# Curling, A Great Physics Mystery

• By Ari Daniel
• Posted 02.28.18
• NOVA

To physicists, curling presents the most puzzling conundrum in the Olympic Games.

Running Time: 04:32

## Transcript

### Curling, A Great Physics Mystery

Published February 28, 2018

Onscreen: Curling. Full of suspense, surprises, and thrills.

Jessica Schultz: Oh my gosh, it's fantastic, are you kidding!?

Onscreen: It's also full of mystery (for physicists).

Don Wade: There is no single theory that explains the behavior that we actually observe.

Onscreen: Remarkably, no one's fully figured out the physics behind the sport.

Schultz: There's just so much that we don't know.

Onscreen: Here's the deal. In curling, the goal is to get your team's stones into the bullseye, or "house," at the end of the ice sheet. You do that by lunging, rotating the stone, and releasing.

Karen O'Neill: The sport is called curling because the stone will curl as it goes down the ice.

Onscreen: That is, it'll turn away from the line of delivery. The game depends on the curl.

O'Neill: There's much more strategy possible when you can curl a stone around other stones on the ice. You can position where a stone hits another stone. You can bump other stones out of the way.

Onscreen: But here's where things get weird. Take any object. Like a bowl. Spin it counter-clockwise and slide it along a surface, and it'll curl to the right. The reason: the bowl tips forward slightly as it moves. That puts more pressure on the leading edge than the trailing edge, so the frictional force is greater in front than back. This pushes the bowl in the opposite direction of its rotation. This is what happens with every object on Earth. Except a curling stone.

Wade: On ice it curls in the same direction as the rotation.

Schultz: Throw it clockwise, then it's gonna curl to the right. Throw it counterclockwise, then it's gonna curl to the left.

Onscreen: Why is this? No one knows for sure. But here are a few ideas. First, the answer may be frozen in the ice. Curling ice is special.

Schultz: The ice guys have kind of like a jetpack. Or if you're thinking, like, the Ghost Busters.

Sean Maw: They spray fine droplets of water over flat ice. Then you get these little bumps on the ice, which are called pebbles.

O'Neill: Those bumps, when they freeze, are not uniform in size. So part two is then we nip, which is to take a big metal blade, shaving off the very top of it so that we have a uniform running surface across all of the bumps.

Maw: We think the pebbling of the ice is very important to the way the curling rocks behave.

Onscreen: Here's the idea. As the rock glides, it catches on each pebble, pivoting around the pebble in the direction of its rotation before detaching and continuing down the ice.

Maw: It sticks and pivots.

Schultz: Grabs the pebble as it goes.

Onscreen: Each pivot is tiny, but the pivots accumulate, and the stone curls. Here's another theory. With a bowl, the friction builds up at the leading edge. However with a curling stone, that extra pressure in front may warm the ice slightly, creating a thin liquid film.

Wade: There's a little bit of melting that happens.

Onscreen: The extra lubrication decreases the frictional force in front compared to the back. This pushes the stone in the direction of its rotation. There's also a third possibility related to the fact that the only part of the curling stone in touch with the ice is a thin ring. Bumps on the front of that ring will scratch the pebbles, scoring them with tiny valleys running in the direction of the curl. Those scratches may guide the bumps on the back of the ring, causing the stone to move in the direction of its rotation.

Wade: Basically the rock is kind of following the scratches.

Onscreen: These ideas go beyond curling.

Maw: Understanding the nature of slipperiness on ice is not a trivial matter at all. It could have practical applications.

Onscreen: From a car skidding out, to someone slipping and breaking a bone. Most curlers on the ice, however, have other things on their minds.

Wade: As long as it curls in the direction we're expecting, we don't really care about what the underlying physics is.

Schultz: My theory is it does what it does. And I just have to accept it, and go with it!

## Credits

### PRODUCTION CREDITS

Digital Producer
Ari Daniel
Production Assistance
Yasmeen Fakhro & Annie Kim
Editorial Review
Julia Cort
Special Thanks
Greg Kestin

### MEDIA CREDITS

Visuals & Videography
USA Curling
Linda Yip
pixels.com
pixabay.com
shutterstock.com
Music
­APM

### POSTER IMAGE

(main image: curling stone on ice)
Curling USA

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