\({{\text{m}}_{\text{B}}} = 4 \times {10^{ - 9}}\,{\text{kg}},\quad {{\text{r}}_{{\text{AB}}}} = {10^{ - 3}}\,{\text{m}}\)

\(\frac{1}{2} M_{B}\, V^{2}=k q_{A} q_{B}\left\{\frac{1}{10^{-3}}-\frac{1}{9 \times 10^{-3}}\right\}\)

\(\frac{1}{2} 4 \times 10^{-9}\, \mathrm{V}^{2}=9 \times 10^{8} \times 10^{-6} \times 10^{-6} \times \frac{8}{9} \times 10^{3}\)

\(V^{2}=\frac{8}{2} \times 10^{9}=4 \times 10^{9}\)