Let's consider a system of three boxes stacked vertically, resting on the ground. There is friction acting between all surfaces involved. I push on the middle box, but without enough force to move the system at all. If we are to analyze the middle box, we'll see a static friction force coming from the boxes both above and below it. The friction from the top box must equal zero, though, because if it were nonzero, the Newton's third law paired friction force from the middle box would cause the top box to accelerate. My question is this: if we were not given the information that nothing in the system is accelerating, and rather simply given the information about the applied force and friction forces, how would we know that no friction were coming from the top box? Also, why is there no friction from this box in the first place?