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

On 10/19/2010 1:10 PM, carmelo@pacific.net.sg wrote:
Dear all,

Upthrust or buoyancy is defined by some as the upward force on an
object produced by the surrounding fluid (i.e., a liquid or a gas) in
which it is fully, or partially immersed, due to the pressure
difference of the fluid between the top and bottom of the object.

However, this definition has a problem: If the object is at rest at
the bottom of the ocean floor (assuming tight fit) with no fluid at
the bottom of this object, do you agree that there is no upthrust or
buoyancy for this situation?

Upthrust or buoyancy should be better defined as the magnitude of the
weight of fluid displaced by the body instead?

Best regards,
Alphonsus

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Imagine a cube stuck to the bottom of a beaker filled with some liquid.
Assume there is no layer of liquid between the bottom of the cube and the vessel.
There would only be sideways forces on the walls which balance and the force on the top face would be downward.
Net force would then be not equal to weight of displaced liquid.

Or consider a book on a table. There is obviously a layer of air between the bottom of the book and the table.
Otherwise even for a book of the size 20 cm x 30 cm the down ward force due to air would be approximately equal to the weight of a 60 kg object

Regards and Best Wishes

Surendranath