Re: Test question /Reality Strikes!

[Brian Whatcott <betwys@SBCGLOBAL.NET>]

Actually, M.E's answer is distinctively in error.

In three of nine clauses under the heading, "logic",
 he mentions taking  the difference between a
volume and a mass.
This is called mixing apples and oranges.

These were the slips:
(M.E.)
(b2) Sinking the cup by adding the second block makes the water level go
up if the volume of water displaced by the second block is more than the
difference between the cup's mass and the cup's material volume.
(b3) Sinking the cup by adding the second block makes the water level go
down if the water displaced by the second block is less than the
difference between the cup's mass and the cup's material volume.
(b4) Sinking the cup by adding the second block does not change the
water level if the water displaced by the second block is the same as
the difference between the cup's mass and the cup's material volume.
(M.E. ends)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


But we knew what he meant   :-)

So I disagree with J.D that this is the way to do things.
It is not!

Then again, until Herb mentioned the question was 'faulty'.
I hadn't realized it.
I am thinking Herb might just possibly have supposed that
the appropriate answer was
that the water level sinks, supposing the relative density of
the plastic cup to be unnaturally high and also that it sinks
 when a second very small light block is added.
But I could be mistaken. Nobody else seriously entertained
an answer involving a lowered water surface.

I did verify that some kitchens might contain a 'pan' of depth
seven inches. This would be a container of perhaps five quarts
 or more. There are stock pots on offer of 8 even 12 quarts
in fact. Not every private kitchen carries them.

There are plastic cups more dense than water: microwave safe ones
(typically thermoset plastics), but they are not much more dense, nor
specially heavy for their size, compared with china cups.

All the same, it's nice to see a light-hearted physics thread here.

Brian Whatcott    Altus OK

At 02:02 PM 6/4/2003 -0400, J.D., you wrote:
> On 06/04/2003 12:56 PM, Michael Edmiston wrote:
> >
> > Answer with given assumptions:
> ....
>
> I approve of M.E's approach.
>   -- He answered the question.
>   -- He documented the assumptions he made in
>      arriving at the answer, and thereby made
>      clear the range of validity of his answer.
>
> This is the way to do things.
>
> If you require an answer in a case outside this
> range of validity, then you must re-ask the question
> and make the requirements more explicit.
>
> =====================
>
> I find the question itself highly entertaining.
>
> Behind a disarming smile it has sharp teeth.
>
> It requires some out-of-the-box thinking just
> to realize that the answer is highly dependent
> on assumptions.
>
> Table with some relevant numbers:
>    http://www.wimancorp.com/Default.ASP?Page=Gravity
>
> I'll file this one away.  It will help me explain
> why it is so hard to build high-reliability systems.
> Assumptions that are true under normal circumstances
> may not be true in all circumstances.