Conservation of Momentum
Fxext on 1 + Fxext on 2 = D(px1 + px2) / Dt .
If the external forces are small, or if the time of the collision, Dt, is small, then we have:
D(px1 + px2) = 0. This can be re-written as:
(px1 + px2)i = (px1 + px2)f .
This is called Conservation of Momentum. This is a vector law, so a similar equation holds for each component of momentum.