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Re: [Phys-l] buoyancy on a submerged pole



For what it is worth, I placed a large shallow screw-on top (like the
lid to a can of baby formula but deeper) on the bottom of a larger
container. I then filled the container with water, holding down the lid
while it filled. I then let go.

No adhesives were used.

You can predict what happened and then look at the pictures:
<http://www.esu.edu/physics/cohen/phys-l/>

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@po-box.esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
Of Chuck Britton
Sent: Friday, November 05, 2010 8:54 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] buoyancy on a submerged pole

Comments interspersed


At 10:55 PM -0700 11/4/10, John Denker wrote:
I'm all in favor of doing experiments.

hear, hear - and in keeping with doable experiments -
I'd like to withdraw the ill-chosen term 'monolayer'. Any
suitable achievable thickness of said water layer will suffice.
Whatever is required to keep the system and the results 'simple'.

(Realizing full well that the ANALYSIS will NEVER be
simple ;-) )

On 11/03/2010 06:42 PM, Chuck Britton wrote:
If we let a monolayer of water seep under the box (but
keep it tied
down with a thread) we can then call the upward force
Buoyancy. Same
amount of force - different name. OK.

Using a "monolayer" to draw a sharp boundary here is a bad
idea. It is
a distinction without a difference. The physics doesn't
care whether
there is a monolayer there or not. One monolayer of H2O
adsorbed on a
surface will normally act more like a solid than a liquid. Another
reason why it is a distinction without a difference is that
operationally it doesn't matter whether there is a fluid between the
buoyant object and the scale that it sits on. The scale reads the
same, whether there is half a monolayer, or one monolayer, or five
layers, or a thousand layers. Conservation of momentum requires it.

ok


In any case, we should be more interested in the physics than in the
terminology. One thing that greatly changes the physics is
whether or
not any suction-cup effect is occurring.

My hypothesis (which I am QUITE ready to have disproved -
experimentally) is that SuperGlue or threads will exert the
SAME upward force on the bottom sheet of glass.
The results might shed some light on the complexities of
suction cups but I'd prefer to keep the smoke and mirrors to
a minimum.


On 11/04/2010 02:36 PM, Chuck Britton wrote:
So your conclusion is that strain gauges attached appropriately
will show compression as the pole is submerged,

I'm not from Missouri - but I DO look forward to some
experimental results.

This was discussed last week. I thought we established that
depending
on details, one could obtain either answer, either compression or
tension. A slightly-porous object might well start out in
compression
but cross over to tension as water gradually leaked in.

Again - I'm only concerned with non-porous, lightweight objects.
Leave the complexities for later analysis.

KISS - Keep It Simple, Students !!


=============================

A good rule of thumb: Fluids are tricky.

hence my use of the term 'magic'.
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