter of speed on impact, but how squarely it hits the player's teeth.

Q: How does the curvature of the hockey stick blade affect the velocity and trajectory of the puck?

A: The blade is shaped in a concave curve that helps the player keep the puck in position as he skates. As the player strikes the puck, the curve helps lift the puck off the ice, and by giving the puck a little twist as it comes off the stick, it makes it fly with less wobble, giving it an angular momentum like a Frisbee, nice and straight.

Q. What would do the most harm, to be hit by a flying hockey puck, a 100-mph fastball, an errant Tiger Woods drive, or Brian Urlacher leaping at you as you attempt to sail over your offensive line for a touchdown?

A: It is going to be a matter of how much kinetic energy the object is carrying. A golf ball, I think, would do less damage. I know a baseball hit off the bat has more kinetic energy than a flying puck, but a puck has about 30 percent more energy than a pitched baseball. I think Urlacher would be worse by a factor of 2 to 3 over a hockey puck if it is a head-on hit. A football helmet might protect you from a puck, but a collision with Urlacher would probably do some damage.

Q: Why is Philadelphia Flyers defenseman Chris Pronger such a jerk?

A: There are some questions that physics simply has no answers for. This is one of them.

 

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