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Re: [Phys-l] Definition of upthrust or buoyancy





-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of LaMontagne, Bob
Sent: Thursday, October 21, 2010 1:32 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

If you will put up with me talking to myself here, let me extend your
example to clarify something for myself. If I slightly revise your
example to, say, two cups put together rim to rim with an O-ring
between the rims and plunge them under water, they will stay together
because of the net compression pushing the two halves together. But if
I glue a string to the bottom of one of the cups, the pair will float
upward because they are less dense overall than water, but the string
stopping them from rising will be under tension - even though the O-
ring is in compression.

If the bottom of one of the cups is glued to the botton of the pond
with no water between the cup and the pond, the cups are still under
net compression along the O-ring. If I am following the arguments
being
made correctly, the claim would be that the glue is actually under
compression as well.

I guess the question I am wrestling with is how small can I make the
glob of glue so it no longer covers the bottom of the cup completely
and now starts acting like a string and becomes under tension. What is
the essence of that transition from compression to tension?

I guess the essence is that if you start with a large glob of glue so
that it is in compression, and make it smaller, as you make it smaller
you are making more of the surface of the bottom of the cup accessible
to the fluid which pushes upward on the bottom of the cup and the
greater the area accessible to the fluid the greater the upward force by
the fluid on the bottom of the cup. If you keep making the glob of glue
smaller, eventually the upward force exerted on the bottom of the cup by
the fluid is as great as the downward net force exerted by the
gravitational field on the cup as a whole and by the fluid on surfaces
other than the bottom of the cup. At that stage, the force of the glue
on the cup has been reduced to zero.


Bob at PC