Science Friction: Curling

Sending the 42-lb. granite curling stone down a long sheet of ice toward the center of a bull's-eye target is all about friction and surface physics, as NSF-funded scientists Sam Colbeck, formerly from the U.S. Army Cold Regions Lab, and physicist George Tuthill from Plymouth State University explain, with help from Olympic hockey player John Shuster, and Iain Hueton, from the Ogden Curling Club in Ogden, Utah.

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LESTER HOLT, anchor: Curling has been in the Winter Olympics since 1998, but still seems a little strange to most of us. We had John Shuster, captain – or “skip” – of the U.S. Curling Team in Vancouver, explain this unusual sport for us, while scientists funded by the National Science Foundation explain how friction makes it all work.

Curling. Perhaps the most unusual sport in the Winter Olympics. A huge rock is thrust down a long sheet of ice. Two players sweep a path in front of it, guiding it to the center of the target, called the “house.” At the end of play, the team with the most points – the most rocks closer to the center -- wins. Olympic bronze medal winner John Shuster is a life-long curler.

JOHN SHUSTER (U.S. Curling Team): My family is big into curling. I remember throwing you know, a rock at the curling club when I was, you know, probably six or eight years old.

HOLT: Getting the curling stone from the start to the house is all physics, starting with the push-off against what’s called the “hack.”

SHUSTER: What you're able to do off of here is to position your foot to allow yourself to accelerate out of the hack with the curling rock. You can push out of this hack with a very high amount of force and you know it's going to transfer into the curling rock.

HOLT: Then the sweepers move in.

Curlers: Hard!

SHUSTER: This is like a synthetic material that has just a little bit of abrasiveness.

Curlers: Yes!

SHUSTER: Curling terminology as far as sweeping goes, it’s anything that you could consider, versions of “yes”…

Curlers: Yeah!

SHUSTER: But usually it’s “hurry” and “hard”…

Curlers: Hard! Hard!

SHUSTER: You want them to, you know, push harder.

HOLT: To help the stone go farther and straighter.

SHUSTER: Back in the old days, people used to think when you swept in front of a rock you'd clear out snow and frost and that allowed the rock to go further. Well, after we got the snow and frost elements removed when we moved indoors, now all of the sudden you're sweeping a rock and its even affecting it more, it's going fifteen feet further.

HOLT: That’s because sweeping wasn’t just removing the snow and ice, it was reducing the friction between the stone and ice. The rapid back-and-forth of the sweeping generates heat.

Dr. SAM COLBECK (U.S. Army Cold Regions Lab): It all just comes down to this [rubs hands] yeah, it comes down to this. They’re warming that ice up maybe enough to generate a melt-water film.

HOLT: A melt-water layer that reduces friction, creates a literal path of least resistance for the stone to glide on more easily. Friction is reduced even further by another characteristic of curling ice: bumps.

IAIN HUETON (Ogden Curling Club, Utah): All of the other ice sports have a smooth, Zamboni surface effectively, and curling requires that the surface of the ice be roughened.

HOLT: Spraying water does the trick.

HUETON: And then when it lands on the ice, it freezes up and forms these nice pebble-shaped bumps, and if you look at it, it looks like an orange peel, that kind of roughness is about the right texture, and that gives us the surface we need.

HOLT: But wouldn’t the bumps, like mini-speed bumps, just slow the stone down? Actually, it’s the reverse: the “points” reduce the total ice surface the stone rides on. The less surface contact, the less friction. And there’s another reason:

HUETON: If we're on smooth, frozen Zamboni ice in an arena, if that's making a nice seal with the bottom of the stone, you're going to eventually form a vacuum as the stone moves across the ice, and it really slows it down. You’ve got air pressure pushing on the stone. Whereas, if you can have it riding on a few points then it's going to slide more smoothly and you won’t get any kind of a vacuum forming and it’ll go roughly twice as far.

HOLT: The make-up of the stone itself plays a role.

SHUSTER: Curling rocks are 42 pounds of granite, and actually the granite only comes from the island called Ailsa Craig that's off the coast of Scotland.

HOLT: This type of granite is “hydrophobic” – it resists water. That means it will absorb very little of the melt-water layer at the surface. Absorption could reduce the lubrication helping the stone glide toward the house. As the curling stone approaches the target, there's a good chance another team's stone is in the way. When the stones collide, another wonder of physics takes place: momentum exchange.

Dr. GEORGE TUTHILL (Plymouth State University): When you see the collision of two curling stones you’re seeing the transfer of kinetic energy between them and you’re also seeing the transfer of momentum, the product of mass and velocity between them.

HOLT: Well, there you have it: the basic physics of curling. From the hack to the house, from the coefficient of friction to the transfer of kinetic energy, curling is a sport that attracts the crowds and delivers excitement with a bump.

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