Understanding The Science behind Curling
Curling stones are thick stone disks made of heavy and polished granite along with a handle attached to the top. The maximum mass of a curling stone is 20 kilograms. Why does a curling stone curve (curl) on the ice? Players “sweep” the ice right in front of the stone to decrease the friction of the stone with the ice. This affects the degree of curl of the stone as it slides down the rink, towards the target.
To understand why a curling stone curls we must analyze the physics of curling. The figure below shows a schematic of the curling physics, for a curling stone rotating clockwise.
Where: G is the acceleration due to gravity. NF is the resultant normal force exerted on the bottom front half of the curling stone, due to contact with the ice .NB is the resultant normal force exerted on the bottom back half of the curling stone, due to contact with the ice. Since the curling stone is in rotational equilibrium in the plane of the page, the sum of the moments about G in this plane must sum to zero. This means that the normal force NR must exert a momentum in the opposite direction to the momentum caused by R. This is only possible if the line of action of NR is to the left of G (on the bottom front half of the curling stone). Consequently, most of the normal force is acting on the bottom front half, and NF > NB.
It is a known fact that contact pressure with ice melts a very thin layer of water on the surface of the ice. This in turn decreases friction. Thus, the friction is inversely proportional to the amount of contact pressure with the ice. So, as a result of NF > NB, the friction force exerted on the (bottom) front half of the curling stone (due to contact with the ice) is less than the friction force exerted on the (bottom) back half of the curling stone (due to contact with the ice).The figure below illustrates this.
Where: FF is the resultant component of friction force exerted on the (bottom) front half of the curling stone, opposing the direction of rotation FB is the resultant component of friction force exerted on the (bottom) back half of the curling stone, opposing the direction of rotation
As stated previously, FF < FB. As a result, the clockwise rotation of the curling stone causes it to move to the right due to the greater friction force FB. If the curling stone is rotated counterclockwise then FF and FB will be pointing in opposite directions and the stone will instead curl to the left. By imparting spin on the curling stone, friction is induced which steers the stone away from its initial straight line trajectory.
The challenge for curlers is to control the level of curl of the stone, in order to get it where they want in the target area. They do this by controlling the level of friction of the stone with the ice.
Image courtesy : wikipedia