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[Phys-L] Re: Buoyancy question



Be careful - the important point is not the amount of water the object is
floating in - it's the amount of water DISPLACED. The remaining water may
only be a small fraction of the displaced water. If you sum or integrate the
hydrostatic forces over the entire submerged outside surface of the object
the result is the weight of the water displaced. So the two approaches -
pressure and displacement - are equivalent. The displacement argument is the
easier one to follow conceptually, because the forces that act on a
particular mass of water are the same ones that act on the object that
displaces that mass of water. Remember that the water was floating in
position before the object was introduced.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf
Of Savinainen Antti
Sent: Thursday, December 09, 2004 4:16 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Buoyancy question

äHi,

Michael Edminstone wrote:

"However, the algebra shows that the object must submerge sufficiently to
displace
water equal to the weight of the floating object in order for the pressure
on
the bottom
of the object to be sufficiently more than the pressure on the top to make
the
upward
force sufficient to balance its weight."

Do you mean that there must be enough water (i.e. the same weight of water
as
the weight of the object) for floating? If so, that might not be true.
Imagine
that a
container would have the same shape as, say, a boat. Assume that the boat
fits
nicely into the container and that there is a little bit room for water
between
the boat
and the container. Now there would be variation of hydrostatic pressure
from
the bottom
to the top of the container. An important point is that hydrostatic
pressure
does not depend on the amount of water. The buoyance force would be
present
since there is
a difference in hydrostatic pressure. Hence the boat could float even if
there
was
much less water than the weight of the boat would take.

If my reasoning is correct (is it? :-)) then Ken would be right when
stating
that the [true] cause of floating is a pressure difference.

Regards,

Antti Savinainen, Ph.D.
Kuopio Lyseo High School
Finland
Homepage: <http://kotisivu.mtv3.fi/physics>