A hockey puck of area A and mass m rides on an air hockey table, so that the puck sits on a cushion of air (which has a thickness d and viscosity η). The puck is connected to a thread whose other end is attached to a vertical small rod, so the puck travels a large circle of radius R (R is much larger than the radius of the puck, as shown in the figure). The rod exerts a small torque to maintain the circular motion over time, since otherwise the puck would gradually slow down and stop. The puck turns with frequency f (revolutions per unit time). The force due to viscosity is Fvis=ηAv/d.
Which expression gives the torque which the rod must exert?