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

. . . - the blob could be on the bottom of a glass beaker.

Except that the upward rigidity force N of the beaker bottom on the blob/object now enters the calculation.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

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From: "LaMontagne, Bob" <RLAMONT@providence.edu>
Sent: Wednesday, October 20, 2010 12:02 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

If you want to imagine the buoyancy on an object, first imagine a blob of water with exactly the same shape as the object. That water is obviously in equilibrium. Integrate the pressure over the surface area of the blob. If in equilibrium, this must give a force that is equal and opposite to the weight of the water. Now replace the blob with the object. The surfaces forces have not changed so they still add to the weight of the blob. Hence, the buoyant force is the weight of the water displaced. This definition of buoyancy does not require water surrounding all surfaces of the blob - the blob could be on the bottom of a glass beaker.

As John D. has pointed out, stickyness is a different issue.

Bob at PC

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