This weekend I was at club curling practice in Utica and the coach was teaching the new members about sweeping. I happened to overhear her explain that sweeping helps to decrease the frictional force on the stone, and realized that I had no idea about the physics behind the sport that I’ve been playing for three years. I was interested in the motion of curling stones and why exactly they curl in way which is so characteristic of the sport. Curling stones move in a counter-intuitive way: if a stone is delivered with a clockwise rotation it will curl to the right, however if you push a cup along a table with a clockwise rotation the cup will travel to the left (no cups were harmed in the testing of this motion). This is an effective comparison because the stone only touches the ice with the running band, which is a thin, round surface on the bottom of the stone similar to the rim of a cup. There are two theories for the cause of this motion: the scratch theory and the asymmetric friction theory. The first states that the rough running band causes microscopic scratches in different directions on the ice. As the stone moves down the ice, the front of the stone scratches the ice in one direction and is then followed by the back of the stone traveling in the opposite direction, thus causing the back of the stone to experience greater frictional forces than the front due to encountering these scratches. This ultimately causes the back of the stone to slow at a faster rate than the front, and causes the stone to follow the direction of motion at the front of the stone. The second theory states that as the stone slows, is begins to “lean forward” slightly in an attempt to tip over. This same tipping motion is observed in the cup analogy, so there must be something else causing the strange motion of the stone. In fact, the increased force on the front of the stone causes the ice at the front of the stone to melt slightly from the pressure (just like ice skates). The thin liquid layer at the front significantly decreases the coefficient of friction between the ice and the stone, so similar to the first theory, the back slows at a faster rate than the front and causes the stone to follow the direction of the front. Both of these theories are convincing from a physics point of view and both may very well be correct. The science of curling is still disputed and emergent data is currently being collected which may prove these theories or come up with a completely new theory to add to the debate.
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