Figure shows a system of two balls of same volume $=0.25{\mathrm{m}}^3$ moving with terminal velocity in a long uniform viscous medium of negligible density. Balls are connected with a thin metal string of cross-section area $0.5{\mathrm{cm}}^2$. Density of upper and lower ball are $1\mathrm{gm}/{\mathrm{cm}}^3\mathrm{and}9\mathrm{gm}/{\mathrm{cm}}^3$ respectively. In this situation metallic string is just about to break. Ultimate stress of material of a string is $\mathrm{n}\times {10}^8\mathrm{N}/{\mathrm{m}}^2$. Find n ( $\mathrm{g}=10\mathrm{m}/{\mathrm{s}}^2$. Coefficient of viscosity is same for both bodies, ignore atmospheric pressure.)