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Re: [Phys-l] Definition of upthrust or buoyancy

The document
contains two solutions to the problem just below. The first solution
"Method 1" uses the definition of the buoyant force as an upward force
equal in magnitude to the weight of the fluid displaced. The second
solution "Method 2" uses the idea that the buoyant force is the net
force of the fluid on an object submerged in a fluid but in that
solution I just call that force the force of the fluid.

A vertical wooden piling is embedded in the flat horizontal concrete
bottom of an artificial pond. The wood of which the piling is made has
a density that is half that of water. That part of the piling above the
concrete is in the shape of a right circular cylinder of cross-sectional
area A and height h. The artificial pond is filled with fresh water to
a height 2h above the surface of the concrete. Find the average normal
stress in a cross section of the piling at height h/2 above the surface
of the concrete. Note that the average normal stress at a cross section
of the piling is "how hard the material of the piling below the cross
section is pulling downward on the material of the piling above it"
divided by the cross-sectional area of the piling. In the event that
the lower part of the piling is actually pushing upward on the upper
part, a minus sign should appear in front of the magnitude of the force.
In such a case, the minus sign in the resulting expression for the
stress tells the reader that the piling is in compression.