\exists\,\,m_{I}\in\mathbb{R}_{+}:\,\,\sum_{I}m_{I}\mathbf{v}_{I}\left(t\right)=\mathbf{p},\,\,\,\,\,\,\textnormal{e}\,\,\,\,\,\,\frac{d\mathbf{p}}{dt}=0.\label{eq:1.5.1}