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The curl in curling is a physics mystery

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Pyeongchang Olympics Curling Rock Baby
Joe Polo, of the United States, launches the stone during practice at the 2018 Winter Olympics in Gangneung, South Korea.
Aaron Favila | AP

If you plan to watch the Olympic curling gold match between Sweden and USA, you might wonder "why does that stone curl on the ice?"

Well, scientists ask the same question, though they have some theories. NOVA on PBS explored the science behind the sport.

"There is no single theory that explains the behavior of what we observe," Don Wade of the Broomstones Curling Club in Wayland Massachusetts told NOVA.

The game is based on the ability to "curl" the stone. By rotating the stone during the release, the stone can curl away from the line of delivery, allowing players to strategize on throwing stones to bump their opponents stones out of the way, or curl a stone behind another stone on the playing field, called the "sheet."

But the stone doesn't curl the way we typically expect it to.

As the video shows, if you spin an object counterclockwise and slide it along a surface, it will curl to the right. Spin and slide with a clockwise rotation, it will curl to the left. There's more friction at the front of the object than the back, because as it moves, it tilts slightly forward. That results in a curl from the opposite direction of the rotation.

Curling stones, however, do the opposite.

"On ice, it curls in the same direction as the rotation," Wade told NOVA.

So with a clockwise rotation the stone curls to the right and a counterclockwise rotation results in a curl to the left.

Why does it do this?

The top three theories that physicists have pondered are:

• Because the ice isn't flat but 'pebbled' with bumps, the pebbling may be part of the reason that stones curl in the same direction of the rotation. It's possible the stone is catching on the pebbles and turns with the stone's rotation.

• Remember how normal items build up friction in front of the item? With a curling stone, the pressure on the front of the stone may warm the ice and make a thin film of water that decreases the friction on the front end. That might cause more friction on the back half of the stone compared to the front, pushing the stone with its rotation.

• The final theory involves the fact that only a thin ring of the stone touches the ice at any time. That thin ring will scratch the ice and create tiny valleys running in the direction of the curl. The valleys then guide the other bumps on the back half of the stone, and curl the stone.