Recently my social interactions led me to watching an invigorating game of men strategically sliding stones of inertia 0.5MR^2 on an iced lane with a low coefficient of friction: Curling. It's probably a sport that most of us in New York have not tried since it's not very mainstream. However, I may have to consider making a guest appearance at the most prestigious Rochester Curling Club. Watching this sport on television led me ponder the physics behind it.
Curling may be the only sport where the player(s) are allowed to affect the trajectory of the object after its release. "Curlers" use brooms to brush the ice off in front of the stone to make the surface smoother for the stone to travel on (lower coefficient of friction). I first though that the purpose of the curlers brushing the ice was to make the stone curl in whichever way they needed it to by increasing or decreasing friction on one side of the stone. It turns out that I was completely incorrect. They brush the ice to warm the ice so that the stone actually curls less. However I did figure out that unlike many objects, the curling stone curls in the direction it is spun. If you were to take a simple dinner glass and spin it clockwise while it slides forward, (don't use an expensive glass) it will end up curling off to the right. This is because it is pushing on the table with the leading edge more, delivering a greater force of friction. However, the runnning band (the concave surface of contact on the bottom of a curling stone) enables it to move in the direction it is spun - for what reason I could not conclude from my research.
The most interesting question I had was: why specifically does the curling stone "curl" on the ice? Which is apparently a hot topic amongst some physicists in curling competitive countries.
I found two interesting but not entirely proven theories: The Scratch Theory and the Asymmetric Friction Melting (ASF). The Scratch Theory says that the scratches made by the leading edge of the running band are hit by the rear edge of the band - sending it in the path of its rotation. ASF says that there is more friction on the leading edge which heats the ice more and provides more lubrication for the stone, while the back has more friction. This process would also theoretically send it in its path of rotation.
While I learned a lot more about curling and the apparent uncertainty that lies behind it, I think its a great time this winter to maybe try it out for myself!
(p.s. A "snowman" is a perfect score of 8 because it looks like a snowman. If you want to learn more in depth about it check out this link. It taught me a lot! https://youtu.be/7CUojMQgDpM)