In figure shown, the spring constant is k. The mass of the upper block is m and that of the lower back is 3m. The upper block is depressed down from its equilibrium position by a distance δ and released at t = 0.
The value of δ for which the minimum normal reaction on 3m from ground is less than mg is
3mg/k
mg+k(δ−mgk)=3mg or δ=3mgk