Hello! My name is Brad Orr.
I am the Chair of the Physics Department at the University of Michigan and today we are going to talk about the Physics of Hockey. We are doing this because on December 11 there is going to be a hockey game between the University of Michigan and Michigan State in the Big House called The Big Chill. On that day we are also going to be having a lecture on the Physics of Hockey in the Physics Department. We will be talking about shooting, checking, and ice. At the core of hockey is ice. To form ice you need water. Water is a wonderful substance found on earth, is responsible for all of life and it’s unique; one of the unique things you see everyday actually. If you go to a party, you might be tailgating before The Big Chill and you put your ice in your soft drink, you will notice something about it right away. The ice floats. If this were almost any other substance, you would take the frozen material, it would sink in the glass. Now, why is it critical? If ice was a normal material, it would form at the bottom of the lake and we would have to be playing hockey with scuba gear on. But we don’t. We play hockey on the surface. That is the critical part about ice floating. There is another part however that people
have mistaken for hundreds of years, what makes ice so slippery. They said, okay, I take a skate, I put it on to the rink, I step on it, I compress it, I have just formed water. This was the theory of why ice was slippery. We had a water layer on the ice for a long time. Turns out that is not the case. It is true
that with pressure ice will melt. Come on over with me, I will show you an experiment. Here we have two experiments to
show that pressure can melt ice. We have a very thin wire and a heavyweight cutting through this ice cube right now. Even though the ice cube is colder than 32°, we are starting to see the wire cut into it. It’s a very thin wire, very high pressure. Just as I said before, if you compress ice, you melt it. The thick wire, however, which is the thickness of a skate blade, is also trying to cut through the ice and it’s not being successful. That is why the pressure melting of ice is
not the correct theory for why ice is slippery. Now, let me show you what really is going on. So now we are going to talk about why ice is really slippery. What is really going on? So here we have a tray of ice and a hockey puck, and like everyone knows, the hockey
puck slides on this ice nicely. Is it just because the ice is smooth? Well, it’s more than that. We have a very smooth surface here of glass and the puck doesn’t slide it all. So what’s the difference between the water and the ice? The difference is that there is a layer of lubricant on the ice, and if I put a layer of lubricant on this glass; this is just ordinary Windex and I smooth it out a bit, now it slides just like ice. The difference is, we have a fluid layer on the glass. So here we have a crystal and everything that’s above absolute zero shakes. So this crystal shakes just the way ice is shaking. The key thing I want you to look at is how
much we are shaking at the bottom and how much we are shaking at the surface. The bottom is not shaking very much, whereas the surface of the ice is shaking a lot. This is just what happens in ice and in fact, this is the naturally occurring liquid layer on the crystal. It just occurs there because atoms shake. So what we are going to do now is we are
going to do this model experiment and turn it into the real thing. We had our ice here and now we have also ice . The difference is this ice has been cooled to liquid nitrogen, to -321° Fahrenheit. Let’s make sure it’s cold enough. Essentially what I am doing is pouring liquid air onto my ice. What we are doing is we are freezing these top atoms. The question is, will a puck slide across this surface? And now the experiment. It doesn’t slide very well. It’s just like sliding the puck on the dry glass. Hockey is all about shooting and scoring. Today I am going to demonstrate shooting a hockey puck and Shawn is going to demonstrate stopping a hockey puck. So we have a piano across the room, which is almost exactly the right size as a goal,
6 feet by 4 feet. Shawn is going to protect it; I am going to score on him. We have a bow and arrow here and a special puck. I am going to shoot Shawn with this, and hopefully I am going to score.
Are you ready Shawn? Yeah! Good save! So just what does this bow and arrow
have to do with a hockey shot? A hockey stick is actually a very elastic object. When you take a shot, you bend your stick. When you hit the puck, and actually good players will hit the ice first, bend their sticks, store energy in it, then when you shoot it, it springs forward just the way
a bow and arrow does. When you combine the strength of a player swinging the stick, plus the elastic energy stored in this, like the bow and arrow, a good hockey player can shoot at over 100 miles an hour. What that means for a goalie is that from the blue line, it takes less than half a second for that puck to get in the net. So now let me demonstrate how the
stick actually stores the energy. Here we have the shaft of a stick, we are missing the blade, but we are going to put a hockey puck on it. Instead of me taking a slap shot, I am going to bend this down and shoot the puck up into the air. I will try to bend the stick down about 6 or 7 inches. That’s what a good hockey player can do. Hockey is a sport of collisions. Some collisions we want, like a stick hitting a puck. Others collisions we want to be careful about, when a player hits the boards or where a puck hits a player. We want to take care about that and pad them so they don’t get injured. I have an experiment here to show you how
padding helps avoid injuries in hockey. One of the most important parts of a person’s vertex is his skull. Every hockey player is required to wear a helmet. The helmet is padded on the inside and we have an experiment here that shows you how padding will help protect someone’s skull. So an egg, which has a hard outside
and a soft inside, is like your head. If we put it into this helmet, tuck in the chin strap. Now we are going to have a collision. Let’s strap our player in and load him up. Imagine these are the boards. We are now going to collide our hockey player into the boards. So look, the skull is fine and we have no concussion. So what happens if a person goes out to play hockey and doesn’t put on his helmet? So now we are just going to put the egg and strap it in, and this can happen very easily if you are on the ice and you fall over, just when you are skating, your head will hit the ice and it’s a big problem. We can understand why padding protects us from a collision by going all the way back to Sir Isaac Newton. Newton told us that you experience a force when you change the momentum over some duration. So when I go into a check, I will be changing my velocity. I will be stopping. The question is, how fast I stop? I hope that you have enjoyed these
discussions on physics and hockey. Hockey is the fastest sport. Physics is demonstrated all throughout the entire game. I hope when you are watching The Big Chill that you will see physics in every shot, every check, and every goal.