1. Suppose a cube of mass m is lying at rest on a horizontal surface. The area of one face of the cube is A. For the equlibrium of cube no net force acts on it. If we consider the vertical direction then the contact force N should be equal to the force due to gravity plus the force due to atmospheric pressure acting on the top face. There is no air between bottom face and the horizontal surface so atmospheric pressure is not acting on the bottom face. The atmospheric pressure is substantial too, so we can't ignore it. Still we say that N = mg instead of N = mg + PA, where P is the atmospheric pressure. Why ?