# Taking the Shot

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##### Above: Image © Letartean, Wikimedia Commons

I love everything about hockey, even the rancid smell of my equipment. As soon as I unzip my bag at the rink and smell that sweet stench, just like Pavlov’s dog, I start to salivate. Soon I will be on the ice playing hockey.

Did you know? There are many different types of bacteria and fungus that thrive on the conditions inside a hockey bag. Perhaps I should clean my equipment…

Besides the sweet, unmistakable, hockey aromas, I love the sounds of hockey too. Some of the best are the sounds of skates carving up the ice, of pucks thumping off the boards or pinging off the post, or of the crash of players getting checked into the boards. But nothing beats the sound of the shot named after how it sounds – the slapshot.

If hockey shots were in the mafia, the slapshot would be an angry don. When an angry don walks in the room, everybody is on edge, because something big may be about to happen, and anything can happen next. The same holds true in hockey. When a known hard shooter winds up to let a shot rip from the point, everybody in the line of fire needs to watch out.

Did you know? A puck shot from the blueline at 100 mph (161 km/h) will reach the net in only 0.34 seconds.

A frozen disk of vulcanized rubber traveling at speeds upwards of 100 mph is about to be sent into flight, and it may be coming their way. While a blast from the point is exciting for a fan (especially if the puck finds the back of the net), it turns out to be compelling from a science perspective as well. Just how does a slapshot make the puck go so fast?

How to Take a Slapshot

The key to a good slapshot is the conversion of angular momentum to linear momentum. Linear momentum is the product of mass and velocity, so, the more linear momentum that a puck has, the faster it is moving in a direction along a line (towards the back of the net if you’re lucky). The mass of the puck refers to how much inertia, or resistance to change in motion it has.

Did you know? The law of inertia is Newton’s first law. It states that an object at rest remains at rest and an object in motion remains in motion with a constant velocity unless it experiences a net external force.

To give a puck linear momentum, a hockey player strikes the puck with a stick that isn’t moving along a line, but that is rotating. This is where angular momentum comes in.

Angular momentum is the product of the moment of inertia and angular velocity. The angular velocity is how fast the motion of the stick sweeps out an angle in a certain direction. The moment of inertia is to angular motion what mass is to linear motion, and it depends on the distribution of mass about a pivot point. The further the mass is away from the pivot, the higher the moment of inertia will be.

By transferring their weight from their back foot to their front, and by rotating their torso, a hockey player generates a significant amount of angular momentum while shooting a slap shot.

Did you know? When spinning, figure skaters change their moment of inertia by changing how far their arms are outstretched from their body. If figure skaters start spinning with their arms outstretched, and then brings them in, their angular velocity will increase so that angular momentum is conserved. I have no idea how figure skaters don’t get dizzy and fall down when they do this.

So now we know that to make a slapshot go fast, a player must generate angular momentum while they shoot, but there’s more to the story…

The Sound of the Slapshot

The slapshot sounds the way it does not because of the sound the stick makes when the stick hits the puck, but because of the sound it makes when it hits the ice. Why would a player hit the ice first and not the puck? It makes sense when you look at a picture of a player’s stick during a slapshot – the stick actually bends.

During a slapshot, a player puts weight on their stick when it hits the ice, causing the stick to deform or bend. This bending stores energy in the stick, just like a loaded spring.

The stored energy is then released as the stick hits the puck and goes back to its natural shape, transferring the energy to the puck. As a result, the puck gains even more velocity than what it would get from angular momentum alone.

Did you know? The oldest known hockey stick sold in auction for \$2.2 million dollars. Is it currently on display in the hockey hall of fame.

While physics helps us understand the slapshot, it’s the countless hours that players spend refining their slapshot technique that allows them to maximize the amount of speed and accuracy of their shot. Let’s hope that all of this work pays off for players on a Canadian NHL team this year so the Stanley Cup can be paraded around a city north of the border for a change!

About bacteria in a hockey bag:

The Physics of Hockey, Alain Haché, Raincoast Books, 2002.

Exploratorium on Hockey

Michael is currently working on his PhD at the University of Toronto where he does experiments and computer simulations of fluid flow caused by a chemical reaction. You can think of this as an extremely weak explosion in liquid. When he’s not in the lab, you can find Michael playing bass with his band or in the gym playing basketball.

#### Michael Rogers

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