Re: Test question

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 12:16 -0400 6/4/03, Herbert H Gottlieb wrote:
> Assume that a plastic cup and very small block of wood
> inside are just floating on the surface of water in a large jar.
> The density of the plastic cup is greater than the density
> of the water and the rim of the plastic cup is only one
> "RCH" above the surface with the rest of the plastic cup
> below the surface.
>
> When a second small block of wood is added, its weight
> is just large enough to sink the plastic cup.
>
> Will the level of the water in the large jar rise, sink,
> or remain the same after the cup sinks to the bottom of the jar?
Herb, you're changing the statement of the problem on us. You originally wrote:

> A plastic cup containing a wood block is floating in a pan of water.
> If another wood block is placed in the cup, what will happen to the level
> of the water in the pan?

There is no requirement her to assume that the cup is *barely*
floating. I think the answers given by the others are correct, given
that a reasonable assumption is that the floating is "normal," that
is, not on the verge of sinking.

But given your latest statement, the answer is "it depends." Depends
on whether the plastic material is less dense or more dense than the
water, and if more dense, then will the volume of plastic displace
more water than that of the blocks of wood that were formerly in the
cup. It all of a sudden becomes a much more contingent problem.

A better way of phrasing it would have been to have all the materials
involved be denser than water. Then, when the cup sinks, the water
level in the pan clearly goes down, since now only the volumes of the
materials need be displaced, not the amount of water equivalent to
the weights of the objects.

Hugh
--

Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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