# The Strange Physics of Curling

It turns out that curling involves some complex physics. [Destin] of Smarter Every Day has jumped in to find out why scientists on opposite sides of the Atlantic disagree about why curling stones curl.

If you’ve been watching the Olympics, you’ve probably seen some curling, the Scottish sport of competitively pushing stones on ice. As the name implies, curling stones don’t go straight. The thrower pushes them with a bit of rotation, and the stones curve in the direction they are rotating. This is exactly the opposite of what one would expect — try it yourself with an inverted drinking glass on a smooth table.  The glass will curl opposite the direction of rotation. Clockwise spin will result in a curl to the left, counterclockwise in a curl to the right.

The cup makes sense when you think about the asymmetrical friction involved. The cup is slowing down, which means more pressure on the leading edge. The rotating leading edge pushes harder against the table and causes the cup to curl opposite the direction of rotation.

The problem is that curling stones don’t do this. Scientists at Uppsala University in Sweden believe it is due to the scratch theory — the leading edge of the stone scratches the ice as it passes, which imparts a force on the trailing edge of the stone.

Dr. Mark Shegelski from Canada’s University of Northern British Columbia disagrees. His theory is that asymmetric friction melting is at play. The stone melts the ice as it passes, creating a thin film of water which the stone rides on.

It could be that both of these theories are correct — and both forces are at play. The hard part is isolating one from the other. Both scientists have done some practical experiments, but not enough to satisfy the scientific community. Can one of our hackers come up with definitive proof as to why curling stones curl the way they do? Let us know down in the comments.

We love stories that still have scientists guessing. A great example is the ball bearing motor, which we featured back in 2014.