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

On 10/19/2010 03:40 AM, wrote:

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?

The meaning of buoyancy is clear. According to Archimedes principle,
the buoyancy is equal to the weight of the displace fluid. This is
all well attested.

Upthrust is not a technical term. It is not defined and should not
be defined.

Thrust is a technical term. It is not equivalent (or even remotely
similar) to buoyancy. Neither the floating body nor the tied-down body
has any thrust.

It is a mistake to mention buoyancy and upthrust in the same breath
as if they were supposed to be equivalent.

The case of a body tightly fitting against the bottom is pathological.
Depending on geometry and chemistry and other details not mentioned,
the body might well /stick/ to the bottom, because of forces that
are inescapably important in this situation -- but that are usually
neglected in discussions of buoyancy because they are negligible in
non-pathological situations.