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Re: [Phys-L] Nice question on buoyancy and balance (correction)



I know JD wants me to think in terms of momentum flow, but at my level, I
hope to understand mechanics problems by thinking about objects being
pushed by forces.

We have two pans. Both are being pushed down by equal amounts of water.
One is also being pulled up by a string.

I don't have to worry about the history that led up to this state if I
don't feel like it. But if I do feel like it here is one history:

On each pan, there were empty beakers. And next to them, each pan had
identical large beakers of water. One beaker also had a ping pong ball,
tied to its bottom surface by a short cord, say 1/3 as long as the beaker
height. The extra mass of the ping pong ball causes the balance to tip
that way. The other beaker has a steel ball suspended from a crane, about
1/3 of the way down but it iis not yet part of the story.

We pour half of the water from each supply beaker into the empty beakers.
The ping pong ball is now more than submerged but the water hasn't reached
the steel ball. Transferring this mass of water changes nothing. The ping
pong ball side is still slightly heavier.

We continue to transfer water. The level reaches and passes the steel
ball. Now the downward force (reaction to the buoyancy force on the steel
ball) matters. And I know that force is greater than the additional weight
of the ping pong ball because I know that ping pong balls float. So the
balance tips the other way.

I still like my first answer better: same water pushing down, one string
pulling up.




On Wed, Jan 29, 2014 at 10:39 AM, Savinainen Antti <
antti.savinainen@kuopio.fi> wrote:

Hi again,

I reread my posting and realized that it contains some embarrassing
language mistakes. I wrote:

"The ping pong ball displaces the amount of water which equals its weigh.
This is
"under the water" volume if the ping pong ball were floating. So one can
image
that this the amount of water is poured to the LHS whereas the water
poured to
the RHS is equal to the volume of the steel ball."

Instead, I should have written (well, it might still contain English
mistakes; hopefully no physics mistakes :-)):

"If the ping pong ball were floating, it would displace the amount of
water equal to its weigh (the displaced water is equal to the "under the
surface level volume of the ping pong ball"). So one can imagine that this
is the amount of water poured to the LHS, whereas the water poured to the
RHS is equal to the volume of the steel ball."

Antti
--
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Viesti on tarkastettu roskapostinsuodatus- ja virustorjuntaohjelmistolla.
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