Abstract
We would like to point out that Jack Allen's description of why a rock follows a curved path in the game of curling is incorrect (March p17). The rock is a granite stone whose contact area on the ice is an annulus. When the rock is released it can be set to rotate clockwise or anticlockwise. Allen's description fails because of the symmetry between the front and back of the annulus. If you define the front and back of the rock using the direction of the instantaneous velocity of the centre of mass of the rock, then for each back portion of the contact annulus there is a corresponding front portion, with relative displacement parallel to this velocity. The size of the velocities of corresponding front and back portions, relative to the ice, are the same. So in Allen's picture the lateral components of the frictional forces acting on the two portions have the same size but opposite directions, and therefore cancel. In other words there is no net lateral force and no curl.