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Re: SR examination question



As a student I went to see my first teacher of SR (30 years ago, at Cape
Town University) in a very confused state. I remember one sentence of
advice: unless you remain clear about which reference frame you are in, you
will get hopelessly confused.

We start the section by emphasizing that "Peter is moving with respect to
Jane" and "Jane is moving with respect to Peter" are equivalent statements.

During the course I've just taught, students asked how when answering
problems of this kind they are supposed to know when to divide and when to
multiply by gamma. We talked about the kind of indicators that identify the
proper time interval.

I'm grateful to John M, John D. and others for confirming that when one is
using the language in the correct mode for SR the question is ambiguous - I
really could not decide whether to divide or multiply by gamma!

Mark

At 14:10 17/05/01 -0500, 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 observes 1 minute to have passed. During
the time that Peter's watch has ticked off 1 minute AS OBSERVED BY HIM, how
many minutes does JANE observe (in her frame--assumed to be at rest) to have
ticked off on her watch? That's what the question says to me. Now if you
want to read in complications--both are moving away from each other at .45c
and Peter is reading Jane's watch and Jane is reading Peter's--feel free,
but the question is still clear enough to me with no sexist (or even theist)
overtones. ;-)

Rick


----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, May 17, 2001 1:17 PM
Subject: Re: SR examination question


> Earlier today, someone who remain nameless wrote:
> >I think it is quite clear what the question intends.
> >Jane's second hand makes 2.29 revolutions while Peter's makes one.
>
> Ahh, so it is _quite clear_ that Peter's reference frame is preferred.
>
> I know how to explain this:
> 1) It is obvious that girls can't be scientists.
> 2) Therefore if we measure something, we must measure it in
> Peter's frame, not Jane's frame.
> 3) Anybody who identifies with Jane (or even gives Jane
> equal standing) will have their homework marked wrong.
>
>
> >... and the observation is a 'God's Eye View' as is
> >often the case when talking about time dilation.
>
> Wow, God prefers Peter, too.

Mark Sylvester
UWCAd
Duino Trieste Italy