Re: SR examination question

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 17 May 2001, Rick Tarara wrote:

> Read the original question again:
>
> Peter and Jane are each wearing a wristwatch with a second hand that takes
> one minute to make one complete revolution and Peter is moving at a speed
> of 0.9c with respect to Jane. When Peter observes the second hand on his
> watch to have made one complete revolution, how many revolutions will Jane
> observe the second hand of her watch to have made?
>
> Peter, moving (linearly) at .9c

No.  Be careful.  At least the original problem, for all its
genuine faults, does not make the mistake of implying that Peter
"is" moving.

> observes 1 minute to have passed.  During the time that
> Peter's watch has ticked off 1 minute AS OBSERVED BY HIM,

No need for CAPS; those last four words go completely without
saying.  If Peter's watch doesn't tic off one minute *during* one
minute as observed by him, then it simply isn't a watch.  (It's
more like all the "clocks" on my campus.  ;-) )  Note, however,
that *by* saying those four capitalized words, you tend to shift
the focus to what *Peter* has to say about things in general. Not,
I think, what *you* want to do!)

> how many minutes does JANE observe (in her frame--assumed to
> be at rest) to have ticked off on her watch?

It's utterly irrelevant whom we "assume to be at rest."  The only
question is according to whom which clock reads what when.
(Oooh!!  I like that.)  That is are you asking

Q1
  "According to Jane, when Peter's clock reads one minute what
   does Jane's clock read?"

or are you asking

Q2
  "According to Peter, when Peter's clock reads one minute what
   does Jane's clock read?"

>  That's what the question says to me.

Even after your restatement it wouldn't be clear to me what you
think the question says.  I gather, however, from your previous
remarks that you opt for my "Q1."  Fine.  I'll just report that
when *I* first attempted to read the author's mind, I came to the
conclusion that it was "Q2."  Note the otherwise completely
unnecessary words early on in the question:

  "When Peter observes the second hand on his watch to have made
        ^^^^^^^^^^^^^^
  one complete revolution, how many revolutions will Jane observe
  the second hand of her watch to have made?"

Had the question read

  "When the second hand on Peter's watch complete's one
  revolution, how many revolutions will Jane observe the second
  hand of her watch to have made?"

I think my perspective might have shifted to yours since Jane
would now be the only one doing any "observing."  The question
would still be unnecessarily (and, therefore, irresponsibly)
ambiguous, but as it is, the symmetry of the "observing" that is
being done makes it impossible even to decide which answer was
intended.  If I were your student and you marked either answer
"wrong," there'd be hell to pay, even more so if I had noticed the
ambiguity and commented on it as Michael Edmiston suggested *may*
have been the author's intent.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm