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-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
Of LaMontagne, Bob
Sent: Tuesday, January 04, 2011 6:47 PM
To: Forum for Physics Educators
Subject: [Phys-l] Buoyancy again
I would like to return to the informative discussion we had
about buoyancy. If my memory serves me correctly, we seemed
to agree that a wooden post that was less dense than water,
driven vertically into a lake bottom such that its top was
submerged, would actually be under compression (with no net
upward buoyant force acting). This brought into question the
interpretation of the common mantra: "the buoyant force
acting on an object is equal to the weight of the water it
displaces." I don't wish to revisit that problem and please
forgive me if my summary is not exactly correct - I realize
there was some dissention on the conclusion and the mantra.
One aspect of the problem was the idea that if an object less
dense than water was submerged on the bottom of a beaker in
such a way to exclude all water beneath it, the net force
acting on it would not be upward. An argument was made that
if a little water was allowed to seep under a portion of the
bottom of the object, there is a point where the ratio of the
area of the wetted bottom to that of the non-wetted bottom
can become large enough so the net vertical force on the
object is zero. If the ratio increases, the object will rise.
My question involves a blob of air in contact with an asphalt
parking lot. That blob of air will attain a significantly
higher temperature (and lower density) than the surrounding
air. Why does it rise? It is in intimate contact with the
asphalt and none of the denser air has seeped under it (or
perhaps it has?) to mimic the case mentioned above of the
object at the bottom of a beaker.
These bubbles certainly do rise - I have used them often as a
glider pilot.
Bob at PC
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