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

I originally thought about cutting apart the piling, tiny slice by tiny slice,
and looking at either the forces involved, or the energy changes. But I'm
liking more and more the idea of *building up* the piling, starting with the
first tiny thickness dh, and then successively gluing more tiny slices onto it.
Every one has to be transported down through the water, and then attached --
somehow -- to the previous slice.

I haven't found, yet, anything that convinces me that the piling is under
compression at any point.

(OK, OK, I'm not counting the pre-stressing that trees naturally have, and which
tended to cause early mast-builders such consternation when their nice new
composite masts started failing all over the place.)

"The four points of the compass be logic, knowledge, wisdom and the unknown.
Some do bow in that final direction. Others advance upon it. To bow before the
one is to lose sight of the three. I may submit to the unknown, but never to the
unknowable." ~~Roger Zelazny, in "Lord of Light"

From: Jeffrey Schnick <JSchnick@Anselm.Edu>
To: Forum for Physics Educators <>
Sent: Thu, October 21, 2010 5:19:40 AM
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

I think that a negative effect of defining the buoyant force to be an
upward force equal in magnitude to the weight of the water dispaced, is
that I would expect that students who have been taught that definition
would have a greater chance of getting the following problem so wrong
that they come to the conclusion that the piling is in tension rather
than compression

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 pond. 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.

-----Original Message-----
[] On Behalf
Of Bernard Cleyet
Sent: Thursday, October 21, 2010 2:15 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Definition of upthrust or buoyancy



bc thinks he'll remember this.

On 2010, Oct 20, , at 09:02, LaMontagne, Bob wrote:

If you want to imagine the buoyancy on an object, first
imagine a blob of water with exactly the same shape as the
object. That water is obviously in equilibrium. Integrate the
pressure over the surface area of the blob. If in
equilibrium, this must give a force that is equal and
opposite to the weight of the water. Now replace the blob
with the object. The surfaces forces have not changed so they
still add to the weight of the blob. Hence, the buoyant force
is the weight of the water displaced. This definition of
buoyancy does not require water surrounding all surfaces of
the blob - the blob could be on the bottom of a glass beaker.

As John D. has pointed out, stickyness is a different issue.

Bob at PC

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