 So the total momentum of this system was conserved, and remember we didn't know anything about these two forces, why they were there, only that they obeyed Newton's third law, and all forces do obey Newton's third law. So no matter what A and B were, we knew that the momentum was conserved, and in fact that's true for any closed system. What do I mean by a closed system? Well supposing we ignored particle B and we just looked at particle A, obviously that momentum is not conserved, because this momentum here, the change in momentum for particle A, is non-zero, it's going to change. But when you look at this force you say, well I know that particle A must be interacting with something, that's the only way forces come about, so I go hunting for that, and when I find particle B you say all right, now I've got all the particles that are interacting, and so a closed system is where you've included all the bits that are interacting with each other, that's not interacting with anything outside the system, and then if you look at that, the total momentum of any closed system has to be conserved. And this is a really really fundamental principle. It's true in quantum mechanics, it's true in relativity, you have to change the form of momentum slightly for relativity, but there is still a thing called momentum, and it's still a special thing that's conserved for any closed system.