Among the mysteries of the ages are burning questions whose answers have eluded great thinkers for decades, centuries, even millennia: What is the meaning of life? How do they get the caramel into a Caramilk bar? And why do curling rocks curl?
For the first two questions I have no answers at this time. For the third, I do, thanks to Dr. Mark Shegelski, a theoretical physicist at the University of Northern British Columbia in Prince George.
He’s had a strong theoretical answer for years, but now he has experimental evidence to back it up, revealed in an article in the current issue of the Canadian Journal of Physics, co-authored by Dr. Erik Jensen, an experimental physicist at UNBC.
For curlers, it just seems to make sense that a rock should curl in the direction it is spinning: to the right if its spinning clockwise, to the left if its spinning counterclockwise.
But to a physicist, that’s counterintuitive. Other spinning things don’t curve that way. If you slide an empty, top-down glass across a smooth surface and spin it counterclockwise, it doesn’t curve left, it curves right.
That’s because as the glass slides it tends to tip forward, which results in greater friction on the glass’s leading edge than on the trailing edge. The counter-clockwise spin pushes to the left at the front of the glass, and the greater friction means that push exerts more force than the rightward movement at the back of the glass. A push to the left results in a corresponding move to the right, just as you move to the right if you’re sliding on ice and push off of an object to your left (the boards, a tree, another person).
If a glass curls the way it does because friction is greater at its front than at its back, then the obvious conclusion is that a curling rock curls the way it does because friction is instead greater at its back than at its front. According to Dr. Shegelski, the difference is that a curling rock sliding on ice actually melts the ice surface, producing a microscopically thin layer of water and slurry. The leading edge rubs harder against the ice than the trailing edge, therefore melting more of it and producing a thicker layer of lubricating liquid at the front of the rock than at the back. As a result, the trailing edge gets more traction on the ice and its sideways movement is thus what determines the direction of the curl.
But there’s more. The spinning of the rock also drags some of the liquid around the rock’s contact ring, because water tends to cling to granite. As the rock slows down, more of this water is dragged to the front of the rock by the spin, allowing the trailing edge to gain even more purchase. That’s why the most pronounced curl occurs at the end of the rock’s slide.
This idea has been controversial with some curlers, who don’t find water on the ice when they lift up their rocks. Dr. Shegelski says that’s because the water layer is so thin that it freezes instantly to the ice surface as soon as the rock stops moving.
Drs. Shegelski and Jensen tested Dr. Shegelski’s theory at the Prince George Golf and Curling Club. The club created a special ice surface underlain with a detailed grid pattern. With a suspended video camera, the researcher recorded four hours of curling shots, from slow-sliding but fast-spinning to fast-sliding but slow-spinning.
The results seemed to confirm Dr. Shegelski’s theories, but that’s not to say there isn’t room for further refinement. Although the observed results and Dr. Shegelski’s mathematical models matched closely, they didn’t match perfectly.
There may also be room for further refinement of curling techniques. In curling there’s a shot called a spinner, used as a knock-out shot, in which the stone is slid very hard and rotated very quickly, so that it travels straight down the ice. The UNBC researchers discovered during their experiment that a slow-moving rock with a very high rotation speed–70 or 80 full rotations down the ice–results in a curl that’s twice as big as that of a similar shot with only five rotations.
And, finally, there may be room for refinement of curling rocks. In fact, on August 31 Dr. Shegelski obtained the Canadian patent for a “Curling stone providing increased curl.” But he’s not saying anything about it until it’s been tested.
Who knows? Maybe the “roaring game” is in for a 21st-century shake-up.
