$(a)\;12.4 \;m/s\qquad(b)\;6.2\; m/s\qquad(c)\;3.1\; m/s\qquad(d)\;None\;of\;these$

Answer : (b) $\;6.2 \;m/s$

Explanation :

$\bigtriangleup P\;.E=\large\frac{-2\times9\times10^{9}\times2\times4\times10^{-12}}{3} + \large\frac{2\times9\times10^9\times2\times4\times10^{-12}}{5}$

$\bigtriangleup P\;E=\large\frac{-144\times10^{-3}\times2}{3\times5}=\large\frac{-96\times10^{-3}}{5}$

Using conservation of total energy as their is no external force

$\bigtriangleup T\;.E=0$

$\bigtriangleup K\;.E + \bigtriangleup P\;.E=0$

$\large\frac{1}{2}\times1\times10^{-3}\times v^2=\large\frac{96\times10^{-3}}{5}$

$v=6.2\;m/s\;.$

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