[Phys-L] Re: Buoyancy question

[turner at MORNINGSIDE.EDU (Gary Turner)]

On Fri, 10 Dec 2004 12:15:36 -0500, Edmiston, Mike <edmiston@BLUFFTON.EDU>
wrote:


> I am now convinced that the only time a floating object physically moves
> an amount of water (from its original position) equal to the weight of
> the object, is in situations where the water level remains the same
> before and after the object is floated.  If the water level is allowed
> to rise when the object is floated, the actual amount of water that
> physically moves is less than the weight of the object.  The tighter the
> fit between the object and container, the more the water level rises,
> and the smaller the weight of water that must physically move.

Furthermore, the less water is actually moved, the less the object can sink
w.r.t. the container (it will reach the same level w.r.t. liquid level, but
the liquid level will be higher with less moved).

An interesting question I have used in the past -

A coal barge is sitting in a lock (ok, I'm originally from Northern England
where canals are abundant, you could also use a toy boat in a water tank).
It dumps its cargo overboard - which then sinks.  What happens to -

1) the height of the barge above the water level? (this is intuitive)
2) the water level in the lock? (less intuitive, but still not too difficult)
3) the height of the boat when compared to the lock walls?



> Is this common knowledge?
>
>
> Michael D. Edmiston, Ph.D.
> Professor of Chemistry and Physics
> Bluffton University
> Bluffton, OH 45817
> (419)-358-3270
> edmiston@bluffton.edu
>
>
> Not in the least. If you remove the 15.6 gram mass and replace it by
> 15.6 grams of water with the same shape as the submerged part of the
> object, the forces on that water are the same as on the object.
>
> Bob at PC