Recently, I've been pondering deeply on relativity (both Galilean and SR) and all of a sudden I find that I don't grasp even the basic concepts of physics (or life) anymore, i.e. I can't go back to my previous, "normal" mode of thinking. Consider Newtonian mechanics, take the ground to be at rest. Imagine a car moving with constant velocity relative to the ground. We have two observers, one standing on the ground and one inside the car. A bob is hanging from a string attached to the ceiling of the car, making an angle alpha with the vertical line (the guy inside the car is applying a horizontal force P to the bob). We want to compare the formulation of Newton's second law for the two observers (and hopefully conclude that both observers find the law in the form of F=ma to be valid). Now I know this is supposed to be a simple, elementary question, but like I said, I suddenly became LOST. How does the observer on the ground know what forces to put in the equation? The forces are "inside the car", how does he know what forces are being applied to the ball? How does he know their magnitude? He's not even in the car, so how is he measuring/feeling these forces? How does he measure the magnitude of g in the car from his own point of view? (recalling that g was 9.81 m/s2 when he was in his own car doesn't count) How does the guy on the ground even know there's gravity in the car? How does he conclude this? Considering he's not inside the car, does he conclude this based on his past experience being in moving cars? No that can't be right, because it involves changing reference frames...does he conclude this based on Newton's second law? That can't be right either since it would be circular reasoning. Does he recognize gravity by considering the behavior of the "whole picture" (the car together with all the objects inside)? How does he know that the bob has whatever mass it has? I mean how does he know what mass to substitute for m in the equation F=ma to check the equation's validity? After some long thinking and mind twisting, I've finally come up with an answer to the above questions that I might share in my replies, but I would like to know what you guys think first. See attachment for pic.