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[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