Why rocks curl

One of sport’s greatest scientific mysteries has been solved, sort of. Two University of Northern British Columbia physicists have explained the centuries-old question of why a curling stone curls, or moves laterally, in a counter-intuitive direction.

The solution – published in the current issue of the Canadian Journal of Physics – isn’t an elegant equation of the kind mathematicians adore, say the scientists, but rather one that involved a lot of experimental sweeping. The explanation, nonetheless, could spark controversy at rinks – and even result in a new super-curl shot. “If you turn a glass over, spin it and slide it down a table it curls in the opposite way compared to a curling stone,” says Dr. Mark Shegelski, an NSERC-funded UNBC theoretical physicist describing his post-game barroom demonstration of the problem. “The curlers think you’re doing some kind of magic, until they do it themselves and see that the glass goes the ’wrong way.’”

Curling is the indoor winter sport popularized by the Scots, and now an official winter Olympic event, in which two opposing teams slide and rotate smooth 20-kilogram (44-pound) ovals of granite (the stone) down a 28-metre-long sheet of ice. The goal is to get your team’s stones closer to the centre of a bull’s eye-style target than the other team’s.

Baseball curveballs and the trajectory of golf balls have long been the stuff of introductory university physics textbooks. But the reason for a stone’s curl, the very thing from which the game gets its name, has remained elusive. “The physics of friction on ice is considerably more complicated,” explains UNBC experimental physicist Dr. Erik Jensen, the paper’s other co-author. Indeed curlers know that different ice surfaces can have an enormous impact on the stone’s movement; a skilled curler is able to “read ice” and anticipate the degree of curl. After a decade of theoretical exploration, Dr. Shegelski recently decided it was time for a scientific bonspeil to gather the experimental information needed to finally resolve the physics of the curl.

Drs. Jensen and Shegelski developed an experiment, and with the help of the staff of the Prince George Golf and Curling Club they were able to create an ice surface underlain with a detailed grid pattern. Using a suspended video camera they then recorded four hours of the widest possible range of shots, from slowly sliding rapid “spinners” to slow-rotation, high-velocity shots all thrown by three local curlers. The results are the first detailed quantitative measurements of curling stones’ behaviour.

So why does the curling stone curl the way it does? Wet friction, say the scientists.

“Our work makes a very convincing case that melting is inextricably involved,” says Dr. Shegelski. “It doesn’t prove that there’s melting, but to explain our experimental results without invoking the existence of a thin-liquid film, well, I would be shocked if somebody came up with a successful theory that involved no melting.”

This quasi-liquid layer – a microscopic slurry of ice and water “as thin as a bubble’s skin” – reverses the dominant frictional force on the stone. The glass on a table experiences dry friction, in which the largest frictional force is on the leading edge. So if it’s rotating clockwise, it will curl left. However, for the curling stone, the liquid layer reduces the friction at the front so that it is less than the friction at the back. Thus a clockwise-turning stone curls to the right.

Moreover, the only way to explain the extent of some of the extreme curls they observed, up to one-and-a-half metres of lateral movement, is that “the frictional force acting on each segment of the rock is directed opposite to the motion relative to this thin liquid film, and not relative to the underlying fixed ice surface,” write the authors.

It’s an explanation that the physicists say has evoked cries of foul from some long-time curlers who insist they don’t see any water under their stones. The water layer is so thin it freezes too quickly to be observed when a stone is lifted.

However, the definitive theoretical explanation of the stone’s curl remains tantalizingly out-of-reach. Even though the observed curls and mathematical models fit closely, there’s still a gap, what curlers would call a biter. “At the end we punted and said we really can’t explain everything from first principles,” says Dr. Jensen, now content to head back to his usual surface physics experiments with lasers.

But in pursuing his quest for curling’s ultimate prize Dr. Shegelski has inspired physics teachers across North America, and as far away as Germany, to take to the rink with their students. And, far from being purely theoretical, the latest experiments have paved the way for a new curling shot. Curlers are familiar with a shot called a “spinner,” used as a knock-out shot, in which the stone is slid hard and rotated quickly so that it travels straight down the ice. “What we found is that if you really slow down the speed but maintain the high rotation rate of 70-to-80 full rotations, the stone’s curl is double that for a similar shot with five rotations,” says Dr. Shegelski. “So that’s a cool thing that I didn’t expect to happen.”

There could even be a new theory-inspired stone. On August 31st Dr. Shegelski obtained the Canadian patent for an idea entitled “Curling stone providing increased curl.” But, after a decade of tangling with the curious curl, he’s remaining silent on this until the stone’s been tested.