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

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From: LaMontagne, Bob [mailto:RLAMONT@providence.edu]
To: Forum for Physics Educators [mailto:phys-l@carnot.physics.buffalo.edu]
Sent: Wed, 20 Oct 2010 12:02:55 -0400
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

If you want to imagine the buoyancy on an object, first imagine a blob of water with exactly the same shape as the object. That water is obviously in equilibrium. Agreed.
Integrate the pressure over the surface area of the blob. OK; I get a net downward force for a blob at the bottom, and a net upward force for one in the middle or at the top.

If in equilibrium, this must give a force that is equal and opposite to the weight of the water.I agree that the net force on the blob must be zero, but at the bottom there can also be a normal force from the bottom. The buoyancy and weight aren't the only two forces.

Am I missing something? (Probably!)

jg

Now replace the blob with the object. The surfaces forces have not changed so they still add to the weight of the blob. Hence, the buoyant force is the weight of the water displaced. This definition of buoyancy does not require water surrounding all surfaces of the blob - the blob could be on the bottom of a glass beaker.

As John D. has pointed out, stickyness is a different issue.

Bob at PC

force________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of M. Horton [scitch@verizon.net]
Sent: Tuesday, October 19, 2010 8:19 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

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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Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l