A bead of mass ' ' slides without friction on the wall of a vertical circular hoop of radius ' ' as shown in figure. The bead moves under the combined action of gravity and a massless spring (k) attached to the bottom of the hoop. The equilibrium length of the spring is ' '. If the bead is released from top of the hoop with (negligible) zero initial speed, velocity of bead, when the length of spring becomes ' ', would be (spring constant is ' k ', g is acceleration due to gravity)