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

On Tue, 18 Apr 2006 01:59:20 +0000, David wrote:

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!


[David said:]
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

Aha, this comment explains a lot.

Two things.

a) If your news service has retention extending back just 30 days,
chances are it's missing a lot of messages right from the start, as
well; that may be why you don't always see them.

SO.... you should use Google Groups to see the full set of messages
(because they're more reliable than your usual news server), and if
you want to be sure everybody sees what you post, you should post
through Google as well.

b) You can access archived messages for (at least) several years
through Google. You've been given a couple links but maybe you
need something more. Here's a link + some directions:

Go here:

http://groups.google.com/advanced_search

Enter "sci.physics.relativity" in the "Group" field

Enter words from the subject, in the "Subject" field

Enter the Author if you want.

Then click the _UPPER_ "Google Search" button (the one at the page
bottom which says "Lookup Message" is only for message ID
searches).

Once you get a message whose thread you're interested in from the
search results, go to the top of the web page, find the link "view
as tree" just below the bold-faced topic header, and click it.
That will give you the whole thread.

And that's that.

[And David said:]
Actually in the train problem I posted you said to show some numbers
so I did and you did not reply.

If you mean the "rubber train" problem ... If I recall correctly, I
asked you to show, mathematically, that you could build a train that
could be stretched arbitrarily (like, to 10x its original length,
maybe?) without breaking, yet which would collapse if one additional
passenger stepped on board. As far as I can recall you made no
attempt to do so, but just asserted that you could. Since I very
much doubt that it's even possible, showing it seems important.

You also seemed to have lost track of whether you had the front and
back of the train glued together or not, and you never clarified
that.

Frankly, that problem was pretty ridiculous, and I was kind of losing
interest anyway. However, if you think you showed that it made sense,
post a link to the message, but I don't recall ever seeing any
calculations from you on that one, just assertions.

[David:]
In the battery problem I posted my explanation Harald

You replied to Harald, but you ignored _my_ last reply to you on the
battery problem, which problem is actually a lot more interesting than
the rubber train, IMHO.

Maybe you didn't see it. You should, again, use Google if you really
want to know whether someone replied to you or not. They seem to be
well-connected to the whole web; they hardly ever drop messages.


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.




Maybe you want me to plug in the length contraction, and
then add the product of the time difference & V (distance traveled) to
see if I get the correct number. This only demonstrates an
understanding of algebra.

I should point out that is no good his saying, 'Yes, yes, yes, I
know all that', he must clearly demonstrate that he fully
understands basic SR before anyone will accept that his questions
are serious.
I suspect this posting still won't result in any physics answers to my
two cylinder questions I posted - this one and the one 4/10/2006. But
we'll see.
David

Will he make a concerted effort to understand the basics and show
that he has done so or will he continue as before? We shall see.

Martin Hogbin




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