Re: Buoyancy on a submerged block on bottom of tank

From: Edward Green (spamspamspam3_at_netzero.com)
Date: 02/06/05 

Date: 6 Feb 2005 10:11:41 -0800

Gregory L. Hansen wrote:

> In article <cu4q0e$ivf$1@sparta.btinternet.com>,
> Franz Heymann <franz.heymann@btopenworld.com> wrote:
> >
> >"Jeffrey" <jhopkins4"no spam"@sc.rr.com> wrote in message
> >news:1PjNd.66535$fE4.11298072@twister.southeast.rr.com...
> >>
> >> How is the Buoyancy on a block determined for the ideal case of a
> >uniform
> >> block submerged in a deep, flat bottom tank of water, where
there's
> >no water
> >> below the block only around it and atop it - the block is placed
> >flush with
> >> the bottom of the tank?
> >
> >The weight lost by the block is always equal to the weight oif the
> >water it displaces, whether it is resting on the bottom or floating
> >freely or suspended by a piece of string.
> >
> >Franz
>
> Hmm... weight lost? Depending on the definition of "flush with the
bottom
> of the tank" (e.g. is it a cork with the bottom exposed to
atmosphere?)
> there may be weight gained.

Right. To spell it out in detail, resting on a normal sort of bottom,
with a bad seal, the pressure under the block is going to be the same
as if it were submerged to that depth regardless of the bottom. If
there is a seal, the pressure under the block may be any value, up to
one that would lift the block.

Along these lines, I was recently admiring the extreme cleverness of
the flapper/float valve found in many toilet tanks. The valve is
bouyant (contains an air pocket), and also seals an opening on the
bottom of the tank normally over air. When handle is activated a chain
pulls the valve off its seat, allowing water to rush down the hole. At
this point the valves natural buoyancy takes over, and it floats on the
surface of the water, so that it does not reseat immediately. When the
tank is sufficiently drained, the valve reseats, and the tank refills.
There are other features I have not described.

Although it seems like cheating, we could take this as an example of
reduced buoyancy for an object flush with the bottom of a tank.

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