Air surrounds any object on all sides. Even if an object is resting on a surface

Question

Air surrounds any object on all sides. Even if an object is resting on a surface there is always air (even if only a thin layer) underneath. The pressure of the air above will exert a downward force. The air below will exert an upward force. Explain and discuss how these forces result in a net buoyant force due to the object being immersed in a sea of air. Why does it work out that the resulting force has exactly the value that it does?

How would the situation be different if there were no air (a vaccum) underneath the object? Then, what would be the net force due to the air pressure?

Explanation / Answer

Let A = Cross-sectional area of the surface P1 = Pressure of air on the upper surface P2 = Pressure of air on the lower surface Case I: ( Object is immersed in sea of air ) Force exerted on the upper surface = F1 = P1 A ( downward ) Force exerted on the bottom surface = F2 = P2 A ( upward ) Net Force = F2 - F1 = ( P2 - P1 ) A ....( upward direction since P2 > P1 ) Here, Net force = Buoyant force = ( P2 - P1 ) A ( upward direction ) ------------------------------------------------------------------------------------- Case II:( No air under the object ) Here, Net force = Force on the upper surface = P1 A ( downward direction )

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