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Re: Fluids Physlet



I also get Bob's result. The block will be in equilibrium only if its density is intermediate between that of oil and the water. Once the oil level decreases below the top of the block, the pressure on the top of the block remains constant while the pressure on the bottom of the block continues to decrease; therefore, the block should sink slightly. If the oil is removed quickly, I would expect the block to oscillate about its changing equilibrium position.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org


-----Original Message-----
From: Bob Sciamanda [mailto:trebor@VELOCITY.NET]
Sent: Monday, August 18, 2003 2:45 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Fluids Physlet


If r = the fraction of wood above water, then I get:

r = (wood - water)/(oil - water), where wood, water and oil are the
densities of these things. This is independent of the oil height. This
looks like A!
Forgive my condensed notation - I am in a hurry. The calculation was also
hurried (???) Will re-check later.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Stuart Leinoff" <Leinoffs@ACC.SUNYACC.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, August 18, 2003 3:35 PM
Subject: Fluids Physlet


Greetings,

Ok, I am stumped by the Physlet on fluids found at (among other place=
s):

http://webphysics.davidson.edu/physletprob/ch8_problems/ch8_11_fluids=
/fluids_2.html=20

The alleged answer is that "B" is the only "physical" animation, but =
I do not see why the block of wood will float higher in this animatio=
n as oil is drawn away but before the block of wood breaks the surfac=
e of the oil.

By Archimedes Principle and the equilibrium principle, the weight of =
the block (constant) should equal the weight of the fluids displaced;=
which would be the sum of the (oil displaced)'s weight and the (wate=
r displaced)'s weight. Why would the ratio of these two weights chan=
ge when the block is less deep under the oil?

If we calculate the buoyancy force based on the difference between th=
e water's pressure pushing up on the bottom surface of the block and =
the oil's pressure pushing down on the top surface, again I do not se=
e why this "difference" would be different before the oil no longer c=
overs the top surface of the block.

Animation "A" does not seem physical to me because once the block is =
no longer under the oil, it should sink deeper into the water.

I would appreciate someone else's insight into this.

Thanks,


Stuart Leinoff
Professor of Physics
Science Division Chair
ACC