[Phys-L] Re: Buoyancy question

[rlamont at POSTOFFICE.PROVIDENCE.EDU (rlamont)]

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>