If a box exerts a normal force on the floor, is this strictly on the floor or can we also regard the floor as the Earth?
Answers and Replies
Think of the Earth as a really huge sphere, which is locally flat. Then imagine a plank of wood somewhere on the sphere (that can be your floor). Finally, you put the box on the plank. There will be a normal force pair between the plank and the Earth, and another normal force pair between the plank and the box. If the plank has mass, then the normal force between the plank and Earth will be slightly more than that between the plank and the box.
Alternatively, you can just take the Earth and the plank to be a single system, and the net force on this system is just the normal force from the box plus the gravitational force from the box.
For mechanics problems, it's often perfectly fine to completely ignore any technicalities about 'floors' and whatnot, and just treat the ground as an idealised immovable surface.