$(4\mathrm{.}2)\backslash (\backslash$underline$\{6\}\backslash ).$ A taut string with three equal masses attached is shown. The tension in the string can be assumed to remain constant for small angles of oscillation. If an excitation Fo sin $\backslash ($ w t $\backslash )$ is applied to the center mass, determine the steady$-$state motion.