The ABCs of Curling - L

is for Line, the path the rock follows as it travels down the ice. In order to hit an opposition rock, or draw behind a guard, the line the rock follows is very important. Because a rock will begin to curl more as it slows down, and sweeping a rock helps it maintain its momentum longer, the players will sweep a rock to make it curl less, or stop sweeping to allow it to curl more, as desired.

This brings us to the age old question: why do curling rocks curl? It is an interesting question, but an even more interesting question is: why do curling rocks curl in the direction that they do? When you throw a curling rock, and impart a clockwise spin to it, it will curl to your right as it moves away from you down the ice. This seems to make sense.

Here's an experiment for you to try. Take a small drinking glass, and turn it upside down. The rim of the glass simulates the running surface of the rock, as we saw in I is for Ice. Slide the upside-down glass down your kitchen counter, while spinning it clockwise, as if you were throwing a curling rock. You will see that the glass curves to the (wait, what?) to the left, as it travels along the counter. The exact opposite to what a curling rock does. What's going on here?
The answer has to do with the only recently understood physics of the interaction between the gaseous, liquid, and solid states of water at the interface between the air and the surface of a sheet of ice, and the changes to that interface under the pressure exerted by the weight of a running rock.

It's pretty sciency stuff, so I'll forgive you if you don't want to click through, but here is an article by University of Northern British Columbia physics professor Mark Shegelski that describes the effect, and here is a neato flash presentation from the University of Toronto that illustrates it.

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