Greetings the venerable PF society, I have a question that has been lingering in my mind for a while, so I thought I'd ask it here. Firstly, how far below does the action-reaction (Newton's third law) principle go down fundamentally? How far below can I take it with me so that I don't make gross mistakes? Or else does it not break down at all? Please enlighten me on the meaning as well: "To every action there is an equal and opposite reaction." (from Wikipedia) What does this mean? Why would you necessarily have an equal an opposite force for every acting force element? I know from informal discussions that it breaks down for some specific cases, but I don't know how severely that's the issue. I just need to connect it up to something I know better, so any help or general remark would be useful. ---------- BTW, the context I am going to use it is the effect of spin polarized electrons flowing through a magnetic body, namely spin-torque. It happens that the magnetic body itself feels a torque because it exerts a torque on the incoming spin-polarized electrons to align their spins. So essentially it's an action-reaction type of interaction. But I don't know if that's truly the most fundamental picture Thanks!