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

["M. Horton" <scitch@verizon.net>]

I agree that the original question may not have been asking what it intended 
to ask.  So, I'll ask what I think the original questioner was intending to 
ask because I've long had the same question.  (Wow, that was confusing.)

Where does the buoyant force come from?  Or to ask it in a slightly 
different way, what causes the buoyant force on an object and how?  I've 
seen answers such as "The difference in density between the liquid at the 
bottom of the object versus the top," "The differnce between the pressure at 
the bottom of the object and the top," "the now-elevated water level pushing 
back," "the displaced water trying to get back in," and other equally 
non-convincing arguments.

Mike


----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, October 19, 2010 8:09 AM
Subject: Re: [Phys-l] Definition of upthrust or buoyancy


> On 10/19/2010 03:40 AM, carmelo@pacific.net.sg wrote:
>
> > Upthrust or buoyancy is defined by some as the upward force on an
> > object produced by the surrounding fluid (i.e., a liquid or a gas) in
> > which it is fully, or partially immersed, due to the pressure
> > difference of the fluid between the top and bottom of the object.
> >
> > However, this definition has a problem: If the object is at rest at
> > the bottom of the ocean floor (assuming tight fit) with no fluid at
> > the bottom of this object, do you agree that there is no upthrust or
> > buoyancy for this situation?
> >
> > Upthrust or buoyancy should be better defined as the magnitude of the
> > weight of fluid displaced by the body instead?
>
> The meaning of buoyancy is clear.  According to Archimedes principle,
> the buoyancy is equal to the weight of the displace fluid.  This is
> all well attested.
>
> Upthrust is not a technical term.  It is not defined and should not
> be defined.
>
> Thrust is a technical term.  It is not equivalent (or even remotely
> similar) to buoyancy.  Neither the floating body nor the tied-down body
> has any thrust.
>
> It is a mistake to mention buoyancy and upthrust in the same breath
> as if they were supposed to be equivalent.
>
> The case of a body tightly fitting against the bottom is pathological.
> Depending on geometry and chemistry and other details not mentioned,
> the body might well /stick/ to the bottom, because of forces that
> are inescapably important in this situation -- but that are usually
> neglected in discussions of buoyancy because they are negligible in
> non-pathological situations.
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