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-----Original Message-----an
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of brian whatcott
Sent: Thursday, October 21, 2010 7:37 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Definition of upthrust or buoyancy
Hmmmm...the smart-ass answer to this one is: zero stress (on non
existent piling...)
B
On 10/21/2010 6:19 AM, Jeffrey Schnick wrote:
I think that a negative effect of defining the buoyant force to be
wrongupward force equal in magnitude to the weight of the water dispaced,is
that I would expect that students who have been taught thatdefinition
would have a greater chance of getting the following problem so
ratherthat they come to the conclusion that the piling is in tension
thatthan compressionhas
A vertical wooden piling is embedded in the flat horizontal concrete
bottom of an artificial pond. The wood of which the piling is made
a density that is half that of water. That part of the piling abovethe
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 waterto
a height 2h above the surface of the concrete. Find the averagenormal
stress in a cross section of the piling at height h/2 above thesurface
of the pond. Note that the average normal stress at a cross sectionof
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
the lower part of the piling is actually pushing upward on the upperforce.
part, a minus sign should appear in front of the magnitude of the
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-----_______________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] 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
Hej!
Excellent!
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, firstimagine 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
Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
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