A Note about Treating Gasses as Fluids

Gasses may often be treated as fluids. This means that, for example, we can speak of the buoyant force on an object due to some volume of air being displaced by that object, just as we spoke of a buoyant force due to water and oil being displaced in the previous sections.  Although the buoyant force exerted by air is usually negligible (compared to other forces acting on the object, such as its weight or normal force), this is not always the case. 

For example, in the case of a hot-air balloon or a helium balloon, the buoyant force which provides the “lift” for the balloon is equal to the weight  of the air displaced by the balloon.  When drawing a FBD for such a balloon, it is important to keep in mind that, while the upward force acting on the balloon is the buoyant force (the weight of the air displaced by the volume of the balloon), the downward force must be a combination of both the weight of the balloon (and anything else tied to it) and the weight of the gas inside the balloon (just as the weight of the air outside the  balloon providing the buoyant force is not negligible in this type of problem, neither is the weight of the gas inside the balloon!).

The buoyant force can also be important for solid objects or substances in some cases.  For example, chemists must sometimes take the buoyant  force into account when measuring the mass of a sample in air due to the great precision to which they must know its mass.  (Will the scale reading be greater or smaller than the actual mass of the sample due  to this systematic error? See homework problem 19 #11.)