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[Phys-L] Re: Buoyancy question



* * * In response to Bob Lamont (below)... * * *

So, when we say the 15.6 gram floating mass displaced 15.6 grams of
water, we don't mean it moved 15.6 grams of water to another location.
We mean that it would take 15.6 grams of water to replace the mass if we
were to remove the mass *and* also require that the water level remain
the same.

That's the way I now perceive it. By that is not what the word displace
means.

* * * Furthermore... * * *

I realize I am dwelling on the semantics of the word displace. The
discussion in the last couple days has led me to conclude that the words
displace, displaced, displacement cannot mean the same thing with regard
to Archimedes Principle as people generally assume they mean.

Note that dictionaries do not all use the same wording, but displace
typically means to move something from its original position. My
Webster's New Collegiate Dictionary actually uses a buoyancy example...
"to remove physically out of position <water displaced by a floating
object>"

In the usual demonstration this is true. Water fills a container to the
brim, or better, to an overflow tube. The object is then floated, the
water is displaced from the container out the overflow tube, and this
displaced water has the same weight as the floating object.

However, in the case of floating the 15.6 gram object in 10 ml of water,
that did not happen. 15.6 grams of water were not physically moved from
its original position. The actual amount of water that moves when my
particular 19-mm-diameter 15.6-gram mass is floated in my 22-mm-diameter
graduated cylinder is about 6 grams of water physically moved (only 6 g
of water displaced).

* * * Insight new to me... * * *

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.

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