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



But all objects subject to buoyancy are under compression. Buoyancy for an
object suspended in a fluid is because the net upward force of pressure on
the bottom is not balanced by the net downward force on the top. The object
is under compression, but there is a net upward force from all of the
pressure combined.

The piling is subject to a net downward force due to the water pressure,
which must be balanced by a net upward force provided by the Earth. So
clearly there is compression.

The simple Archimedes rule has a big problem. It does not reveal the
physical mechanism for why there is a buoyant force. Giving students these
types of rules before the understanding of why they work, is very bad
pedagogy. This is obvious from the reactions of various people on this
list.

I always ask questions, and when I show them large suction cups capable of
lifting a table, there is a lot of back and forth about how they work. None
of the college students even remotely came up with the correct mechanism on
first try. They all think the cups are pulling up on the object. But once
they realize that a vacuum can not exert any forces, and there is air
surrounding the object which is the only other thing touching the object,
some of them come up with a good answer.

Incidentally Archimedes rule is often taught in middle school and even in
intro college courses, they don't use it when first confronted by buoyancy.
Actually buoyancy has some good proportional reasoning problems which can be
helpful in raising student thinking, but of course they must figure them
out. I never do problems for them.

John M. Clement
Houston, TX


If I took a saw to the piling and cut it along a cross section, wouldn't
the upper segment float to the surface? Seems to be under tension.