Last Friday I joined a contingent of Caltech physicists (including the Time Lord) for an afternoon of curling -- yeah, you heard me, curling -- in honor of Caltech theoretical physicist Mark Wise's 60th birthday. Turning 60 is a big deal among physicists, often marked by a special conference commemorating the honoree's work. Wise's past and present students decided to do something a little bit different. The Canadian-born Wise is a fan of curling, so they organized a group session via the Hollywood Curling Club at an ice rink in Van Nuys, and we all gamely trooped out to try our hand at what amounts to boules, bocce ball, or shuffleboard on ice. The only person who'd ever played before in our group was Wise himself -- once, apparently, a long time ago. So the comic potential was huge.
Scoff if you will, but we had a blast, even if our collective skill was a bit lacking. For those unfamiliar with curling, it's that sport where one person on a team slides heavy granite stones (also called "rocks") down the ice ("curling sheet"). Then two other team members madly sweep the ice in front of the stone with little brooms, trying to get as close as possible to the center ("button") of the "house" (a target of four colored concentric rings). The teams take turns throwing eight curling stones each, per "end" (there are usually eight to ten such "ends" in a game). Whoever gets the most stones closest to the tee wins.
Curling is hugely popular in Canada, and it's only been an Olympic sport since 1998, but its roots lie in medieval Scotland, where it once was known as "the roaring game" -- because that's the sound you hear as a curling stone rolls down the ice. The earliest known written reference, according to Wikipedia, dates back to 1541, but there is an inscribed curling stone with the date 1511. Those early stones looked nothing like the "rocks' used in curling today, and because they were so irregular in size, shape and texture, players had far less control over the stones' trajectories along the ice.
Today's curling stones are made of a special kind of granite from Scotland, with a handle attached to the top, the better to grip and rotate (ever so slightly) as a player releases the stone. That's how you get the slow gentle curl of the stone's trajectory, from whence the sport derives its name.
As with any sport, there's a good amount of physics involved in curling, and there is tons of online information about the basic mechanics. There's inertia, for starters: curling stones are pretty heavy (around 45 pounds each), so once a stone is thrown and gets some momentum, it can slide quite a long ways before the friction builds up enough to slow it to a stop.
The better players can control the friction, the better they can control the curl of the stone as it travels down the ice and position it right where they want it in the house. The ice itself is special: the surface is sprayed with droplets that then freeze, forming a pebbled surface that reduces friction. as does a circular "running band" along the bottom of the stone -- the only part of the stone that actually touches the pebbled ice, because the stone's weight is concentrated on a very small area compared to regular flat ice.
The frenzied sweeping with the little brooms helps reduce friction even further, slightly heating that segment of the ice very briefly before it refreezes. The stone curls more if you leave it alone. The point of the sweeping is to make the stone curl less and travel further. How much or how little you sweep depends on where you want to the stone to end up. There's a lot more skill involved than you think, which is why the sport is sometimes dubbed "chess on ice."
But why does a curling stone curl at all? Even weirder, why doesn't it curl in the opposite direction of how you spin it, which is what happens with, say, a glass sliding across a table? Instead, it curls in the same direction, and it's not entirely clear why. Canadian physicist Mark Shegelski of the University of Northern British Columbia has given this question a great deal of thought, publishing several scientific papers analyzing what he thinks is the physics at play. As he told Business Insider earlier this year:
Take a rock that is rotating counter-clockwise. At the front half of the stone, the direction of motion is to the left and the opposing force of friction is to the right. At the back half, the direction of motion is to the right, so the opposing force of friction is to the left. Importantly, the amount of friction at the front and back are not equal. That's because the curling stone has a tendency to tip forward as it slides down the ice. The leading half pushes down harder on the ice than the back, generating more friction at the front. If the same experiment was performed using an upside-down cup on a table, the cup would spin right because the force of friction at the back (where the sideways motion is to the right) is less than the force of friction at the front (where the sideways motion is to the left).
But the opposite happens in curling due to another phenomenon. Shegelski believes that the high pressure warms the ice more in the front, which creates a very thin, liquid film. The melted ice acts as a lubricant to reduce the force of friction at the front of the rock. The friction at the front of the stone, which is exerted to the right, is now less than the friction at the back, which is to left, so the rock curls left.
That seems straightforward enough, but as Destin from Smarter Every Day points out in the above video, a group of researchers from Sweden's Uppsala University beg to differ with Shegelski's conclusions. They published the results of their own study last year in the journal Wear, claiming that the stone curls because of asymmetrical friction created by tiny microscopic scratches on the pebbled ice. Per PhysOrg:
As the stone slides over the ice the roughness on its leading half will produce small scratches in the ice. The rotation of the stone will give the scratches a slight deviation from the sliding direction. When the rough protrusions on the trailing half shortly pass the same area, they will cross the scratches from the front in a small angle. When crossing these scratches they will have a tendency to follow them. It is this scratch-guiding or track steering mechanism that generate the sideway force necessary to cause the curl.
The scientific debate rages on, and I look forward to seeing more work on this in the future. We of the Caltech Curling Contingent, however, were less concerned with why the stone curls last Friday and more focused on trying to keep our balance while throwing the stones in the first place. It's harder than it looks. You have one foot placed in the "hack," the better to push off, and the other on a special "slider sole" as you crouch into a squat. You use either a broom or a special stabilizer for balance in one hand, and grasp the handle of the stone with the other. Then you push off and slide across the ice in a lunge position before gently releasing the stone. The momentum should ideally come from your body, not from pushing the stone with your arm, but I didn't figure this out until the end of the session.
All in all, it was great fun, and I think we should do this every year on Mark Wise's birthday. I have a much deeper appreciation for the sport having tried to play it. But knee pads should be required equipment, since I now sport an enormous bruise on my right knee from all that sliding and kneeling on the ice.
Jensen, E.T. and Shegelski, Mark R.A.. (2004) "The Motion of Curling Rocks: Experimental Investigation and Semi-phenomenological Description," Canadian Journal of Physics, 791-809.
Nyberg H., Alfredsson, S., Hogmark, S., and Jacobson, S. (2013) "The asymmetrical friction mechanism that puts the curl in the curling stone," Wear 301 (1–2): 583–589.
Nyberg H., Hogmark, S., and Jacobson, S. (2013) "Calculated trajectories of curling stones sliding under asymmetrical friction: validation of published models," Tribology Letters 50(3): 379-385.
Shegelski, Mark R.A. (2000) "The Motion of a Curling Rock: Analytical Approach," Canadian Journal of Physics, 857-864.
Shegelski, Mark R.A. and Matthew Reid. (1999) "The Motion of a Curling Rock: Inertial vs. Noninertial Reference Frames," Canadian Journal of Physics, 903-922.
Shegelski, Mark R.A., Matthew Reid, and Ross Niebergall. (1999) "The Motion of Rotating Cylinders Sliding on Pebbled Ice," Canadian Journal of Physics, 847-862.