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



Scott, how DARE you sully a nice conversation by comparing our navel-gazing to
reality!


/**************************************
"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"
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________________________________
From: Scott Orshan <sdorshan@aol.com>
To: phys-l@carnot.physics.buffalo.edu
Sent: Thu, November 4, 2010 5:22:07 PM
Subject: Re: [Phys-l] buoyancy on a submerged pole

I've been watching this discussion go on for days. It only took me 10
minutes to set up an actual experiment.

I took a bowl from my kitchen, and an empty soda cup I had. If you push
an empty cup down into water, of course it tries to float and resists
your push.

I put a ring of ordinary Colgate toothpaste around the bottom of the
cup. I pushed the empty cup against the bottom of the bowl. See picture.
(Vaseline worked as well.)

http://picasaweb.google.com/lh/photo/D0wwsGFspEkxg40eL5zatA?feat=directlink

Toothpaste is not glue, and it's not particularly sticky. As you see in
the picture, the empty cup is resting on the bottom of the bowl of
water, with only a layer of toothpaste between them. Any significant
tensile force would pop the cup free.Therefore, there is no net tensile
force.

If you try the toothpaste experiment, you will feel that the cup resists
your push as you submerge it, but as soon as you establish the seal, it
stops pushing back.

However, there *is* the compressive force of air pressure, which is the
way your basic suction cup works. If the whole bowl and cup were placed
in a vacuum, would there be a buoyant force on the cup? I don't see how.
Water that isn't flowing can not exert a parallel force to a surface.
Since the cup is tapered, there is probably a small upward force from
the vertical component of the water pressure. A cylindrical cup (or the
original pole of the topic) would not have that upward force because
there is simply no mechanism for the static water to interact with the
smooth sides to produce a net force in any particular parallel direction.

Even if you consider friction, at each infinitesimal vertical section of
cup, the random motion of the water molecules would produce just as much
downward force as upward force.

There *is* a general inward pressure, and if the cup were shaped out of
something weak, like aluminum foil, it would collapse inward when submerged.

Scott





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