[Phys-L] Re: Buoyancy question

[edmiston at BLUFFTON.EDU (Michael Edmiston)]

I agree with what Hugh says (below).  But I don't think it is what most
people picture when they say a floating object displaces its weight in
water.  I think most people believe a 15 g object must literally push
away 15 g of water in order to float.  Everybody I asked today thought
of it that way, and they all said that a 15-g object could not float in
10 g of water.  How can you displace 15 g of water if there isn't that
much water present?

The usual dictionary definition of displace is "to move something away
from its normal position."  When my 15-gram object was placed in 10 g of
water, it did indeed move some water away from its prior position, but
it did not move 15 grams of water away, and thus, according to the
dictionary definition of displace, it did not displace 15 g of water.

This is why I stated in my earlier post that we have to be careful how
we define "displace" as used in Archimedes Principle.

Herb suggests I redefined floating.  Nope.  My hollow aluminum cylinder,
mass 15.6 g, volume a tad more than 15.6 ml, density a tad less than 1,
was floating in the normal sense of the word in only 10 ml of water.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu


> But when you were finished, you still had 10 grams of water in the
> beaker, it was now occupying a total volume of 25.6 ml, because the
> 15.6 ml object was floating in it. Of course it displaced 15.6 ml of
> water, and that 15.6 ml is not there--because it has been displaced.
> If you had had 20 g of water in the beaker, the water level after
> floating the object would have been at 35.6 ml. The amount of water
> actually there is irrelevant until there isn't enough to get the
> object off the bottom, but as soon as that minimum is reached any
> more water added to the beaker is superfluous.

> Hugh Haskell