The rotation of the stone is directly related to the direction in which a stone will curl. A stone rotating in a clockwise direction will curl to the right and a rock that is rotating counterclockwise will curl towards the left. What does this mean? Well, this tells you that there is a difference in the forces felt between the edge that is rotating in the direction of motion of the stone and the other that is rotating in opposition to the rotation of the stone.
See Diagram #1 below.
Why does a curling stone curl?
In order for the stone in Diagram #1 to curl to the right, the opposing side must be experiencing a greater force of friction that the favouring side. A curling stone is supposed to make three to four complete rotations in its total trip. This means that the low rate of rotation has a powerful effect on the nature in which a stone curls.
Factors Affecting Curl
Factors that affect how much a curling stone curls are the curling stone's running surface and the surface of the ice.
See Diagram #2 below.
A flat or unusually 'smooth' surface will tend to curl more towards the last few metres of travel than a stone that has a 'sharper' running surface. It can be explained in terms of the force of friction felt by the different types of running surfaces. The smooth running surface will have less frictional force and will not stop as quickly or will tend to 'slide further' in the last few metres of travel than a sharp stone.
The ice surface mainly depends on the way in which the ice is made. Pebbling the ice involves walking down the centre line of the ice, evenly spraying small droplets of water in a side to side motion. As rocks pass over this pebble, they wear it down, and make the surface of the ice flat. Because the majority of curling rocks are thrown down the centre, the pebble on the sides of the ice is rarely worn flat. The pebble gets built up at the sides of the ice, creating a bowl like shape on the surface of the ice. An exaggerated view of this is shown in Diagram #2 above.
Suggestions for further study that would provide a greater understanding of the physics of curling are:
A. How does the rotation rate affect how much a rock curls?
B. Find the static and kinetic coefficient of friction for ice using a Newton force metre and a curling rock.
David D. Babcock