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    CURLING: The Game For All Ages and Abilities

THE SCIENCE OF CURLING

 

We at ICING receive many requests for information about the "science" of curling: "What makes a rock curl?", "Why does sweeping make the rock run straighter?", "Why does a heavy pebble make the ice slower?", etc., etc.

Since we are not physicists, (some of our club-mates say we shouldn't even call ourselves curlers), we must depend on those who are trained and certified in the study of physical forces and principles to provide us with explanations.

Theories which have been tested, supported and substantiated by qualified, independent referees may be submitted through any of the addresses at the bottom of this page.

ICING reserves the right to evaluate and accept/reject, without notice or explanation, any items submitted!

         

WHAT PUTS THE CURL IN A STONE?

 by Mark Shegelski

Dr. Mark Shegelski is a social curler, a curious curler, and [an] Associate Professor of Physics at the University of Northern British Columbia in Prince George. He and his co-workers, fascinated by the whys of curling, have published four scientific papers on the physics of the curl in curling. He was recently interviewed by the Discovery Channel and the CBC's "Quirks and Quarks".

Canadian Curling News, March, 2000 issue.. Reprinted with permission 

Any curler knows that a curling rock, rotating counter-clockwise (when viewed from above and behind) curls to the left. But to a scientist new to the game, it is surprising. Why so?

Consider an overturned drinking glass sliding over a smooth surface and rotating counter-clockwise: the glass will curl to the left? No, it curls to the right! This may be surprising to the curler (ed. note: an empty overturned glass may be even more surprising) but it is fairly easy for the scientist to explain.

As the overturned glass slides over the smooth surface, it tends to tip forward. Consequently the front of the glass pushes harder on the surface than the back does. Thus, the friction on the front of the glass is greater than the friction on the back. For a counter-clockwise rotation, the "sideways" motion of the front of the glass is to the left, so the sideways component of the friction on the front of the glass is to the right, and he glass curls to the right. You can easily check this out, and when you do, you will see that the glass does indeed curl opposite to a curling stone.

Why then is the curl of the curling stone opposite to that of the drinking glass? The reason is that the friction on the front of the rock is less than the friction on the back. How can that be? Part of the explanation is the following. Like the overturned drinking glass, the curling stone tends to tip forward as it slides down the ice, and so the front exerts a greater pressure on the ice than the back. More pressure on the front means that the front of the stone causes more melting (momentarily) than the back. Consequently, the front of the stone will have less friction than the back. For a counter-clockwise rotating rock, the sideways motion at the back will be to the right, and the friction at the back (which is greater than on the front) will be to the left, and bingo, there it is. The rock curls to the left. (See diagram below.)

Simple, eh? Well, not quite! If that was the whole story, curling rocks would not curl nearly as much as they do. The friction on the front is not only less than on the back: it is much less, especially when the rock is slowing down, coming over the hog line and into the Free Guard Zone or the house. This explains why curling stones curl most at the end of their motion.

Due to the motion of the rock over the ice, there will be a momentary melting of the ice and the formation of a thin film of liquid just beneath the running surface (contact ring of the rock. As the rock slides and rotates, the thin contact ring will tend to drag some of the thin liquid film around it as it rotates. There is a force of attraction between
granite and water: water tends to cling to granite. Thus, the thin liquid film under the rock tends to get dragged along with the rock.

As the rock slows down, this thin liquid film is dragged around the rock, from the back along the side and eventually to the front. Consequently, the front of the rock will have even less friction on it than the back (as the rock slows down) and that is why we see most of the curl happen near the end of the rock's travel.

These main ideas were the key ingredients in a scientific model I developed, along with my co-investigators at UNBC. All of the details are given in four papers we published (three in the Canadian Journal of Physics; the other in the Australian Journal of Physics). It is important to note that our work was carefully evaluated by other scientists before publication. We didn't just come up with an idea. Our ideas and calculations were carefully tested and have passed those tests. That is why we can say, quite confidently, that our explanation is correct.

In science, a good model is one where predictions can be tested: our model makes two significant predictions that have been tested and have passed with flying colours.

Our model concerns the motion of a rapidly rotating, slowly sliding curling rock (in curling parlance, a "spinner"). The other concerns the shape of the pattern of contact between the rock and ice.

Suppose you took a curling rock and spun it as fast as you could manage, and pushed it only slightly, so that the rock was rotating very rapidly and sliding over the ice so slowly that it would only move from one side of the house to the other? What would you see?

Our model predicted that because the rock was sliding so slowly, the contact ring would have ample time to drag some of the liquid film around it. In fact, the liquid would be circling around the rock at an appreciable fraction of its rotational speed. The result would be that the frictional forces would change so that friction would stop the rock sliding long before it stopped rotating!

In conclusion, we tested our ideas by predicting specific results, and these were then confirmed by experiments that supported our ideas. Why does a curling stone curl the way it does? Because (1) melting occurs as the rock slides over the ice, and (2) the rock drags some of the thin liquid film around it as it rotates, making the friction much less at the front than at the back of the stone, especially when it is in its final feet of travel. 

  In a counter-clockwise rotating rock, the "sideways" motion at the front is to the left (dashed arrow), and the sideways friction on the front is to the right (solid arrow).

The sideways motion at the back is to the right, and the friction is to the left.

Because the friction at the back is greater than at the front, the rock curls to the left.
 
  References:
  1. "The Motion of a Culing Rock", Shegelski, Niebergall, and Walton,
    CANADIAN JOURNAL OF PHYSICS, volume 74, pages 663-670, 1996.
  2. "The Motion of Rapidly Rotating Culing Rocks", Shegelski and
    Niebergall, AUSTRALIAN JOURNAL OF PHYSICS, volume 52, pages 1025-1028,1999.
  3. "The Motion of Rotating Cylinders Sliding on Pebbled Ice", Shegelski, Niebergall, and Reid, CANADIAN JOURNAL OF PHYSICS, volume 77, pages 847-862, 1999.
  4. "The Motion of a Culing Rock: Inertial vs. Noninertial Reference Frames", Shegelski and Reid, CANADIAN JOURNAL OF PHYSICS, volume 77, pages 903-922, 1999.

 

   Other contributions requested!

 


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