[[Linear Momentum]]

Total momentum of system stays the same when there is no external force acting on it.

# Summary
* Total momentum is vector sum
* If net external force is 0, then momentum stays the same.
* Momentum is conserved in every action
* Nonzero net external force transfers momentum in or out of system, which is impulse.
* Center of mass velocity stays constant when no external force.

# Center of Mass Velocity
$$\vec v_{cm}=\frac{\Sigma \vec p_i}{\Sigma m_i}$$
Treats whole system like momentum is concentrated in one point.