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



Two thoughts...

1. This reminds me of an older thread about what holds up a car or bicycle. Consider the spokes of a bicycle wheel: they are all in tension. It is the difference between the tension in the upper and lower spokes that balances the weight of the bike. Similarly, it is the difference in pressure on the upper and lower surfaces that produces the buoyancy force. Same for the upward force that you apply with a suction cup. After all, the inside of the cup does not have to be a complete vacuum for the cup to work. But if there is air inside the cup, it exerts downward pressure. But less downward pressure than the upward pressure coming from the atmosphere.

2. The argument that "if you cut off the piling it floats so it must be in tension" had me convinced for a while. Or the reverse argument that if you wanted to add another layer to your piling, you would need glue. But you wouldn't need glue -- you would just need to have smooth enough surfaces to push out all the water (which brings us back to the question that started the thread). It is not a realistic way to make a piling, but it does show that without any upward component to the water pressure, the piling is in fact in compression. Cutting part of it off introduces another surface for water pressure to act on.



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Thursday, October 21, 2010 10:42 AM
To: 'Forum for Physics Educators'
Subject: 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.


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