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Re: [Phys-l] Definition of upthrust or buoyancy

Let me muddy the waters:

Consider the flap at the bottom of a filled toilet tank.
It snugly covers a hole in the tank bottom. The hole leads into the toilet bowl.
The flap is held in place by the tank water pressure on its upper surface, preventing water flow from the tank into the bowl. The flap's underside is in air.
When the (pivoted) flap is raised by an added, momentary upward mechanical force (via the manual flushing mechanism) the tank begins to empty into the bowl.
The flap is now surrounded by tank water and floats in an "open position" as the tank empties.
The flap is "spring loaded" so than it can sink to its original "closed position" when the tank water level has decreased sufficiently. Its lower surface is again in air and the water pressure on its upper surface holds it in place.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)