A block of mass M 1 resting on a frictionless horizontal surface connected to a spring of spring contant K that is anchored to a wall .A block of mass M 2=?M 1 is placed on the top of the first block.The two bodies move as a unit with S.H.M .What is the maximum amplitude of oscillation so that the two bodies move as a unit? The coefficient of friction between two bodies is ?.