[Phys-L] Re: Buoyancy question

[hhaskell at MINDSPRING.COM (Hugh Haskell)]

At 20:37 -0500 12/9/04, Michael Edmiston wrote:
> 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.
Suppose you had 15 g of water in there in the first place. Then the
water level in the beaker would be raised by another 15 ml when you
put the cylinder into it. But it will be floating some distance above
the bottom, and, technically, the water below the bottom of the
cylinder would not have been disturbed, hence no displaced. You could
then, if you had a drain plug at the bottom of the beaker, drain off
all the water in the bottom until the cylinder was just short of
touching the bottom. I don't know how much water you would be
draining off, but the fact is that there is now less than 15 g of
water in the beaker and the cylinder is still floating, hence it has
displaced 15 g of water. In other words, it is occupying the 15 ml of
volume that would otherwise have been water if the water level would
be the same with the cylinder gone as it is with the cylinder present.

Lewis Epstein deals with this problem in a very nice way in his
clever book, "Thinking Physics." He points out there that all of the
water beyond that thin layer that surrounds the floating object is
unnecessary, so the object will float as long as the container is
large enough to hold at least an amount of water equal to the weight
of the floating object (plus a little bit to provide that layer of
water to surround the object).

Hugh
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Hugh Haskell
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