Swedish friction experts unravel curling mystery

Researchers in Sweden have revealed the mysterious mechanism responsible for the curved trajectory of curling stones as they slide down the ice in one of the country's more popular winter sports.

Swedish friction experts unravel curling mystery

In the sport of curling, the players slide large, 20 kg stones along the ice so that they slowly slide towards a target area nearly 30 metres away. The stones often travel on a curved path, sometimes knocking other stones out of the target area as each teams attempt to place their stones as close to the centre target as possible.

Achieving the optimal curved trajectory of the stones is an important part of a team’s strategy in the sport. If a player puts a clockwise rotation on the stone, for example, it curls to the right, while a counter-clockwise spin results in the stone curling to the left, allowing them to reach open spots behind previously played stones, or take out opponent stones.

Despite years of speculation by those active in the sport, as well as several scientific articles, no one has been able to offer a good explanation to why the curling stones actually curl – until now.

According to the findings of a team of friction researchers at Uppsala University in eastern Sweden, a curling stone’s curved path is due to the microscopic roughness of the stone producing microscopic scratches in the ice.

“As the stone slides over the ice the roughness on its leading half will produce small scratches in the ice,” the researchers said in a statement.

“The rotation of the stone will give the scratches a slight deviation from the sliding direction.”

Analysis carried out by reseachers Harald Nyberg, Sara Alfredsson, Sture Hogmark and Staffan Jacobson, described the phenomenon as “scratch-guiding” or “track steering” whereby the rough protrusions on the trailing half of the stone cross over the initial microscopic scratches, crossing them at a small angle.

As the front-end scratches are crossed, they will have a tendency to follow their direction, generating the sideway force necessary to cause the curl.

While curlers have long known of the importance of having the right degree of roughness on the sliding surface of the stone, the roughness had not previously been connected to the steering mechanism, the researchers explained.

The findings have been published in a paper entitled “The asymmetrical friction mechanism that puts the curl in the curling stone”.

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