A system consists of three masses  ${\mathrm{m}}_1$and  ${\mathrm{m}}_2$ and  ${\mathrm{m}}_3$ Connected by a string passing over a pulley P. The mass  ${\mathrm{m}}_1$ hangs freely and ${\mathrm{m}}_2$ and ${\mathrm{m}}_3$ Are on a rough horizontal table (the coefficient of friction = $\mu$ ). The pulley is frictionless and of negligible mass. The downward acceleration of mass m1 is : (Assume  ${\mathrm{m}}_1 = {\mathrm{m}}_2 = {\mathrm{m}}_3 = \mathrm{m}$)