Force, Impulse & Collisions

NHL hockey pucks are made of vulcanized rubber and weigh between 5.5 and 6 ounces (160 - 170 g). During a game, every movement of the puck follows the laws of physics and illustrates the concepts of force, impulse and collisions. "Science of NHL Hockey" is a 10-part video series produced in partnership with the National Science Foundation and the National Hockey League.

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LESTER HOLT, reporting: The swings, the slams, even the saves.

In the National Hockey League, things collide, but nothing on the ice takes as many hits as the puck. Weighing in at 6-ounces, this 1-inch by 3-inch black cylinder is made of vulcanized rubber. It's often kept in the freezer before games to minimize its elasticity.

ERIC JOHNSON (Defenseman, Colorado Avalanche): When the pucks bounce around, we call that ‘grenades.’ It's just kind of bouncing everywhere. You don't know where it's going to go and it blows up-- it blows up and then it hits your stick.

HOLT: During a game, every movement of the puck follows the laws of physics and illustrates the concepts of force, impulse and collision.

Force is the push or pull that can accelerate an object by changing its velocity or by changing its shape.

Dr. JIM GATES (University of Maryland): In order for something to change its state of motion, there has to be an unbalanced force.

HOLT: Jim Gates is a theoretical physicist at the University of Maryland and has been supported by the National Science Foundation.

GATES: Suppose that, as the hockey puck is sitting there, I suddenly come along and hit it with my hockey stick. I exert a force in this direction. There's nothing pressing back in that direction. That unbalanced force causes the acceleration.

HOLT: The force on an object is equal to its mass multiplied by its acceleration, commonly expressed as "F-equals-M-A." But scientists often write it as "A-equals-F-divided-by-M", which makes it more obvious that the greater the force applied to an object, like a puck, the faster it accelerates.

GATES: When you come with a stick and you rap that hockey puck really hard, you're applying a big amount of force to it. And so, it accelerates off really fast. On the other hand, you just give a gentle tap, and it just leisurely slides across the ice.

HOLT: The passing and shooting of a puck are also examples of an impulse, which is the measure of a force applied for a time.

Dr. ROBERT GEHRZ (Minnesota Institute for Astrophysics, University of Minnesota): The impulse is an important concept in hockey because most of the time you are applying forces for only a very short time.

HOLT: Robert Gehrz is an astrophysicist at the University of Minnesota and has been supported by the National Science Foundation.

GEHRZ: For example, when you shoot a puck by swinging your stick at it, the force you apply to the puck acts for just a split-second, then the puck is gone. That's an impulse.

HOLT: To calculate an impulse, the applied force is multiplied by the amount of time it's applied, represented by Delta-t.

On a graph indicating force versus time, a larger force... or a longer time for the force to act, creates a bigger impulse. The bigger the impulse, the bigger the change in the speed of the puck. Conversely, a smaller force or shorter time creates a smaller impulse.

GEHRZ: So when you're dribbling the puck, you're applying little impulses to move the puck back and forth. But you don't want to apply a big impulse because the puck will get away from you.

HOLT: Once the puck starts, it will continue to move until another force acts on it - like a collision with the boards or a goalie's glove.

JAROSLAV HALAK (Goalie, St. Louis Blues): All the goalie coaches, they’re trying to teach us, stop the puck first, you know. But the best thing is to catch the puck and with no rebounds.

HOLT: A collision is when one object touches another object. It's called an "elastic" collision if the object bounces off and retains its energy of motion.

GEHRZ: Well, the boards are pretty hard and they’re pretty massive. And so when the puck hits the boards, it basically bounces back with the same speed that it was coming in at.

HOLT: During an "inelastic" collision, some of the object's energy is transformed into heat through friction or deformation, or into some other type of energy.

GEHRZ: An inelastic collision would be exemplified by a puck hitting a player. The player has soft padding on and so the puck just sort of dies when it hits. This is what a goalie wants--a totally inelastic collision so the puck drops on the ice in front of him and he can smother it.

PEKKA RINNE (Goalie, Nashville Predators): You're going to trap a lot of pucks against your chest protector. You're going to use your pads that you wear on your legs. Even sometimes you use your skates. So it's, uh, anything you can get in front you're going to use, even your head.

HOLT: Whether the forces and impulses are working with it or against it, for the puck, a hockey game is one collision after another, all the way to the net.