Believe it or not, scientists are not exactly sure why curling stones move as they do. More on this below, first some background on the sport of curling.
Curling is a fun sport to watch and has been an Olympic sport since 1998. Curling consists of a member of a four person team sliding a granite stone down a a sheet of ice. Two other team members sweep the ice with special brooms in front of the stone while the fourth team member directs their sweeping.
Points are awarded by being close as possible to the center of a large target (the “button”) and having your stones within the target (called the “house”). Scoring is somewhat complex depending on the placement of each team’s stones.
Curling stones weigh between 42 and 44 pounds and are made of granite. There are a few factors that help the curling stones travel the long distance down the ice. First, the weight of the stone gives it inertia. Second, the stones have a concave shaped bottom which causes the stones to travel farther because less area is making contact with ice.
Third, the ice sheet is is sprayed with water to create small frozen droplets on the ice surface – referred to as “pebbles” of ice. The curling stone rides these pebbles of ice which allow them to travel farther than they would on smooth ice by reducing the surface area by which the stone makes contact with the ice.
A curler can cause the stone to curve as it slides along the ice by making the stone slowly rotate either clockwise or counter-clockwise. The curving trajectory of the stone is called “curling” – which is why the sport is called curling.
The sweepers change the curve and make the stones travel farther. “Sweeping in front of a running stone accomplishes several things. First, it cleans the path of any debris that may be on the ice that could otherwise alter the stones travel. Secondly, by applying pressure to the broom while sweeping in front of the stone, the ice is slightly warmed, creating less friction between the ice and the stone – this can help the stone travel further than it would have, and it can also affect the curl of the stone.” (from Anchorage Curling Club). Thus, the point of the sweeping is to cause the stone to curl less and travel farther.
Now for the physics controversy: why does a curling stone curl? The stone travels slowly on ice and only rotates 2-3 during its 25 second journey. This is too slow to curve due to the reasons why a ball in sports such as baseball, ping pong and soccer can curve.
Canadian physicist Mark Shegelski has published several papers on this subject. His conclusion is that the curling stone melts the ice a bit as it moves forward and thus creates lubrication at the front which produces unequal friction between the front half and back half of the stone. This is what causes the stone to curl.
Not everyone agrees with Dr. Shegelski. Researchers in Sweden published a study a few years ago that concluded that the stone curls due to microscopic scratches created in the ice sheet by the front edge of the stone and the rotation of the stone gives the scratches a slight sideways deviation. When the back half of the stone pass the same area they hit the scratches at an angle which causes the curl.
The debate continues. As of yet, there is no clear answer why curling stones curl.