Projectile Motion

Unlike a slap shot, an NHL wrist shot isn't about brute power. It's about precision - putting the puck in the exact spot where the goalie can't reach it. A wrist shot is also a perfect example of what's known in physics as projectile motion. "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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transcript

LESTER HOLT, reporting: If an NHL slap shot is about brute power, then an NHL wrist shot is about laser-like precision, the ability to put the puck in exactly the spot where the goalie can't reach it.

BRENDEN MORROW (Left Wing, Dallas Stars): Probably the biggest advantage of a wrist shot is accuracy over power. If you want to get something done quick, you can just take a quick wrist shot that's very accurate.

HOLT: Brenden Morrow is left-winger and captain for the Dallas Stars. His wrist shot is more than just a display of quick reflexes and pinpoint accuracy, it's also a great example of what's known in physics as projectile motion. Projectile motion is the change in position of an object after it's propelled into the air. In this case, the hockey puck is the projectile. Its motion is caused by the original velocity supplied by Morrow's stick and the downward force of gravity.

Dr. ROBERT GEHRZ (University of Minnesota): The wrist shot is the most accurate shot in hockey because the player can direct the puck exactly where he or she wants it to go. And this is done by a combination of carefully pointing the stick when you shoot in the right direction and putting the spin on the puck to keep it stable as it's flying.

HOLT: Bob Gehrz is an astrophysicist at the University of Minnesota and has been supported by the National Science Foundation. At night, he plays left-wing for the Bent Blades, an over-60 hockey team in the Minnesota Wild Adult Hockey League.

GEHRZ: When we place the puck at the base of the curved stick and flip the stick over very hard with the wrist, as we move the stick forward the puck will gain rotation and spin and become a little gyroscope.

HOLT: To understand the physics of projectile motion, we filmed Brenden Morrow shooting his wrist shot with a special high-speed camera called a Phantom Cam. This technology allows us to capture an image of a spinning puck at 10,000 frames per second. That's about 160-times faster than the human eye can see, making it possible to examine each rotation of the puck as it flies through the air. The tight spin results in the puck acting like a gyroscope because it maintains its orientation as it moves through the air, a phenomenon known as the gyroscopic effect.

GEHRZ: A gyroscope is something that's spinning. So, when you turn the puck into a gyroscope by making it spin very rapidly, it wants to stay oriented the way it left the stick.

HOLT: Gyroscopes, even toy ones, have amazing properties, such as the ability to stay upright and balanced on a thin piece of string without tipping over. Beyond the gyroscopic effect, a spinning puck is faster and more stable because it has less surface area exposed to the drag of the passing air. It's the same concept as when a quarterback throws a football with a tight spiral.

GEHRZ: When you're looking at the wrist shot and you're watching the player put spin on the puck, that's what we call angular motion.

HOLT: Angular motion is the change in direction of an object around a fixed point or axis, such as a planet around its axis. The more angular motion, the greater the gyroscopic effect. For Morrow, to impart angular motion on the puck, he positions the puck at the heel of his stick blade. As he snaps his wrist, the puck rolls off the blade and towards the target.

MORROW: When you're following through, you're rolling your wrist and creating the force and momentum to guide the puck just with the strength of your wrist.

HOLT: The longer the puck is in contact with the blade, the faster it will spin. A puck with more spin has greater gyroscopic stability, which keeps it flying in the same direction it was aimed.

GEHRZ: The shooter wants to put spin on the puck because that rotation causes the puck to be very stable in the orientation that you let it leave the stick in.

HOLT: When a puck slides across the ice, it has what's called linear motion, the one-dimensional movement of an object along a straight line. But once a puck is launched into the air, a force begins to work against it: gravity.

GEHRZ: When something's in projectile motion, you have to take into account the fact that it's going like this. It's curving because of gravity. It goes up until it reaches its highest point and then it starts to come down again.

HOLT: This curved path is called a parabola. Even as the puck rises, the gravitational force between the earth and the puck slows it until it's no longer going up, and then causes it to fall faster and faster to the ice.

GEHRZ: All things flying through the air have a shape called a parabola and it looks like this. The height of the parabola is determined by how much energy you give the puck when you shoot it.

HOLT: During a game, it's almost impossible to see the parabolic path, much less the spin of a puck, but truth be told it's there.

MORROW: I’ve never been able to tell a difference but maybe goalies can, maybe that's how they cheat and know where the pucks are going but when I shoot, I'm just hoping that it hits the net.

HOLT: The physics of projectile motion, unleashed with just a flick of the wrist.