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

A student of mine investigated this effect. He put some material from
the bottom of a natural body of water (very fine-grained loose
sand/silt) in a five-gallon bucket and added 3-4 gallons of water. He
tied a string to a brick. The brick was placed on the sand in the
bottom of the bucket. The string extended up from the brick to a force
sensor. A string attached to the other end of the force sensor extended
up to a 3/4 inch outer diameter pipe loosely supported horizontally
above the bucket by means of some clamps. He used the pipe as a reel to
lift the brick off the bottom by gradually increasing the tension in
the string attached to the brick. The force sensor indicated that the
maximum tension in the string while the brick was at rest on the bottom
was noticably greater than the tension in the string while the brick was
suspended at rest, fully submerged in the water, above the material on
the bottom of the bucket. The few trials carried out by the student
suggested that the longer the brick was at rest on the bottom (in the
longest case it was about a week) the greater the force required to
break it loose from the bottom. I would say that the definition of
buoyancy as the net force being exerted on the submerged object (at
rest) by the fluid (macroscopically at rest) in which the object is
submerged is just fine, it is just that the buoyant force is not always
equal in magnitude to the weight of the displaced fluid. I would say
that any time the object is resting on the bottom, the upward buoyant
force is, in principle less than the weight of the water displaced, and
with the definition just above, the buoyant force can be downward. The
degree of the effect depends on the degree to which the fluid is unable
to come into contact with the part of the object that is in contact with
the bottom. I think the effect is pretty ordinary and that
nonphysicists refer to the effect as suction.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of
carmelo@pacific.net.sg
Sent: Tuesday, October 19, 2010 3:41 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Definition of upthrust or buoyancy

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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