8.4: Law of conservation of momentum
If there are no external forces acting on the system, and no masses leave or enter the system, the system is called a closed system. Since \(\overrightarrow{\boldsymbol{J}}_{i, \text { ext }, 12}=\overrightarrow{\mathbf{0}}\) in such a system, it follows from Eq. 8.10 that we have:
The total momentum of a closed system is conserved. This means that the total momentum is constant, equal to the momentum of the centre of mass \(\overrightarrow{\boldsymbol{p}}_{G}\), and does not change in time.
\[\overrightarrow{\boldsymbol{p}}_{G}=\sum_{i} \overrightarrow{\boldsymbol{p}}_{i}=\text { constant } \tag{8.12} \label{8.12}\]
Because \(\overrightarrow{\boldsymbol{p}}_{G}=m_{\mathrm{tot}} \overrightarrow{\boldsymbol{v}}_{G}\), momentum conservation also implies that the velocity \(\overrightarrow{\boldsymbol{v}}_{G}\) of the centre of mass of the system is constant. Since Equation 8.12 is a vector equation it can be applied along each of the coordinate axes:
\[\begin{align} \sum_{i} p_{i, x} & =\text { constant } \tag{8.13} \label{8.13}\\[4pt] \sum_{i} p_{i, y} & =\text { constant } \tag{8.14} \label{8.14}\\[4pt] \sum_{i} p_{i, z} & =\text { constant } \tag{8.15} \label{8.15}\end{align}\]
Note that if external forces only operate along one of the coordinate axes, conservation of momentum along that axis does not hold anymore, but along the other two axes momentum will still be conserved.