Work, Energy & Power

The slapshot is one of the fastest projectiles in team sports. In order to generate a 100 mile-per-hour (160 km/h) slapper, NHL players depend on three important physics concepts: work, energy and power. "Science of NHL Hockey" is a 10-part video series funded by the National Science Foundation and produced in partnership with the National Hockey League.

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LESTER HOLT, reporting: It's the shot heard round the rink. When a player in the National Hockey League rips a slapshot, he unleashes one of the fastest projectiles in team sports, with some pucks reaching speeds of more than 100-miles-per-hour, or about 160-kilometers per hour.

BRENDEN MORROW (Left Wing, Dallas Stars): The last time I had it measured was a long time ago, I think I was in the eighties somewhere. I was nowhere near the 105 that the top guys shoot at nowadays.

DR. JIM GATES (University of Maryland): A slapshot is perhaps one of the most dramatic demonstrations that you can see in hockey because at that point you see a player generating an enormous amount of force and energy to make a puck move as quickly as possible.

HOLT: Jim Gates, a professor of physics at the University of Maryland who has been funded by the National Science Foundation, says that from wind-up to follow through, a slapshot is a perfect illustration of the concepts in physics known as work, energy and power.

HOLT: To understand these concepts, we filmed Dallas Stars captain Brenden Morrow with a Phantom camera, a high-speed digital camera that shoots at up to 10-thousand frames per second.

DIRECTOR: And action!

HOLT: The first ingredient to Morrow's stinging slapper is energy - the power that will propel the puck into action.

GATES: If you want an object to move fast, you want to put as much energy in its motion as possible.

HOLT: In an NHL slapshot, two types of energy can be seen - kinetic and potential. Kinetic energy is the energy of movement.

GATES: Kinetic energy is always connected with motion. So, the hockey player moves his arm. There's motion of the arm. Therefore, there's kinetic energy associated with the arm. The stick that the hockey player holds is moving. Therefore, there's energy in the motion of the stick.

HOLT: As Morrow shoots, he generates kinetic energy as he rotates his body and swings his stick at the puck, shifting his weight from his back leg to his front. But just before hitting the puck, Morrow's blade actually strikes the ice first. Instead of slowing his shot, the brief collision with the ice causes his stick to flex, loading it with potential energy. Potential energy is energy that is stored in an object. Although it happens in the blink of an eye, NHL players are conscious of this trick and do it intentionally to increase the speed of their shot.

BRENDEN MORROW (Left-Wing, Dallas Stars): You want to get as much torque in, in your stick as possible so you hit behind the puck causing that that torque in your stick and the whip forces the puck in the action. And then when you let it go from the ice, the stick does all the work, it just whips.

HOLT: As the flex in the stick is released and the blade makes contact with the puck, the potential energy of the stick is converted into kinetic energy, some of which is added to the puck.

GATES: So all that energy that was in the arm and in the stick has now been converted to the energy of motion of the hockey puck.

HOLT: The Law of Conservation of Energy states that energy can be neither created nor destroyed, but it can change form. In the slapshot, much of the energy of the swing goes into launching the puck, but some of it is converted into other forms of energy, such as sound waves or heat from the friction of the collision. Once Morrow strikes the puck, he has done work, and we aren't talking about his day job. In physics, work is when a force acts upon an object resulting in the displacement of the object. It's possible to calculate work by multiplying the amount of force applied by the distance that the force is applied. For example, if we know the amount of force the stick applies to the puck and the distance that the stick is applying the force to the puck, we can figure out the amount of work done.

GATES: For a physicist, the ability to do work is how we measure energy.

HOLT: The final measure of the slapshot is power. While we can see that a slapshot is powerful, we can use physics to quantify it. Power is the measure of the amount of work done over time.

GATES: The smaller the amount of time that it is in contact, the more power the hockey player generated to cause the motion. So it's actually change in energy over change in time.

HOLT: If we know the amount of work done and how long Morrow's blade is in contact with the puck, we can calculate the power of his slapshot.

DIRECTOR: Woohoo! Cut!

HOLT: Work, energy and power. Three key components behind the physics of an NHL slapshot.

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