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

["Dan L. MacIsaac" <danmacisaac@mac.com>]

consider how a suction cup functions

Dan M

On Oct 19, 2010, at 5:44 PM, surendranath wrote:

>  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
> >
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@carnot.physics.buffalo.edu
> > https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
> 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
>
>
>
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