Re: Another Rotating Cylinder Problem - explain from moving frame view

On Wed, 19 Apr 2006 18:50:28 +0200, "Harry" <harald.vanlintel@xxxxxxx>
wrote:


"David" <dseppala@xxxxxxxxxxxxx> wrote in message
news:7qh842hdgtchu8ita0vbbo8iu957s68i4t@xxxxxxxxxx
On Mon, 17 Apr 2006 11:17:38 -0400, sal <pragmatist@xxxxxxxxxx> wrote:

On Mon, 17 Apr 2006 03:16:54 +0000, David wrote:

On Sat, 15 Apr 2006 11:02:17 +0100, "Martin Hogbin"
<goatREMOVETHIS123@xxxxxxxxxx> wrote:


"David" <dseppala@xxxxxxxxxxxxx> wrote in message
news:2781421lvufjvsa14pmj5fnvk52d3mutdd@xxxxxxxxxx
On Fri, 14 Apr 2006 18:29:50 +0100, "Martin Hogbin"
<goatREMOVETHIS123@xxxxxxxxxx> wrote:


"David" <dseppala@xxxxxxxxxxxxx> wrote in message
news:5e5v32hhd167p57ikjfcvnfm82o1thjecu@xxxxxxxxxx
Can anyone explain this rotating disk problem from the point of
view
of a moving observer?

Davis Seppala is one of the mysteries of this group. Unlike
Spaceman, for example, he is smart enough to dream up endless SR
puzzles, many of them involving accelerating reference frames,
yet by his own admission he has practically no understanding of
SR.

Is he really an expert on the subject testing posters'
understanding?

Is he a bunch of psychology students performing some kind of
experiment on us all?

Is he just a troll who delights in stirring up discussion and
argument?

Any suggestions?
He is none of the above. David's understanding of Einstein's notions
so far is much like David's comprehension of E. M. Escher's drawings.
He gets to points in problems where there seems to be contradictory
results as in his posting on 4/10/2006 where a moving rigid rod is
always parallel to the x-axis and loops about the x-axis in a
circular
pattern at a 10 meter diamter circle yet no forces are applied to the
rod to make it continue in this circular pattern. Or in this posting
where as tension on a straight wire increases the center of the wire
moves away from a straight line. This is opposite to typical
experiences - wires form straight lines when stretched from two
points
with nothing in between them to interfere with the straight line.
This
does not make sense to David.

Then what David should do, as he has been told many times, is to
make sure he fully understands basic SR _in inertial frames_ with
only _inertial motion_ involved.

If he could demonstrate a sound understanding of Einstein's
postulates,
I cannot demonstrate a clear "understanding" of Einstein's postulates.
I
know the two main hypotheses stated in relativity are that all physical
laws are the same in any given inertial reference frame and that the
speed
of light is constant and independent of the motion of the emitting
source.
The translated text I read actually used the word "velocity" of light
instead of speed. We all know the velocity of light (speed and
direction)
must vary with the motion of the light source but the speed can
possibly
be constant. Although stated as a definition and not as a hypothesis
Einstein states that "time" at two points cannot be defined at all
unless
the "time" required to travel from A to B equals the "time" required to
travel from B to A. I readily admit that this statement of time that
Einstein characterizes as "true by definition" seems to me more like a
hypothesis than something true by definition.
what an inertial frame is,
An inertial frame is one in which no accelerations or higher order
changes
in position of objects occur. In in a real sense we do not find
inertial
frames except in rare situations since any kind of motion causes the
frame to accelerate slightly. But these are negligible in a practical
sense.
the train experiment,
I'm not certain which train experiment you are referring to. To
demonstrate that one inertial frame is identical to every other frame
there is a real life experiment where the acceleration of a train is so
small that passengers on either of two trains cannot easily tell which
train is moving out of the station or not. This is not much of a
physics
experiment since it merely demonstrates lack of needed sensitivity in
measuring devices, so I don't know if this is the experiment you are
referring to or not.
and the pole and barn paradox the others would be much more willing to
help him.
The pole and barn paradox. What am I supposed to do here? Plug in
values
in the time and length separation formulas to show in one inertial
frame
where the two door closings are simultaneous and the pole just fits
gives
values that in another inertial frame the two doors open and close at
different times and this is compensated precisely by the legnth
contraction?

Yes, this would be a great start. Actually do the calculation and post
the result, thus demonstrating that you've gotten to first base.

You frequently request others to post calculations for you. Do one
yourself, and post it.

If you already understand it, it won't be hard to do and won't take
much time. If you _don't_ already understand it, you'll actually
learn something useful from it. So, the exercise will be
either easy or valuable (or both) and it'll impress everybody who's
currently throwing tomatoes at you, and give you a chance to say "See
I told you all along I understood (at least) this much of it!".

So, yeah, do it, Dave!
Actually in the train problem I posted you said to show some numbers
so I did and you did not reply. In the battery problem I posted my
explanation Harald replied saying he found my error but it appeared he
mis-interpreted what I had said. So I clarified the response and he
no longer replied.
I don't know how to search for archived threads and my
reader/service only shows the past 30 days or so, so I can't get the
full thread to post as a reference for you.
David

Hmm, your reply didn't show up in my newsreader. I now do see your reply,
but it doesn't appear as if I significantly misinterpreted what you said.
I'm not sure to recall the whole story correctly, but Bill's above remark
seems to be spot-on, for you wrote (if I'm not mistaken) about an
*accelerating* point: "From the moving end's point of view [...] "time" is
measured to be slower at the other end".
In that problem the rates were constant, and then there was an
acceleration as in the twin's paradox and constant rates were resumed.
That's no good: such statements are valid for inertial frames, but
not in
general for accelerating points.

Harald


.

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