A small bead of mass $0.20\ \text{kg}$ is threaded on a smooth circular ring of radius $0.50\ \text{m}$, fixed in a vertical plane. A light spring of force constant $4.0\ \text{N/m}$, whose natural length equals the radius of the ring, has one end fixed at the top A of the ring and the other end tied to the bead. The bead is released from rest at P, level with the centre O, as shown in the figure. Take $g = 10\ \text{m/s}^2$. What is the force from the ring on the bead when the bead passes the lowest point B?

Fig. 1: A fixed vertical circular ring of radius R = 0.50 m has centre O, top point A and lowest point B. A bead of mass 0.20 kg is on the ring at point P, level with O on the right, and a spring of spring constant k = 4.0 N/m runs straight from A to P.