|
|
THE GAME: SCIENCE
LE JEU: SCIENCE
|
What Puts the Curl in a Curling Stone ?
by Mark Shegelski
Canadian Curling News, March, 2000 issue
Reprinted with permission
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".
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
the 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.
|
Contact ICING : Author / Auteur: Peter M. Smith
E-mail / Courriel: psmith@icing.org
Postal mail / Courrier postal: 73 Appleford Road, Hamilton, Ontario, Canada. L9C 6B5
Telephone / Téléphone: 1 905 389 7781
Fax / Télécopieur: 1 905 318 7515
|
