One end of a massless rope, which passes over a massless and friction less pulley P is tied to a hook C while the other end is free. Maximum tension that the rope can bear is 840 N. With what value of maximum safe acceleration (in ${\mathrm{ms}}^{-2}$ ) can a man of 60 kg climb on the rope? ($\mathrm{g}=10\mathrm{m}/{\mathrm{s}}^2$ )