inner and outer diameters of the ring – Note that the meter stick lies across the center of the ring such that one side of the measurement is aligned with the 30 cm hash. In the Background Material, w...inner and outer diameters of the ring – Note that the meter stick lies across the center of the ring such that one side of the measurement is aligned with the 30 cm hash. In the Background Material, we derived an expression for the moment of inertia in terms of the hanging mass, the radius of the hub, and the acceleration of the hanging mass.