Atwood’s machine was an apparatus that allowed a direct verification of Newton’s second law. It could also be used to measure g. Two blocks of equal 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 calculated by measuring time taken to the fall through a distance D. t is the time for which it moved at constant speed. Then find the value of ‘g’ in  $m/s^2,$  assuming ideal small pulley. (The diagram is not drawn to scale) 
[Given : M = 4 kg, m = 1 kg, D = 2m, H = 1m, t = 1s]