Date: Fri, 3 Dec 2004 13:42:22 -0500

Author: Wolfgang Rueckner

Subject: Re: buoyancy

Post:

I kind of like this way of thinking about it, Andrew. If fact, I was
just mulling over a "what if" scenario which is related. What if one
were to accelerate the container of water with object floating in it
upward, and the object did bob down? Then additional water would be
displaced and (because it's in a container) be pushed upwards in the
direction of the acceleration. That water would then experience an
acceleration even greater than the acceleration of the system, and that
wouldn't make sense. Therefore the floating object does not bob down
into the water. Same argument for accelerating downwards (but only up
to the value g at which point everything is in freefall and buoyancy is
not longer a meaningful concept). Does that reasoning make sense? And
many thanks to all of you that are weighing in on this one.
-- Wolfgang


On Dec 3, 2004, at 10:07 AM, A Gavrin wrote:

> I would look at it another way. Forget about "bouyancy"; instead,
> think of hydrostatic pressure. Bouyancy is just a fancy name for the
> net upwards force. At any given depth, this is due to the pressure,
> given by rho*g*h, so changes in g (due to acceleration, trips to other
> planets, etc.) change the pressure, and thus the force. This change is
> equal to, and opposite from, the changes in the weight of the floater.
>
> Changes in the pressure due to acceleration of the container would be
> "communicated" around the container very fast. Roughly, at the speed
> of sound in the liquid. For water, v of sound is about 1500 m/s, so
> all of the water in a 20 cm cup "hears about" changes in pressure in
> about 0.13 milliseconds.
>
> - Andy
>
>
> Wolfgang Rueckner wrote:
>
>> 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
>>>>>
>>>>
>>>>
>>>
>>
>>
>
> --
> Dr. Andrew D. Gavrin
> Department of Physics, 402 N. Blackford St.
> Indiana Univ.-Purdue Univ. Indianapolis
> Indianapolis, IN 46202-3273
>
> 317-274-6909 (Ph) -2393 (FAX)
> agavrin@iupui.edu
>
From agavrin@iupui.edu Fri Dec 3 14:08:33 2004

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