A block of mass M at rest on the floor is attached to a ball of mass M / 2 via a light inextensible string of length L which passes over a light small frictionless pulley. The pulley is at a height of 0.6 L from the floor. The ball is raised to a height of L / 6 and released (see figure). The speed of the block at the instant the string gets taut (not slack) will be