Date: Thu, 03 Jun 2004 09:36:03 -0400

Author: Machele Cable

Subject: Re: buoyancy

Post:

Perhaps the graphs were noisy due to the low viscosity of the liquid
(water assumed)? Thus, the liquid can slosh around. I wonder if a higher
viscosity fluid would make less noisy graphs. Now, what fluid to use
that isn't overly messy???? How many boxes of gelatin would you have to
heat/dissolve into 5 gallons to make a noticable difference?

Also, was were the volume and mass ratios of the block to the water in
the 5 gallon bucket? Changing those would probably affect the graphs too.

Chele

Jerry DiMarco wrote:

> Back when this topic was discussed I didn't have time to think
> about it (a beaker of water on a scale, a piece of wood tied to the
> bottom of the beaker, what happens to the scale reading when the
> string breaks?). The final vote was: 3 thought the scale reading
> would momentarily increase, and 3 thought it would momentarily
> decrease. Well, I've had time to try it out by now so I'd like to
> break the tie.
> Someone suggested that the key to understanding this experiment
> is to realize that the center of mass of the system is falling when
> the wood block is rising. That led me to try the experiment another
> way. I put a dowel across the top of a large cylinder and hung a
> weight from it, in the water. This is the reverse of the original
> situation, but when the mass falls, the center of mass of this system
> will also be falling. So the result at the scale should be the same.
> The cylinder was hung from a Vernier Dual Force Probe, with the
> results graphed by LoggerPro. When the weight was released, the scale
> reading dropped while the weight was in free fall. This result is no
> surprise since it is so much like the example of a mass hanging in a
> box on a scale. This version was not only easier to do, but also
> easier to understand.
> Next I performed the experiment as originally proposed, using a
> variety of buoyant objects taped to the bottom of a large cylinder or
> beaker. At first, they were hung as described above, but then I found
> the experiment could be done more easily with a 5 gallon bucket on
> Vernier's Force Plate. The result was always the same though, the
> scale reading dropped when the object began to rise. The waveforms
> were not nice and clean, but the initial move was always unmistakably
> down.
> I would like to understand why the graphs were so "noisy", but I
> think a high speed camera would be needed. It will have to wait for
> another time. So does this break the tie?
>
>
> Jerry
>
>
> At 10:27 AM 2/6/2004, you wrote:
>
>> I am confused by this question taken from Physics (Walker, p. 492, #14).
>> Basically, a piece of wood is tied to the bottom of a water filled
>> beaker,
>> and is completely submerged. The beakers is on a scale. When the string
>> breaks (and the wood rises to float), what happens to the scale reading?
>>
>> One could argue the total mass is the same, so the total weight (scale
>> reading) stays the SAME. But there's also tension in the string, which
>> pulls up on the bottom of the beaker. String breaks, no tension, scale
>> reading goes UP.
>>
>> Well, I tried the experiment using a Ping Pong ball. I found that the
>> scale reading APPEARED to stay the same. However, there was some water
>> splashing (due to the ball reaching the surface), and my scale was not
>> accurate enough to register any relatively small change in mass.
>>
>> My other physics colleague and I are puzzled. Has anyone ever tried
>> this,
>> or know what result should be produced?
>
>
>
> <><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
>
> Jerry DiMarco
> Manager of Lecture Demonstrations and Instructional Labs
> Montana State Univ., Physics Dept.
> Bozeman, MT
>
> Our Motto: "There's a demo in there somewhere."
>
>

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Machele Cable Lab Manager Physics WFU
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From doughera@lafayette.edu Thu Jun 3 09:49:23 2004

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