Final answer:
The magnitude of the normal force of the elevator floor on the dog is 400 N.
Step-by-step explanation:
The magnitude of the normal force of the elevator floor on the dog can be determined by considering the forces acting on the dog in the vertical direction. Since the dog is not accelerating vertically, the sum of the vertical forces must be zero. The normal force, which acts perpendicular to the elevator floor, counteracts the weight of the dog. Therefore, the magnitude of the normal force is equal to the weight of the dog.
In this scenario, the weight of the dog is given as 400 N. Therefore, the magnitude of the normal force of the elevator floor on the dog is also 400 N.