Date: Fri, 3 Dec 2004 08:32:57 -0500

Author: Wolfgang Rueckner

Subject: Re: buoyancy

Post:

I guess my hang-up is in switching from an accelerated frame of
reference to the "lab" frame. In the accelerated frame, the upward
acceleration (for example) is taken care of by the fact that the
buoyancy force has increased by the appropriate amount (g+a). I think
my confusion lies in trying to imagine the mechanism or sequence of
events in the lab frame. I too imagine that the object would sink
lower for an "instant" but that really doesn't seem to happen. It's so
easy to invoke the equivalence principle and simply say that g has
changed. But visualizing it otherwise is not so easy (for me). --
Wolfgang


On Dec 2, 2004, at 7:10 PM, John Welch wrote:

> If the accelerated frame were due to freefall, it would be the
> gravitational
> force that accelerated both the fluid and the floating object. If the
> acceleration were due to a force that only acted on the fluid but not
> the
> object, like pushing the 'tank' upward against gravity, then it does
> seem to
> me that the object would sink lower for an instant until an
> equilibrium was
> reached. No?
>
> ----- Original Message -----
> From: "Paul Nord"
> To:
> Cc: "Paul Nord"
> Sent: Thursday, December 02, 2004 2:59 PM
> Subject: Re: buoyancy
>
>
>> Sure. The water pressure changes. Put a pressure gauge in the fluid
>> and you should find that the absolute pressure changes. This would be
>> the same change you would observe if you increased the gravitational
>> attraction of the earth.
>>
>> But only to the compressibility limit of water... Hmmm, good question
>>
>> Paul
>>
>> On Thursday, December 2, 2004, at 04:38 PM, Wolfgang Rueckner wrote:
>>
>>> I have a conceptual question pertaining to buoyancy in an accelerated
>>> frame (for example, Dick Berg's demo found under
>>> http://www.physics.umd.edu/lecdem/services/demos/demosf2/f2-22.htm
>>> I understand the argument why the buoyant force of a floating object
>>> increases by the same amount the the object's weight increases in the
>>> accelerated frame, and therefore the object doesn't float any
>>> differently than when it's sitting still. My problem is visualizing
>>> the mechanism of what's going on to accelerate the floating object.
>>> The container of fluid is accelerated by some outside force and that
>>> force has to be communicated to the floating object by the liquid
>>> it's
>>> floating in so that it too accelerates by the same amount. But the
>>> only force of the liquid on the object is the buoyant force. So
>>> doesn't the buoyant force have to be a little different from the
>>> apparent weight of the floating object to produce an acceleration?
>>> Where is my thinking hanging up? -- Wolfgang
>>>
>>
>>
>
From rueckner@fas.harvard.edu Fri Dec 3 08:33:59 2004

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