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[Phys-L] 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>