Canada rocks the physics of curling
One thing is certain about Canadian athletes: we are very good at throwing stones. With one gold medal already in mixed doubles, thanks to Kaitlyn Lawes and John Morris, Canada has the inside track on two more in the men’s and women’s events. This could have something to do with our inherent understanding of ice. For most of the country a significant portion of the year is spent dealing with snow and ice. Part of “being Canadian” is learning how to walk on ice from an early age!
So it is perhaps not surprising the term “hurry hard” and “raise take out” have extra meaning to Canadians. Indeed, watching curling at the Olympics, it is amazing to hear teams from around the world speaking English language terms in their discussions.
Curling is a sport which is all about science. Everything from the material used in sliders to the design of the broom to the construction of the ice to the rocks themselves has been affected by scientific experiments and advances. The sport has certainly evolved over the past few decades.
Gone are the days of corn brooms with their distinctive “thwacking” sound on the ice. Even the bristled push brooms are obsolete. Modern fabric covered brooms provide better action and allow for more control of both the distance and the direction of the rock. Indeed, some broom heads employing directional fabric have been removed from the game.
But the purpose of using a broom remains the same. It is intended to clean any debris from in front of a rock. Something as tiny as an eyelash can significantly alter a stones path. Brooms also clear the frost which builds up on the surface.
Frost can cause rocks to stick or slide in an unpredictable fashion.
By applying pressure to the broom head, the sweeper can melt the ice a tiny bit.
Generating the first few layers of water molecules makes for a smoother sliding surface with less friction. Anyone who has ever tried to walk on wet ice knows just how much friction is reduced as ice gets wet.
Curling has become sophisticated enough for sweepers to be almost able to steer a rock depending on how fast or hard they are sweeping and where they are sweeping relative to the front face of the stone.
Sweeping from the outside of the curl in exaggerates the amount of curl. Sweeping from the inside out keeps the line straighter.
Current thinking is sweeping has one more effect on the ice. It creates microscratches on the surface. A curling rock will curl regardless if it is thrown properly but
By applying pressure to the broom head, the sweeper can melt the ice a tiny bit. Generating the first few layers of water molecules makes for a smoother sliding surface with less friction.
the micro-scratches can exacerbate the effect by giving the running surface purchase against the ice. The direction of the scratches matters which is why at the top level you will see only one of the sweepers sweeping the rock down the ice.
When curling was in its infancy, any sheet of ice would do. Pebbling added to the roughness of the surface and allowed for more control. But now icemakers have a number of tools at their disposal. Deionized and de-aerated water generates ice which is more reliable and uniform. Gone are small pockets of air and mineralization which could potentially affect a stone.
Temperature control across the surface allows icemakers to fine tune the running line. Making good curling ice is much more than simply pouring water over a cold surface. Some of the best icemakers in the world are Canadian because ice is something we know a great deal about.
The rocks used in curling are all made of the same granite and shaped with exacting precision. Each has a concave base with a narrow running band which is typically only 6 mm wide. It is along this narrow rim which the rock slides.
The curl of a rock is a complex mixture of physics and chemistry. Water is an unusual substance. It is the only compound known which melts under pressure.
This has to do with the hydrogen-bonding network joining the individual molecules into the lattice structure.
When a stone is thrown, it will typically travel at speeds of around 1.5 to 2.0 m/s and complete somewhere between 2 and 4 rotations as it travels down the ice.
Under these conditions, the entire shot is a gentle curve. For a rock thrown with a counter-clockwise rotation (an “out-turn” for a right-handed curler), the rock will curve to the left.
The curve arises from the difference in the amount of melting between the front and back edges of the running band. In the front, extra melting leads to less friction.
The net result is the stone slip-pivots all the way down the ice.
Curling is just one of many sports with a lot of science. But it is a sport where Canadians have become masters of the forces involved.