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Re: [Phys-l] Errata for FCI?



That the buoyancy force = weight of displaced fluid is derived by observing that an identically shaped water blob would be supported by the pressure of the adjacent water. This observation gets you out of having to do the integral over the entire surface of your shape -- you already know how it will come out. But it assumes that there is in fact water COMPLETELY SURROUNDING your object (and your water blob). It seems to me that if the object is only partially surrounded by water, then there is ambiguity in the question. In other words, the "buoyancy = weight of displaced fluid" rule does not apply bc you are not integrating over the entire surface.

If I wanted to design an FCI type question to see which of my students have mastered this subtlety (by then end of their first semester of physics?) I would not have the object rest on a surface. I would maybe ask about a helium balloon tethered by a thin string.