This article is from the In-Depth Report Science at the Winter Olympics
Science Talk

Ice, Ice, Baby: The Physics of Curling

Mark Shegelski of the University of Northern British Columbia talks with podcast host Steve Mirsky about the physics of curling, currently taking its turn on the world stage at the Vancouver Olympics. (Shegelski is also the author of the new sci-fi collection "Remembering the Future.") Plus, we test your knowledge of some recent science in the news

Mark Shegelski of the University of Northern British Columbia talks with podcast host Steve Mirsky (pictured) about the physics of curling, currently taking its turn on the world stage at the Vancouver Olympics. (Shegelski is also the author of the new sci-fi collection Remembering the Future.) Plus, we test your knowledge of some recent science in the news. Web sites related to this episode include and

Podcast Transcription

Steve: Welcome to Science Talk, the weekly podcast of Scientific American posted on February 18th, 2010. I'm Steve Mirsky, and you know what the hottest sport in the world is right now, thanks to the Winter Olympics? It is, of course, curling—that's the one where the guy slides the big, heavy rock down the ice wall, two other guys furiously sweep the ice in front of the moving rock—or maybe you've seen this fairly hilarious commercial on NBC promoting Olympic curling.

[advertisement excerpt from NBC]

The spike in curling interest prompted David Letterman Wednesday night to say, "it's all the excitement of shuffleboard curling plus household chores. You know it's just …" In his top 10 list of surprising things about curling number one was: no one cares; but we know that's not true. We know that you care because curling is all about physics, which is why four years ago, after the last Winter Olympics, I interviewed a scientist named Mark Shegelski from the University of Northern British Columbia—he studies curling physics. Back then this program had a different format of shorter interviews, and I ran only five minutes of the conversation I had with Shegelski. But with more time now to talk, I went back and I reedited the entire long conversation I had with him. So sit back with some hot cocoa and maybe an intro physics textbook, because here we go!

Steve: You are not a full-time curling physicist. What do you do with most of your time?

Shegelski: Well, most of the time I'm teaching; roughly 30 percent of the time or 40 percent of my time goes to research, and my research is currently in the area of quantum mechanical tunneling and decay.

Steve: People might not have heard of quantum tunneling and decay. Can you briefly explain what that is?

Shegelski: Well, if you imagine a marble inside of a [bowl], and it's rolling around in that [bowl]. If it doesn't have enough energy to get right up to the top, it's going to stay in the [bowl], and it won't ever get outside. If you then consider the same problem, but on a quantum mechanical scale, so you're thinking of, for example, of a particle such as an electron, if its experiencing the same kind of a potential, it's in a well or a [bowl] and there's a barrier that the electron needs to overcome to get outside. What is found is that, it is well known is that particles emerge from inside of this potential [bowl] or potential well, and they emerge outside with less energy than we would need to have to clear the barrier. In other words, the energy of the particle is below the maximum of the potential well for the particle.

Steve: So they've snuck right through the wall.

Shegelski: Oh yeah. The term tunneling comes from, it's as if—in thinking about from our every day experiences—it's as if the particle somehow tunneled through this barrier with potential that it had to get over, and it got out with less energy than it had to get over. So from an everyday point of view, you couldn't really understand that except that somehow they've tunneled through. So one of the details of the quantum mechanics is to find out how this happens and there are a lot of problems that are still needed to be solved in this area, and that's the area that I am looking at.

Steve: It's one of the really weird things about quantum mechanics.

Shegelski: Yeah, pretty much most things about quantum mechanics are in [opposition to] our regular everyday experiences, and so one of the first thing students have to do in learning quantum mechanics is they have to give up the regular everyday ideas that we have and accept a new way of looking at things. And in that sense, quantum mechanics is quite difficult, because it isn't something that you experience on a daily basis.

Steve: And that brings me to curling believe it or not! I know that you have done some scholarly studies of the physics of curling, and the reason I got in touch was I was watching curling during the Olympics, and you see these people furiously sweeping the ice in front of this big heavy granite rock, as its going down the ice and, what are those sweepers actually accomplishing from a physics point of view?

Shegelski: Well, there are several things. The most important one is that by sweeping in front of the ice, you are reducing the friction between the rock and the ice and let's just stay with that for a bit; with a reduced friction the rock still slows down, but it doesn't slow down as quickly, and so therefore if you sweep the rock vigorously in front, you sweep the ice in front of the rock, you can actually make the rock go quite a bit further than it would go if you didn't sweep.

Steve: And what is actually happening when you sweep—are they melting the surface?

Shegelski: Okay, [to get an] answer to that is unequivocal is quite difficult, but there are some things that are generally agreed upon. First of all, by sweeping the ice, you're putting energy into the ice and so, you know, the important thing [is] that [what] would govern the motion of the rock is what's going on between that thin contact ring of the rock and the ice that it's touching. And, you know, if you turn over a curling rock and look at it, you will see that it is not the case that there's a circle in contact with the rock; it's a thin ring, and the ice itself is also not flat, its pebbled, it has little hills and valleys, so that the actual area of contact is quite small and therefore there is a large pressure of the rock on the ice. Now in sweeping in front of the ice here, you are bringing the temperature of the ice up and that reduces the friction, but you're also creating a thin film of a quasi-liquid type [of] material. This is something that is not fully agreed upon by everybody, but you know, the work that we've done strongly supports the idea that the key thing going on is the friction that is due to the thin liquid film. In fact, we did a number of experiments, Dr. Eric Jensen and I—[he's] my colleague at the University of Northern British Columbia—and we did a number of experiments and then applied the theory that I had developed, and we modified that theory and applied it; and we could explain many things using the idea that there is a thin liquid film there as opposed to that there's not a film there. So I'm strongly convinced that there is a thin liquid film there and it plays a key role. With the liquid film, you have better lubrication for the rock, and so you have less friction and that's how the sweeping makes it go a further distance.

Steve: And your most recent curling publication—you've had, I think, four that I found, is that right? How many scholarly papers on curling have you published?

Shegelski: More than I'd like to admit. It turned out that, you know, when first starting to look at the problem, I didn't anticipate so many interesting questions coming up, and I don't actually remember how many I've published, but its something like eight or something like that. But the one that's the most important in my opinion is the most recent paper that Dr. Erik Jensen and I collaborated on and published in the Canadian Journal of Physics in November 2004.

Steve: And that's "The Motion of Curling Rocks: Experimental Investigation in Semi-Phenomenological Description".

Shegelski: Exactly.

Steve: And tell us what you studied there and the kind of conclusions you came to?

Shegelski: Well, what I was facing at that time was a need to refine the theory, to bring it in line so that it could agree better with things that were known about the motion of curling rocks on pebbled ice. And so I wanted to do every possible experiment that we could do within reason, and I'll talk about this for a little bit and tell, you know, give you a fairly [full] description of what we did. First of all, we had the ice technicians at Prince George Golf and Curling Club, they made a flat sheet of ice by simply flooding, and then they placed a grid down with ribbons and threads of different colors, red and black, and then fronted over that, so that it was actually a grid underneath the flat sheet of ice, and later they pebbled that sheet of ice. So what we did was we looked at all sorts of motion of curling rock[s] on the flat sheet of ice, the flooded sheet of ice, and also the pebbled sheet of ice. So the flooded sheet of ice is not what you have in a curling game, but it's definitely important from a scientific point of view. You want to know what's going on on the flooded sheet, the flatter sheet as opposed to the pebbled sheet, and we compared a lot of motions and compared the results between the flat sheet and the pebbled sheet.

Steve: The flat sheet is acting as your control.

Shegelski: Actually I don't know if I'd say that, but I'd say that it allowed us to investigate all possible motions that we could think of—within reason, of course, we can always do much more,—but for example, you can take a curling rock and simply rotate it and don't slide it and just have it go on a pure rotational motion, so we could look at that on flat ice as opposed to the pebbled ice. And then we could do shots where you would push the rock, but not very hard, so it would not travel very far, and you'd have it spinning very rapidly, and at the other extreme, we would have the rock going all the way down the ice and rotating maybe two or three times. And then another shot that we did was having the rock go all the way down the ice, but rotating between two and 80 times, going down the ice.

Steve: So the rock is either rotating exceptionally slowly as it makes its way in a linear motion or it's rotating very quickly as its going down the linear motion.

Shegelski: Yeah, yes that's right. One of the key things for us was to look not just at the shots in a curling game, but other shots that you definitely don't see. So for example, when we would videotape the motion by having a camera on a boom above the rock, follow the rock down the ice and so the camera was almost above the rock at all points. There'd be a slight variation because the rock would move side to side, but in the standard curling shot, you'd have two or three full rotations of the rock going down the entire sheet of the ice and we did study that motion. We also studied a case where you have a strong component of rotational motion. For example, if the rock rotates 80 times in going down the sheet of ice, the rotational motion and the rotational energy is much greater than in a regular curling shot and so it becomes interesting to see what you get as a result of this higher rotation.

Steve: So one of the big deals is that the rock, if I understand this correctly, the rock doesn't move in the direction that you would expect it [to] move based on the rotation as its going down the ice path, its breaking in a way that you would expect not to happen based on the direction that the rock is spinning.

Shegelski: Correct.

Steve: So if it's spinning, let's say the rock is spinning clockwise, you would expect it on dry ground, if you had something spinning clockwise on dry ground, it would break in what direction, to the left?

Shegelski: Yeah, if you take a drinking glass, that has a nice cylindrical symmetry, and you turn the drinking glass over and rotate it clockwise and push it, so its sliding away from you. This glass will curl to the left as it goes away from you and that's exactly the opposite of what curling rock does. If you take the curling rock rotating clockwise and send it away from you, the curling rock curls to the right. So from a physics point of view, it's easy to understand that the drinking glass curls to the left. People who curl, they are surprised that the drinking glass when rotating clockwise and moving away from them curls to the left.

Steve: They're so used to seeing what happens on the ice that they expect the dry ground situation to be the same.

Shegelski: Yeah, based on their experience they sometimes think that they you're doing a magic trick or something like that and then, you know, you give the drinking glass to the curler and make the curler do the shot of the drinking glass for themselves and then [they] see that, yeah, this happens every time. So there's something very different going on with the curling rock on pebbled ice as opposed to this drinking glass overturned on say a countertop or a tabletop.

Steve: Okay, so what's going on—do we know?

Shegelski: We investigated that rather [thoroughly], as I said, and the idea that we had used to explain what we saw, these ideas worked very well. And one of the main things about this was that there would be more melting of the thin liquid film at the front than at the back. And this thin film is very, very thin, so you can't observe it directly. But having more melting at the front of the rock [then] at the back makes the friction at the front of the curling rock less than it is at the back and so you can understand this by looking at the drinking glass and the curling rock—lets take them both to be rotating clockwise, and lets look first at the drinking glass as it slides on a countertop. The front of the drinking glass has a sideways motion to the right and therefore the friction at the front will be to the left.

Steve: Right. I mean, correct, to the left.

Shegelski: Yeah.

Steve: Got you. The motion is to the right as its turning clockwise so the friction that it's encountering is going to the left.

Shegelski: Correct. And the drinking glass has a tendency to step forward, it doesn't actually liftoff, the back does not actually lift off unless the friction is very high, but it has a tendency to push harder, it does push harder on the tabletop at the front and the back and therefore the tabletop pushes harder back on the drinking glass [at the] front as compared to the back. And that means that the friction at the front is greater, is a stronger force than the friction at the back.

Steve: So [that] component is pushing it to the left as it moves forward.

Shegelski: The back component, okay it's rotating clockwise.

Steve: The front is stronger than the back, the friction at the front [is] stronger than the back and pushes it to the left because that's the direction the friction is pushing.

Shegelski: That's right. At the front of the drinking glass, the sideways motion is to the right, the friction is to the left, and it's greater than what goes on at the back. At the back, the clockwise rotation, the sideways motion is to the left and the friction is to the right so the friction at the back is to the right, friction at the front is to the left, but the friction at the front is stronger than it is at the back and therefore the rotating drinking glass curls to the left.

Steve: Right. But in your rock situation on the ice because of what you just explained about the thin film up front, you have the opposite frictional situation.

Shegelski: Exactly. The roles are reversed—instead of the friction at the front of the curling rock is [less] than it is at the back and so therefore the friction force, the sideways component of the friction force at the back is to the right and therefore the curling rock curls to the right.

Steve: Very interesting. Now you are a curler yourself, is that right?

Shegelski: I'm no longer active in curling, but I was for quite sometime and, you know, that's what got me interested in trying to understand what was going on with the rock and the way it moved.

Steve: Sure. In the curling, I don't know anything about it other than the little I've seen on TV, but were you the person who actually throws the rock or were you the one of the sweepers or what was your position?

Shegelski: The positions are, you have four people. The person [who shoots first is] called the first and the second person is called the second, the third is called [the] vice-skip or the third, and the last person is called skip. Now the skip needs to have a very good understanding of the game and the strategy and ha[s] to be very good at observing how the ice condition changes as the game goes on. So when I started curling, I started at the lead position and a little bit later I moved to the second position and I was comfortable in that. So I was doing, you know, the third and fourth shots for the team and doing a lot of sweeping; the lead, the person [who shoots first is] the lead and then also the third, the vice-skip, and then the final shooter would be the skip.

Steve: And the skip has to be more expert because there's so much more going on the ice at that point with all the rocks in various positions.

Shegelski: Yeah. Curling is, you know, often called "chess on ice" and you really have to know, really understand well what kinds of shots the other team can make and therefore you plan your shot in accord with that strategy. And the other thing that is really important is that the ice conditions change as the game goes on. They're not the same at the end of the game as they are at the beginning, so the skip has to have a good understanding of how these ice condition[s] change and what, you know, which path you would take to curl more at the end of the game as opposed to beginning of the game and all sorts of things like that.

Steve: And do you anticipate that the research that you've done into curling physics might have any applications outside of just understanding curling?

Shegelski: I think that there's a possibility that the work we have done on the motion of curling rocks, the considerations of friction there may have a bearing on the understanding of friction in other situations, where you have two materials sliding relative to each other. For example, you could look at paper sliding over top of paper and if the paper is created in a certain way, the friction could be small and so the paper could slide easily, one sheet relative to the other; or another way of doing it is to make it so that the friction is big and therefore the paper does not fly quickly relative to the other. So in a general sense, you'd be looking at things where you have two surfaces that are moving relative to each other, something sliding over some other surface.

Steve: And that's the kind of problem that you find in manufacturing all the time.

Shegelski: Yeah. So I think there is a large number of examples that it could potentially apply to and there is a lot of working done by other people with regard to this using their, you know, other approaches.

Steve: I really appreciate your time. It's fascinating.

Shegelski: Yeah, it is very interesting stuff.

["Ice Ice Baby" song plays]

Steve: Now its time to play TOTALL……. Y BOGUS. Here are four science stories; only three are true. See if you know which story is TOTALL……. Y BOGUS.

Story number 1: People with sleep apnea, where you stop breathing repeatedly during sleep, also experience fewer nightmares.

Story number 2: A former Microsoft bigwig has developed a laser system that shoots down mosquitoes.

Story number 3: Carbon emissions are changing the atomic nature of maple syrup.

Story number 4: A new DNA analysis of King Tut's mummy found not only his DNA, but the DNA of plague bacteria, meaning that he most likely died of bubonic plague.

Time is up.

Story number 1 is true. People with apnea do have fewer nightmares, which really isn't that surprising because apnea disrupts sleep, leading to less REM sleep, which is when dreams usually happen, including nightmares. That's according to a study in the Journal of Clinical Sleep Medicine. Only 21 percent of people with severe apnea said that they remembered at least one nightmare a week as opposed to 71 percent of people without apnea; it's still not worth having the apnea.

Story number 2 is true. The mosquito laser system burns the wings of the skeeters. It was developed by Intellectual Ventures, the company run by Nathan Myhrvold, Microsoft's former chief technology officer. The thing can even target just female mosquitoes of specific species because the timing of the wing beats is gender and species specific and that's what gets targeted. Myhrvold thinks a laser shooter could ultimately cost only about 50 bucks, so it might even find a place in mosquito control in regions with malaria. I've seen slow motion footage of the mosquitoes being taken down, you can find that on the Web. It's the only thing I've ever seen that actually made me feel sorry for the mosquitoes.

Story number 3 is true. Today's maple syrup has a different carbon-isotope ratio than the maple syrup of 30 or 40 years ago because of more carbon emissions, some of which get incorporated back into growing vegetation. That's according to a study in the journal Nature. Oil and coal emissions are low in carbon 13, which has seven neutrons. The more common carbon 12 has six neutrons. The ratio of carbon 12 to carbon 13 in foods in usually a good clue about where the ingredients originated. In fact food scientists can tell or could tell if allegedly pure maple syrup had been doctored with additional sweeteners by measuring the carbon-isotope ratios, but what's now an environmentally changed isotope ratio could make it harder to see if maple syrup or other foods are being altered before being sold.

All of which means that story number 4, about King Tut dying from bubonic plaque is TOTALL……. Y BOGUS. Because the new DNA and CAT scan analysis of Tut did not turn up any plague genes. The work appears in The Journal of the American Medical Association and is covered in an article you can find on our Web site. The new autopsy results, if you will, did find malaria bacterium DNA meaning that Tut might've had malaria in addition to numerous bone problems. He had a tough, short life showing that in some cases it's not good to be the king.

Well that's it for this week. I'm off to San Diego for the annual meeting of the American Association for the Advancement of Science, so we'll probably have material from that big conference next week and for a few weeks after. In the meantime, get your science news at, where you'll find the In-Depth Report on where the stimulus money went in the world of science and technology. Oh, and you can follow us on Twitter where you'll get a tweet every time a new article hits the Web site. Our Twitter name: @SciAm. My Twitter name is the same as my actual name. For Science Talk, the podcast of Scientific American, I'm Steve Mirsky. Thanks for clicking on us.

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