با توجه به اصل پايستگی انرژی داريم: $\Delta {{U}_{AB}}+\Delta {{K}_{AB}}=0$ $\Rightarrow -\left| q \right|E{{d}_{AB}}\cos \theta -mg{{d}_{AB}}+{{K}_{B}}-K_{A}^{0}=0$ $\xrightarrow{\cos \theta =1}{{K}_{B}}=\left| q \right|E{{d}_{AB}}+mg{{d}_{AB}}$ $\Rightarrow \frac{1}{2}mv_{B}^{2}=\left| q \right|E{{d}_{AB}}+mg{{d}_{AB}}$ $\Rightarrow V_{B}^{2}=\frac{2\left| q \right|E{{d}_{AB}}}{m}+2g{{d}_{AB}}$ $\Rightarrow V_{B}^{2}=\frac{2\times 1\times {{10}^{-3}}\times 10\times 0/1}{1\times {{10}^{-3}}}+2\times 10\times 0/1$ $\Rightarrow V_{B}^{2}=4\Rightarrow \left| {{V}_{B}} \right|=2\frac{m}{S}$