At wood’s machine was an apparatus that allowed a direct verification of Newton’s second law. It could also be used to measure g. Two equal blocks of mass M  hang at either side of a pulley ; see figure. A small square rider of mass m is placed on one block. When the block is released, it accelerates for a distance H till the rider is caught by a ring that allows the block to pass. From then on the system moves at a constant speed which is measured by timing the fall through a distance D. t is the time moved at constant speed. Find value of 'g'.

[Given  :   M = 4 kg, m = 1 kg, D = 2 m, H = 1m, t = 1 s]