State the law of conservation of momentum.

(N/A) The law of conservation of momentum states that if the net external force acting on a system of particles is zero,then the total linear momentum of the system remains constant.
Mathematically,if $\vec{F}_{ext} = 0$,then $\frac{d\vec{P}}{dt} = 0$,which implies $\vec{P} = \text{constant}$.
This means that the total momentum before an interaction (such as a collision) is equal to the total momentum after the interaction.