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Why a curling rock curls

Swedish researchers who specialize in studying friction and wear in industrial systems have uncovered the secret to why a curling rock curls.

The globally popular sport gets its name from the slightly curved or “curl” path the stones take when released by the player towards the target area nearly 30 metres away.

As soon as the player releases the stone, it is only affected by the friction against the ice. The friction can be slightly reduced, and therefore the sliding distance somewhat increased by intensively sweeping the ice just in front of the sliding stone.

The stone, which weighs nearly 20 kg rotates slowly based on the spin the player applies upon release and typically revolves two to three times during the 25 seconds it takes to travel the length of the sheet.

Despite years of speculations among the curlers and several scientific articles, so far no one has been able to present a good explanation to why the curling stones actually curl. Interestingly, other rotating objects sliding over a surface curl in the opposite direction (make a simple test by sliding, for example, a glass turned upside down over a slippery floor).

However, a team of Uppsala University researchers in Sweden found that the curved path is due to the microscopic roughness of the stone producing microscopic scratches in the ice sheet.

As the stone slides over the ice the roughness on its leading half produces small scratches or grooves in the ice. The rotation of the stone will give the scratches a slight deviation from the sliding direction. When the rough protrusions on the trailing half shortly pass the same area, they cross the scratches from the front in a small angle and have a tendency to follow them.

The importance of having a proper roughness of the sliding surface on the stone to give it the expected trajectory, is since long known among curlers, the Uppsala release says.

However, this has not previously been coupled to the steering mechanism. While working on their model the Uppsala researchers experimented with pre-scratching of the ice in various ways, and could then observe that also non-rotating stones could be guided.

Stones with very smooth, polished sliding surface were, however, not affected by the scratches.

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