A pulley with radius R is free to rotate on a horizontal fixed axis through its center. A string passes over the pulley. Mass m1 is attached to one end and mass m2 is attached to the other. The portion of the string attached to m1 has tension T1 and the portion attached to m2 has tension T2. The magnitude of the total external torque, about the pulley center, acting on the masses and pulley, considered as a system, is given by: (m1 – m2)gR Can someone explain how to derive this relation? Why is the distance just R?