Conservation of Momentum

Conservation of linear momentum

In physics, it is confirmed by abundant experiments that total momentum of a system does not change, if the mass of the system does not change and the external net force of the system is zero:

$$\sum\limits m_i=const.\text{ and }\sum\limits \vec{F}_{ext}=\vec{0}\Rightarrow \sum\limits \vec{p}_i=const.$$

Collision is the most common case to apply this law, because external forces such as friction are very small compared to contact forces during the collision.

Conservation of angular momentum

In physics, it is confirmed by abundant experiments that total angular momentum of a system does not change, if the mass of the system does not change and the external net torque of the system is zero:

$$\sum\limits m_i=const.\text{ and }\sum\limits \vec{\tau}_{ext}=\vec{0}\Rightarrow \sum\limits \vec{L}_i=const.$$

Besides collisions that involve rotation of objects, orbital movements such as earth moving around the sun are also examples of conservation of angular momentum.