OK, first of all, I wasn't really sure where to post this; it's not really homework because we covered the material weeks ago and moved on, leaving me totally in the dark. My physics teacher has this annoying habit of saying something is "ALWAYS true" or "NEVER true" and then beginning the next class with an exception, so I need to know if this is true here or not. My question is this: Is Newton's third law valid only in inertial frames of reference? That is, can the reaction force ever be unequal to the action force, and under what conditions? We did the classic "elevator experiment", wherein we weighed ourselves at different points of the elevator's motion and found that our apparent weight change is caused by a differing strength of the normal force of the floor on us. I realize that this is an experiment designed to verify Newton's second law, and not the third, but it seems like should be more consistent - the reaction force in these cases is very clearly not equal to the action force. I can't help but feel like I'm missing some element in this that's keeping it all from "clicking". I asked my teacher, and he went on some tirade about pseudoforces (???) and circular motion ... thus cementing my decision to become an English major when I enter college. Any and all help would be appreciated, and try to be gentle - physics is not my forte.