A body of mass $1 \mathrm{g}$ and carrying a charge $10^{-8} \mathrm{C}$ passes from two points $\mathrm{P}$ and $\mathrm{Q}$ . $\mathrm{P}$ and $\mathrm{Q}$ are at electric potentials $600 \mathrm{V}$ and $0 \mathrm{V}$ respectively. The velocity of the body at $\mathrm{Q}$ is $20 \mathrm{c}\mathrm{m} {\mathrm{s}}^{-1}$ . Its velocity in $\mathrm{m} {\mathrm{s}}^{-1}$ at $\mathrm{P}$ is