C&B Notes

As the Stone Turns

We are not surrounded by winter sports down here in Alabama, but that fact does not diminish our love of curling that is revived every four years during the Winter Olympics.  We enjoy the game strategy but a group of scientists are more focused on exactly how and why a stone moves in curling.  Turns out the answer is not easy to agree on.

Spin an overturned glass clockwise on a smooth counter top and it drifts to the left. Spin it counterclockwise and it drifts to the right. A curling stone in play does the opposite. Spin that glass harder and it makes a sharper turn. In curling, the faster the stone spins, the more straight it goes.

“We have no idea why a rock curls,” says U.S. women’s curling team captain Nina Roth.  Neither do scientists, who have been stumped for close to a century.  “Anything unusual gets us excited, and a curling rock has that,” says Ray Penner, an astrophysicist at Vancouver Island University, who also studied the physics of golf.  He took up curling and got hooked on finding an explanation.

The curling stone is completely smooth except for a raised ring on the bottom called the running band.  That is where the mystery’s rubber hits the road.  The earliest known theory to explain the stone’s apparent misbehavior surfaced in 1924, the year of the first Winter Olympics, held in Chamonix, France.  University of Saskatchewan physicist Ertle Harrington attributed it to friction differences between the stone’s right and left sides as it moved on ice.  Writing in the journal Nature in 1930, researchers William Macauley and G.E. Smith said the friction differences were actually between the stone’s front and back.  Mr. Harrington responded that their idea was “non-concordant with the known behavior of curling stones.”  You get the drift.

University of Northern British Columbia physicist Mark Shegelski recently teamed up with Edward Lozowski, an emeritus professor at the University of Alberta, to arrive at the stick-slip friction theory.  Curling’s playing surface is pebbled with droplets of frozen water that get their tops flattened by a machine called a nipper, making a tiny landscape of miniature buttes.  Messrs. Shegelski and Lozowski say parts of a stone’s running band briefly stick — for billionths of a second — to the pebbles. But the right side of a clockwise turning stone is a bit stickier than the left. Going back to the overturned glass, it is as if the right side of the rim gets stuck on a bit of jam on the table; the glass would rotate around the sticky spot to the right…

The inquiries have since spread among scientists in Scotland, Japan and Russia.  Harald Nyberg was doing graduate work on industrial friction at Uppsala University in Sweden when a World Curling Association ice expert introduced him to the puzzle.  Mr. Nyberg, who now conducts research on friction for parts used by heavy-duty truck maker Scania, says he and his grad-school colleagues “got stuck on this question.”  The problem with Mr. Shegelski’s theories, including the latest one, he says, is that “they are nice and accurate math models for how a rock would move, but never really get into where those forces come from.”  Solving that question led to Mr. Nyberg’s own aha! moment. He and his team found that curling stones left behind tiny scratches on the ice pebbles.  Those scratches cause the curl, the Swedish researchers say.  Here’s how: The leading edge of a rotating stone engraves curved scratches on the ice.  As the back edge passes over the scratches, the etched grooves gently push the stone in the direction it is rotating.  Imagine the grooves in a vinyl record. If you lightly dragged a finger tip across the record’s surface, it would be pulled in the direction of the grooves.

Not surprisingly, Messrs. Shegelski and Lozowski have problems with the scratch theory.  First, they say, the Swedish researchers haven’t presented a formal mathematical model to explain it.  And wouldn’t scratches on the ice left by previous stones affect the ones that follow?

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