Curling: The Friction Sport (Technical)

The modern game of curling consists of a 41 pound granite rock sliding across some 42 m of ice to a target called a house. The rock, technically termed a “stone,” is typically preceded on the ice by two players with “brooms,” who vigorously sweep the ice immediately in front of the stone to influence its trajectory (Willoughby et al. 2005, Bradley 2009, Esser 2011). This unique winter sport, affectionately termed the “roaring game” because of the sound the stone makes as it slides on the ice, has its roots in 16th century Scotland, where river bottom rocks were the stones which slid across ice-covered lochs to a target (Clark 2008). Now, play is more refined, utilizing pebbled ice rinks, brooms, curling shoes, and carefully formed granite stones.

The ice rinks are pebbled by spraying water onto the ice to make a slightly bumpy surface as the droplets freeze into little protrusions on the ice surface, without which the stone could not curl (Shegelski 2001). Brooms are now typically made of synthetic materials such as nylon, and curling shoes consist of one shoe with a larger friction coefficient than the ice and another shoe which has a friction coefficient smaller than that of the ice, thus providing players with a “gripping” shoe on one foot and a “sliding shoe” on the other. A friction coefficient is the measure of how much a substance resists sliding on another substance. The granite stones are highly polished, very hydrophobic, and have a small hollow on their underside so that only a small ring on the bottom of the stone actually touches the ice at any one time.

Each of the pieces of equipment needed to play curling is designed to maximize the impact it has on friction coefficients. Style of play also influences friction coefficients. For example, some sweepers use a conventional style of sweeping, which consists of the sweeper standing to the side of the stone while they sweep. The sweepers employ more force closest to their feet, so there is a greater rise in temperature on the side of the stone closest to the sweeper, which causes asymmetric friction acting on the stone. This matters because the sweeping causes the uppermost layer of ice to melt, thus providing a lubrication layer for the stone to glide on. On the other hand, a high-angle style of sweeping consists of the sweepers being behind the stone while it travels, giving a more even distribution of heated ice, and so lessening the curl of the stone (Marmo 2006). Both can be useful, depending on where the stone is desired to go.

There is much scientific debate about the role friction plays in causing the stone to curl. The friction acting on the stone is undoubtedly asymmetric, but how this results in the stone’s curling trajectory is not yet fully understood. Some believe that as the stone twists, it pushes the water layer to the side, creating a lubrication film for the rock to slide on (Bradley 2009). Others say that the rotating stone not only pools water one its one side, but also chipped ice and other debris (Denny 2002). As curling continues to gain popularity, it can be expected that further scientific inquiry will continue to be directed at exploring the role of friction in the curl of the stone.

Learn more about the technical aspects of friction and curling.

By: Jessica Egan, University of Utah
Jessica Egan began her love of chemistry under her mom’s direction with homeschool experiments in middle school and then up into high school through her high school chemistry teacher. She pursued a chemistry and art double major at Hillsdale College and decided to continue her chemical education by attending graduate school at the U. She has recently graduated with a M.S. from the U in Analytical Chemistry and is looking for a career in industry.

References

Bradley, J. L. 2009. The sports science of curling: a practical review. Journal of Sports Science and Medicine 8:495-500.

Clark, D. 2008. The roaring game: a sweeping saga of curling. Key Porter Books, Toronto, Ontario, Canada.
Denny, M. Curling rock dynamics: towards a realistic model. 2002. Canadian Journal of Physics 80:1005-1014.

Esser, L. 2011. Swept away: exploring the physics of curling. Science Scope 35:36-39.

Marmo, A. A., I. S. Farrow, M-P Buckingham, and J. R. Blackford. 2006. Frictional heat generated by sweeping in curling and its effects on ice friction. Proceedings of the Institution of Mechanical Engineers, Part L.: Journal of materials: Design and Applications 220:189-197.

Marmo, B. A., M-P Buckingham, and J. R. Blackford. 2006. Optimising sweeping techniques for Olympic curlers. The Engineering of Sport 6 3:249-254.

Shegelski, M. R. A. 2001. Maximizing the lateral motion of a curling rock. Canadian Journal of Physics 79:1117-1120.

Willoughby, K. A., and K. J. Kostuk. 2005. An analysis of strategic decision in the sport of curling. Decision Analysis 2: 58-63.

Articles by Kenny Morley.

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