Block A of mass $m$ lies on a smooth horizontal table. A light inextensible string tied to A passes over a light frictionless pulley at the edge of the table, goes down and under a light movable pulley, and its other end is tied to a fixed support above, as shown in the figure. Block B, also of mass $m$, hangs from the movable pulley. The system is released from rest. Taking $g = 10\ \text{m/s}^2$, the acceleration of block A is

Fig. 1: Block A on a smooth table, joined by a string over a pulley at the edge, down under a movable pulley carrying block B, and up to a fixed support.