A system consists of three masses $m_1$, $m_2$ and $m_3$ connected by a string passing over a pulley P. The mass $m_1$ hangs freely and $m_2$ and $m_3$ are on a rough horizontal table (the coefficient of friction = $\mu$ ) The pulley is frictionless and of negligible mass. The downward acceleration of mass $m_1$ is (Assume $m_1$ = $m_2$ = $m_3$ = m)