Air Pressure Acting Below Object Consider an object resting on a surface. If I had to find the net force on the object, I would write an equation as follows: weight = normal force.
But what about the air pressure that's acting on the surface of the object? Since object is resting on a surface, there is no air pressure acting from below the object to cancel out the air pressure acting on top of the object. So shouldn't the equation be: weight + air pressure force = normal force.
Is this correct?
 A: Yes. However, it usually works out the same as if you ignored air pressure. Homework problems in beginning physics courses are often simplified so you can concentrate on the part of physics in the current chapter. So it is certainly true for your problem. So let's see why you can ignore it.
Consider an object not sitting on a surface. It has a top and bottom surface with area A. The force from air pressure on both surfaces is $pA$. The forces are in opposite directions, so the total is $0$. The object is not accelerated by air pressure.
Consider a surface with no object sitting on it. Often the back side of the surface is not shown in the problem. But it always exists. The surface is just the top surface of another object. And that object is always in air. This is true if the object is a table, or even if it is the entire Earth. So air pressure squeezes that object, but does not accelerate it.
This means you can think of an object sitting on a surface as an object sitting on another object. Air pressure acting on the pair together does not accelerate the pair.
Air pressure squeezes them together, but that is no different from how they are squeezed when sitting apart. The force on the table changes from $pA$ to $pA + w$. The $pA$ part cancels a similar $pA$ on the back of the table. The net force on the table is $w$.
Likewise, the force on the bottom of the object changes from $pA$ from air pressure to $pA + w$ from the normal force of the table. Again. $pA$ from the top and bottom cancel. leaving a net force of $w$ upward.
A: Your equation is correct only if the surface and the object are perfectly flat, so that they press flush against each other. However, this is almost never the case in real life,as there are minute gaps between the surfaces. Therefore, the difference is pressure between the top and bottom surface is negligible(for small objects) and we dont consider it.
