9. A small sphere of mass m is attached to the end of a cord of length R which rotates under the influence of the gravitational m force (mg) in a vertical plane about a fixed point O. Assume that the tension T is zero when the small sphere is passing through the top of the circle (0 = 0). Please write down the force equation along the tangential direction and find the angular speed ( w = d0 / dt ) as a function of the angle 0. (10%)