Why Do Curling Rocks Curl?

March 13, 1997 For Immediate Release

The curl of the rock is an essential component to the sport of curling. It can allow a rock to hide behind a teammate's guard stone, or move around an opponent's rock to the button. The curl is created by giving the rock a slight initial rotation when it is first released. Why does it curl and why does it curl more when it isn't moving as quickly? Two University of Northern BC professors and a colleague from the University of Lethbridge think they have it figured out.

"The friction between the rock and the ice actually produces a thin film of water as the rock moves down the ice. The curl is a result of wet and dry friction acting on the rock at the same time, but with different effects when the rock is moving fast or slow," says UNBC Physics professor Mark Shegelski. "In contrast, if a person were to take something like a can of salmon and spin it down a slippery floor, it would curl in the opposite direction of a curling rock."

The curling rock curls more in the the last few metres of its travel, when the rock is moving slowly.

"The curl of the rock is increased once the speed decreases because the rock actually drags some liquid around the rock as it rotates, especially at the front and (if the rock is curling left) the right side," adds UNBC Mathematics professor Ross Niebergall. "We all know from trying to walk on an icy sidewalk that a thin layer of water reduces the friction. That's why the rock curls more as it slows."

The theory also explains why sweeping the ice affects the curl and the distance the rock will travel. The professors worked out nearly two pages of mathematical formulae to outline the components of both wet and dry friction.

The study was motivated simply by personal interest in curling and the absence of a satisfying explanation of why curling rocks curl.

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