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Professor of the Puck

75-year-old astrophysicist & longtime adult-leaguer teaches about the physics of hockey & how a puck reacts to a slapshot

by Bob Condor / @NHLSeattle_ /

SEATTLE -- As the last NHL goalie to play his position without a mask, the late Lorne "Gump" Worsley was a one-man test lab to prove out the physics of a hockey puck.

"Gump played his final seasons here with the Minnesota North Stars," says Robert Gehrz, an astrophysicist at the University of Minnesota in Minneapolis. "He would say he did get hit in the face with the puck, but always on the flat side [top or bottom), so it didn't hurt much."

Gerhz is a 75-year-old full professor who plays left wing in two leagues operated by the Twin Cities Adult Hockey Association, one for 60-plus players and other for "the 70-plus old guys." 

Who better to explain the physics of the puck?

"Gump mostly played in the era of the flat-bladed stick," explains Gehrz. "The puck tends to flutter off a straight stick blade."

The result is a puck that loses speed and force as it approaches the goalie, who benefits from even a split-second more time to react. But with a curved blade, Gerhz says today's players can control the puck and "point the end of the curved blade to where players want the puck to go."

What's more, with the curved blade shot, the puck spins tightly in the same plane-"maximizing velocity" says Gehrz and conserving what physics students know as "angular momentum." It makes for a faster, more accurate shot with the puck spinning like a disc or gyroscope with the edges of the puck as first contact. It can be compared to a quarterback throwing a tight spiral on a football; there is less mass to create drag in the air.  

The conservation of angular momentum is "what happens when a competitive figure skater tucks in her arms when spinning to finish a routine," says Gerhz. The pirouette is more dramatic and spins faster and longer. 

Some facts about the puck: It is made of vulcanized rubber. This process of hardening rubber allows for a high level of wear and tear on the puck during a game. Well, plus "vulcanized" sounds cool, right, Mr. Spock?

The regulation puck weighs six ounces and is one-inch thick and three inches in diameter. It is black to make it easier for players and on-ice officials to see the puck against white ice. The color was once a point of contention for television viewers and broadcasters because it could be hard to follow the puck. High-definition screen technology has solved that problem.

You may have heard hockey pucks are frozen prior to being used in NHL games. Same for many other leagues in organized hockey, including the American Hockey League, which features top development teams affiliated with NHL clubs. The NHL protocol is to keep pucks frozen between 14 and 20 degrees Fahrenheit in a cooler stored and available ice level. The home team is responsible for freezing and providing the pucks.  

Back to physics: The freeze treatment reduces the level of friction the puck encounters on the ice surface, enabling it to travel farther without sticking (one reason why all of our outdoor rink games without the precisely frozen puck tend to slow down). The sweet spot high-teens puck temperature greatly reduces bouncing. Players are the beneficiaries, working with pucks easier to keep on the stick or control when receiving a pass.

NHL referees and linesmen will swap out pucks during breaks in the action to keep the puck movement at a maximum. The typical NHL game requires 15 to 20 pucks due to officials swapping in a new one or a puck flipping over the glass into the crowd. 

During this spring's NHL playoffs, pucks with a thermochromic coating were used to help officials recognize pucks that were too warm. The printing on the postseason frozen pucks was purple, changing to clear when the puck's temperature goes above freezing. Before faceoffs, officials could more easily determined if it was time to swap out pucks. 

While players themselves bump into each other dozens of times per game, the puck is colliding the most: sticks, boards, off players. There are two kinds of puck collisions: elastic and inelastic.

The elastic collision basically means the force of the puck is met with equal reaction, such as puck hitting the side or end boards and bouncing right out. The inelastic collision is what goalies crave; it means the force of the puck is mostly absorbed. In the case of a shot on goal, the goaltender is looking for the most inelastic collision, say, hitting a piece of padding that deadens the action and stays near the goalie to hold or clear. A less elastic collision, maybe off the stick blade, can provide enough of a reaction to create a rebound or second shot. 

Gehrz is a leading expert in the study of novas and supernovas, a class of stars that show a sudden gigantic increase in brightness and then slowly return to its original state over months. A supernova can shine as bright as an entire galaxy. 

READ: Sticking to it - Why Stickhandling is Trending Up

But Gehrz also knows his fair share about the physics of a slapshot, the wind-up above the shoulders and let-it-rip attempt to score. Some NHL players can top 100 miles per hour with their slapshots and the league-wide average is somewhere between 85 to the mid-90s, depending on who is measuring. Boston's six-foot-nine defenseman Zdeno Chara has the fastest slap shot on record, topping 108 miles per hour. 

"What lots of fans don't know is all slapshots follow a parabolic arc," Gerhz. "It means the shot will drop. Gravity makes it so."

To help with the visuals, Gehrz points out the baseball home run follows a parabolic arc. Pretty much a what goes up, must come down proposition. The difference between home runs and slap shots is effectively distance traveled; it is much easier to track the drop of long fly ball. 

Players with the best slap shots are actually striking the ice with their stick blades about an inch or two behind the puck. While this seems counterintuitive for maximum miles per hour, what's happening is the striking of the ice first transfers energy to the stick, which then releases and rapidly shoots the puck in whip-like fashion. The player is simultaneously rolling the wrists to open the launch angle of the stick blade to lift the puck off the ice.

Luckily for us, including any would-be slapshot artists out there, Gehrz has calculated just how much a slapshot will drop.

"Let's just say the shot travels 100 miles per hour for simplicity," says Gehrz. "If you shoot from 30 feet [halfway between the blue line and goal line in the offensive zone], the puck will drop seven inches. You actually aim the shot higher to allow for the drop. If you shoot from the blue line or 60 feet [commonly performed by defensemen on the offensive end], a 100-mile-per-hour slapshot will drop two and a half feet. "

Many coaches like a shot from the blue line, aka the point, to stay low. In this case, a player will modulate the force and torque applied to the puck to create a lower parabolic trajectory.

How's that for a physics lesson? There will be no quiz, but allow Professor Gehrz one more set of calculations.

"I figured out that I have played more than 3,000 games of adult league," says Gehrz. "I have averaged once or twice a week for a long, long time. I used to play defense, but with two artificial joints [hip and ankle], I can't skate backwards so well anymore. So I stick to playing left wing. It is so much fun to still play a kids' game."

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