A plastic ring of radius R is rubbed all over with wool, and acquires a negative charge of -Q, distributed uniformly over the ring. The ring is held horizontally by an insulating stick (not shown). A light, point-like plastic ball (mass m) is suspended vertically from an insulating string along the vertical axis, a small distance z (z << R) above the ring. The plastic ball is positively charged with +q. Neglect gravity. The string hanging the plastic ball is suddenly cut. The plastic ball will be oscillating along the vertical axis about the center of the ring. What is the period of oscillation?