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



Bravo, Scott!

Ken

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Scott Orshan
Sent: Thursday, 04 November 2010 7:22 PM
To: phys-l@carnot.physics.buffalo.edu
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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