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

There is a practical case that may be helpful to consider.
To salve a sunken hull, heavy duty elastomer balloons
are secured by straps round the hull.

When the balloons are secure, they are inflated with compressed air to the ambient
water pressure: their density being comparatively low, they pull up, and stretch the straps
The internal air is compressed, which can be a problem when the hull is levitated from
appreciable depth. The internal pressure becomes considerably more than the ambient
pressure, so the balloons could burst.

To summarize: flotation balloons are in compression: lift straps are in tension.

Brian W

On 10/21/2010 6:40 AM, chuck britton wrote:
You are absolutely right.
I would indeed be led to conclude that any 'floatable' object that is
being held forcefully underwater by some attachment to it's underside
- would be in tension.

Would you agree that the underwater attachment mechanism is under tension??

At 7:19 AM -0400 10/21/10, Jeffrey Schnick wrote:
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-----
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, 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