## Monday, November 7, 2011

### Curling

Curling is a unique sport in that not only is physics an active part of everything that takes places, but the competitors have to be aware of physics principles for every shot they take.  The topics of friction, energy, conservation of momentum, and circular motion all come into play in curling.

Perhaps the most interesting example of physics at work in curling is how it explains why the stones actually curl.  First, the stone only contacts the ice along a thin ring on the bottom of the stone called the “running surface,” and the ice is pebbled with frozen droplets of water that the stone actually runs on.
Above is a free-body diagram for a rotating curling stone.  To ensure the rotational forces all sum to zero and maintain equilibrium, the normal force must counteract the force of friction, so the normal force is greater in the front of the stone than in the back.  This means that the pressure is greater on the ice at the front of the stone, which melts the pebbling and produces less friction.  With less friction in the front and a rotation already imparted on the stone, the front half of the stone will rotate more quickly than the back, producing the curl, as shown below.

Sweeping in front of the stone is done to take advantage of physics principles.  By sweeping, the pebble on the ice is briefly melted, reducing the coefficient of friction.  This in turn allows the stone to curl less by producing a more uniform level of friction beneath the entire stone by having melted pebble below the entire stone.  This also allows the stone to travel further by reducing the force of friction acting against its motion forward.

Other instances where physics enters curling is in the delivery, where transfer of energy throughout the curler’s body allows the curler to build up enough speed to deliver the stone down the sheet.  When stones collide, principles such as conservation of momentum (including angular momentum) and loss of energy allow the curlers to predict where stones will end up and assist them in their shot selection.

To see these principles in action: http://www.youtube.com/watch?v=-EswFKNXjMo

-Post written by Rich Grey