# Welcome to physics of ice hockey

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Dr. John Blondin is a professor and head of the department of physics at N.C. State. Here, he breaks down some of the scientific principles that underlie the popular winter sport of hockey. Questions and answers have been edited.

Q. How do the laws of physics come together to help a player skate so quickly and adeptly on ice?

A. The simple answer is ice is slippery – but it is actually much more complex than that. You have to go down the atomic level to see how individual water molecules at the surface of the ice are able to easily break free of the ice and reattach. The result is that ice has very little friction under a variety of conditions – from the relatively warm Olympic arena to the extremely cold frozen pond in northern Saskatchewan. The blade of a skate allows the hockey player to control this friction. She can turn her skate blade sideways to cut into the ice without slipping and push forward with maximum force, or point the blades in the direction of motion to minimize friction and glide with high speed.

Q. Why are hockey sticks shaped the way they are?

A. Hockey sticks have changed in two major ways since I played as a kid, when \$4 would buy you a hefty stick of solid elm made with a flat blade. First came the creation of the curved blade, which gives a player better control of the puck, particularly when shooting. During the motion of the shot, the puck will tend to slide along the blade toward the center of the curvature. In this way, the puck will always be leaving the stick from the same point, giving the player better control over the shot. The second big change was the material, supposedly prompted by the wrath of Dutch elm disease that diminished the source of hockey players’ preferred wood. Today’s sticks cost closer to \$200 and are made of high-tech composite materials that are very light and have carefully controlled stiffness along the shaft to improve shooting.

Q. How is it possible that a hockey player can hit a slap shot up to 100 mph?

A. At first glance, the slap shot looks much like hitting a baseball with a bat, or driving a golf ball down a fairway. But the flexibility in the hockey stick changes the physics of the collision with the puck. In a good slap shot, the hockey stick will hit the ice slightly before the puck. The motion of the player will then bend the stick with considerable force, much like pulling back on the bowstring when shooting an arrow. As the blade snaps forward and makes contact with the puck, it is moving forward faster than just the motion of the player’s arms, allowing the shooter to transfer enough momentum to the puck to accelerate it to over 100 mph.

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