I have always been under the impression that Newton's third law and the law of conservation of linear momentum are really the same thing; synonymous, so to speak. But then I was wondering if Newton's Third Law holds for a non-isolated system. I mean, I know for sure that momentum conservation is only applicable for an isolated system (At least, this is what I have been taught at A level). But, be it an isolated or non-isolated system, isn't it always the case that the force body A exerts on body B is equal and opposite to the force body B exerts on body A when they interact? The question here is, does Newton's Third Law apply for all systems, whether or not an external force is acting? Of course, my first stop was the internet, where I came across this: http://physics.stackexchange.com/qu...on-of-momentum-really-imply-newtons-third-law Now I have more questions really. For a system of more than two bodies, the forces between the bodies may not necessarily be equal and opposite? Long story short, my questions are: 1) Does Newton's third law hold for non-isolated system? If it doesn't, why? 2) If there's a system of more than two bodies, does Newton's law hold? 3) Is the sum of internal forces in a system always zero? I would be very grateful if someone would help me out with this.