Re: Can this be true?

["JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE, IL 60439" <JLU@HEP.ANL.GOV>]

        I think that Glenn Carlson unnecessarily complicates a simple
exercise in logical thinking-
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What if the string had mass, but was horizontal?  Wht if the string w=
as
supported from below, not from above (the string is perfectly straigh=
t
and plumb)?  What if the string is not acted on by gravity?  Change t=
he
assumptions; we change the answer.

Real bodies break at imperfections (stress concentrations).  Perfect
bodies don't break, or we wouldn't call them perfect.

There is no paradox here.  The problem is accepting an idealization
("exactly the same all along") as describing reality ("a true
statement").  We can reach any conclusion we want if we make
sufficiently unrealistic assumptions.  It is quite easy to reach
unrealistic conclusions from ideal (unreal) assumptions (e.g.,
"perfectly incompressible fluid," "frictionless surface," "all things
being equal," "if I were king of the world," "if pigs could fly,"
etc.).

As a physics teacher, it's important that help the students understan=
d
the difference between reality and models of reality.  Models are not
reality.  Models only approximate reality.  Models are tools which he=
lp
us describe (understand?) reality. Any model will fail to accurately
describe reality or even contradict itself if we extrapolate it far
enough.  The goal is help students learn how to model and recognize t=
he
limits of the model.
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        Accept "the same all the way along" as a description of the
string properties.  But the setup of the string is clearly not the
same "all the way along" because the string has two distinguished points -
the end points.  Therefore, depending on how each point is fastened,
I would expect the string to break at one of the end points.
                                        Regards,
                                                Jack
        Who believes that physics is the art of finding the elegant
idealization.

        "I scored the next great triumph for science myself,
to wit, how the milk gets into the cow.  Both of us
had marveled over that mystery a long time.  We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
                Mark Twain, Extract from Eve's
                Autobiography