On Sep 28, 7:55*pm, Bruce In Bangkok
wrote:
On Mon, 28 Sep 2009 17:40:21 -0400, Jeff wrote:
KLC Lewis wrote:
Never argue bouyancy with Roger Long. ;-)

Why? *I think Roger is making a big deal of a very fine distinction.
Its true that an object that is said to be "buoyant" does not generate a
force by itself, the force really comes from water pressure which in
turn is caused by gravity. *But, the force is real and buoyancy is
simply a convenient way to aggregate the net pressure on an object. *If
there were no force (regardless of what we call it) holding up a ship,
it would sink.

There are, of course, imaginary forces, such as Coriolis which appears
in non-inertial reference frames, but that is a different thing.

There are many words in the English language that aren't proper
scientific explanations of a phenomena. Try "beautiful" or "ugly" for
two examples.

Is the fact that there is no scientific justification for the term
"beautiful" *reason to stop using it?

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

Roger has the same mistaken impression that many people have, that it
takes energy to hold an object up against gravity. NO, it takes no
energy for a rigid body to hold an object up against gravity. If his
son thinks it takes energy for a table to hold a book up, he needs to
re-learn physics. If you hold a book out with your arm extended, you
ARE using energy because your arm is not a rigid body and you are
exerting energy to keep the muscles tensed. This is equivalent to
holding something up with a leaky pneumatic cylinder. This is
completely different from buoyant forces holding a boat up which takes
NO energy. A buoyant situation that would require energy would be an
object flating within a container at a constant level where the
container has a leak. In that case energy IS required to keep the
object at a constant level because you have to pump in liquid to
replace that that leaks out.
His observation that buoyancy is the result of unbalanced pressure is
trivial in some cases and not entirely accurate in others. A
floating boat has pressure from below balancing the weight from above
and the net buoyant force is simply equal to the weight of water
displaced. A balloon floating in air (or water) has no unbalanced
pressure and the buoyant force is simply equal to the weight of the
medium displaced. A buoyant force is not imaginary simply due to
being a "net force". Forces are vector quantities and the total force
acting on a body is the vector sum of all forces.