Two blocks of masses 2 kg an 4 kg are connected to the free ends of a spring of force constant 800N/m. The blocks are pulled apart so that the spring is stretched by a length ‘x’m. and are released simultaneously, When the spring acquires its natural length, velocity of 2 kg block is 4m/s. Then ϕ elastic potential energy stored in the spring just after the blocks are released is (the surface is smooth)