Curling Quandary: Why Stones Curl the "Wrong" Way

Brian Handwerk
for National Geographic News
February 23, 2006
Canadian physicists may have answered a question that's been around for curling's entire 500-year history: Why does a curling stone curl in a direction that seems contrary to the laws of physics?

Interest in curling is enjoying a growth spurt, highlighted by this winter's Olympic competition in Turin, Italy.

Tomorrow Canada battles Finland in the men's gold-medal game, while the United States men's team goes for a first ever curling medal in a bronze matchup against Great Britain.

But even with increased attention to the sport, many athletes and fans have been largely unaware of one of curling's biggest quandaries.

In the modern version of this ancient Scottish game, competitors slide 42-pound (19-kilogram) "stones" or "rocks" down a clean sheet of ice. Each team hopes to place its own stones closest to the center of a bull's-eye target known as the house.

Spin is a crucial part of every shot. Olympic curlers generally try to spin the stone the same way each time—three rotations from the start of a shot to its finish.

"The spin actually makes the stone go to a specific spot," said Shawn Rojeski, a member of the U.S. Olympic Curling Team from Chisholm, Minnesota.

"When we put the spin on it, one way or the other, we're more or less guiding in which direction we want the rock to go.

"If you were to throw the rock with no spin, it would be nearly impossible, and it would be hard to control where it goes," he continued. "Any little feature on the ice would make the rock change direction."

Sliding Surprise

But when University of Northern British Columbia physicist Mark Shegelski took up the game, he noticed that the spinning stone curled, or moved to one side, in a counterintuitive direction as it slid down the ice.

Shegelski often demonstrates the problem to curlers after a game.

"I take a nice drinking glass, rotate it clockwise, and slide it down a counter," he said. "Everyone thinks that it will go to the right. That's the natural reaction from curling.

"But it goes to the left, and the initial reaction [from curlers] is that I'm performing some kind of magic trick."

Indeed, Olympian Rojeski did not know that his expert spins present an apparent physics mystery.

"I've never sat down and thought about it," he said. "I bet if you talk to the average curler, nine out of ten people would be unaware of it."

So why does the stone curl the "wrong" way?

Shegelski believes that the answer lies in wet friction, courtesy of a thin layer of meltwater that temporarily forms under the stone.

But testing his theory presented its own share of problems.

"When I'm talking about a thin film, I mean a really thin film" of meltwater, he explained.

"There's no way to measure it directly. If you turn a curling rock on its side, you're not going to see" a puddle on the ice.

Lacking a way to observe the watery film, Shegelski and fellow physicist Erik Jensen recruited volunteers at their local curling club and created a precise under-ice grid system to help track the stones' movement.

They then filmed a variety of shots from an overhead angle using a suspended video camera.

Their data provided a likely explanation.

When a glass or other spinning object slides along a surface, its forward edge holds the highest frictional force.

Thus, when a moving stone is rotating clockwise, it's front edge moves to the right. Sideways friction should cause the object's path to angle, or curl, to the left.

But that extremely thin layer of melted water changes the effect of friction on the stone. It creates better lubrication, and therefore reduced friction, at the front edge.

The sideways friction on the stone's rear edge—where the meltwater has already refrozen—dominates the stone's curl instead and makes it move in the opposite direction.

The results are experimental—they don't definitively prove that meltwater is creating wet friction responsible for the stone's spin.

But theories that don't involve wet friction seem unable to explain the experimental results.

Enduring Mysteries

John Williamson, a retired ice technician from Clintonville, Wisconsin, knows the science of the sport as well as anyone.

For years he prepared ice for curling matches by tracking a myriad of variables, such as humidity levels, dew points, and temperatures at different positions all over and above the ice.

He also prepared surfaces with tiny drops of water called pebble, which are sprayed on the ice to control friction.

Williamson believes the friction created when the stone moves over the pebbled surface is key to creating the meltwater and subsequent asymmetry that is responsible for the stone's unusual trajectory.

"If Jenson and Shegelski are right—and I think they are—what makes the rock curl [the 'wrong' way] is that asymmetry," he said. "There's meltwater at the front of the rock and not at the back."

Williamson explained that, among those curlers who did ponder the reasons behind the curl, the research was welcome.

"There were a lot of arguments going on, because after 500 years nobody could totally understand why these stones curled as they did," he said. "Nobody could get it down to a science."

Shegelski has returned to his day job studying quantum mechanics, leaving some curling mysteries unexplained.

"It's hard to understand why curling rocks curl as much as they do," he said.

"If they only curled half as much it wouldn't be hard to understand. There's at least one mystery that I don't think we've found the answer to."

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