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\begin{eqnarray}
\sf 
\dot{\vec{r}} & = & \frac{1}{m}\vec{p} + \frac{1}{3V}\vec{r} \dot{V} \nonumber \\
\dot{\vec{p}} & = & \vec{F} - \frac{1}{3V}\vec{p} \dot{V} \nonumber\\
M\ddot{V} & = & p-p_{ext} \nonumber
\end{eqnarray}
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