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



Again, I'm asking this question because I honestly don't know the answer.

Let's say you have a pole submerged in water, but not touching the bottom (neutrally buoyant). It is aligned vertically. I have been told that the buoyant force is caused by water molecules pusing up harder on the bottom of the pole than the water molecules are pushing down on the top of the pole. (Nevermind, I answered my own question. But I'll finish it because others might have the same question.). I was contemplating that if the pole gets longer, the vertical surfaces don't change and that explanation wouldn't allow for a change in the buoyant force.

What I failed to consider is that a longer pole will have a bigger pressure differential from one end to the other and therefore, a higher buoyant force. I haven't written out the math yet, but I've almost figured out in my head why this always results in a buoyant force that is equal to the mass of the water displaced. I was thinking of rotating the pole horizontally. Now, there's much more horizontal surface area, but there's much less difference in pressure between the bottom and top of the pole (I'll make it a rectangular pole for ease of calculations).

Thanks for listening to me talk to myself, but I hope that this monoscussion helps someone else's understanding as well.

Mike



----- Original Message ----- From: "brian whatcott" <betwys1@sbcglobal.net>
To: <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, November 03, 2010 10:08 AM
Subject: Re: [Phys-l] buoyancy on a submerged pole


On 11/3/2010 9:33 AM, Chuck Britton wrote:
(an off-list exchange - posted without permission - hope it's ok)

At 10:14 AM -0400 11/3/10, bennett bennett wrote:
The way I see it, the force of fluid pressure on solid is normal to
the surface at all points, so the anchored pole, (with no water
pushing on the bottom surface), is not lifted by the water on its
side, unless the water is viscous and moving upward.

If there is a notch, the pressure on the non-vertical surfaces of
the notch will stretch only the thin part of the pole, but the up
and down forces will be equal, since the vertical components of the
forces on the surfaces of the notch are equal.

And the way _*I*_ see it is that the top of the (totally) submerged
pole doesn't give a flying-flip what's going on at the bottom of the
pole.
The top of the pole 'wants' to float, and it WILL if given a chance.

How does the complicated contact force at the bottom change what's
going on with the rest of the object??
(Still scratching my head vigorously)

By affecting one of the several orthogonal stresses which may exist in
a material? :-)

Brian W
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