The Physics of Skating and Slap Shots

Fairbanks, Alaska - 

When two NHL hockey players collide, their pads and muscles can absorb
enough energy to power a 100-watt light bulb for a minute and a half.
During the 60 minutes of a hockey game, players can burn 6,000 calories and
lose up to 15 pounds.

These are the calculations of Alain Haché, a physicist at the University of
Moncton in New Brunswick, Canada. An amateur goalie, Haché combined
two of his passions in his books “The Physics of Hockey” and “Slap Shot
Science.”

Haché examined the physical properties of ice, one of Alaska’s most
abundant natural resources.

Friction at the contact points between surfaces is what slows most sports
down, but the low friction coefficient of ice makes hockey players faster on
their feet than the athletes of any other team sport.

Many NHL players like Connor McDavid and Nathan MacKinnon skate
faster than 20 miles per hour every game. Hall of Famer Bobby Hull was
once clocked at 29.3 miles per hour. Scientists once tracked Hull during a
game and calculated that he had skated more than eight miles during his 29
minutes on the ice.

While everyone agrees that ice is slippery, scientists have debated why.
Haché wrote about a study in 2000 by researchers at the University of
California at Berkeley that revealed one of ice’s slicker secrets.

The California scientists used an atomic microscope to find a thin, wet layer
of quasi-fluid water on the surface of an ice crystal. That layer provides a lubricant, even when ice is very cold. Without it, the friction of ice would be
comparable to concrete, Haché wrote. This microscopic wet layer and other
physical properties of ice make a sharpened skate on ice one of the fastest
non-motorized ways of propelling the human body.

Though downhill skis move fast, the friction between a waxed downhill ski
and snow is 10 times greater than that between a steel skate blade and ice.
Using the magical surface of ice to combat friction, hockey players are able
to generate impressive amounts of energy. Haché calculated that a 180-pound
NHL forward has more kinetic energy while skating at full speed than does a
charging National Football League lineman of 300 pounds. He also
calculated that a hockey player’s pads and flesh will compress as much as
four inches during a collision with another player.

A goaltender on a series of teams since he was a pee-wee, Haché devotes a
chapter of his first book to facts about that position. A goalie’s huge leg pads
and other equipment cover 60 percent of a shooter’s target, the 4-by-6 foot
goal made of red pipe and netting. Those leg pads, which cover 20 percent of
the net space all by themselves, are made of leather and light synthetic
padding because a hard plastic shell would allow the puck to bounce too far.

Hockey players have blasted slapshots faster than 100 miles per hour using a
three-stage process, Haché wrote.

First, a player rotates his or her torso and stick in an accelerated motion
toward the puck. Next, the stick blade touches the ice and the puck as the
stick bends for a fraction of a second, storing energy like a loaded spring. The
puck accelerates off and the stick returns to its original shape as the player
completes the swing. The puck rockets toward the goal as the player converts
angular momentum (stick and torso swing) to linear momentum (the puck
traveling a straight course for the goal).

Current players who demonstrate the extreme end of this technique include
Tage Thompson of the Buffalo Sabres, whose fastest slapshot registered 106
miles per hour.

Since the late 1970s, the University of Alaska Fairbanks Geophysical
Institute has provided this column free in cooperation with the UAF research
community. Ned Rozell ned.rozell@alaska.edu is a science writer for the
Geophysical Institute. A version of this story appeared in 2004.