Re: If NOT a reactionless drive, will it work?
From: vernonner3voltazim (vnemitz_at_pinn.net)
Date: 07/16/04
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Date: 15 Jul 2004 17:21:04 -0700
"Andrew B. Park" <novakyu@yahoo.com> wrote:
> > "vernnoner3voltazim" wrote:
> > Thanks for the analysis. I will try to clarify the
> > issues below.
>
> Hmm, the clarification changes everything. I think it
> might be beyond my reach to do a detailed analysis,
> but there are a few clarifications I want to propose
> --see if it conforms to the intention of the author.
OK.
> (snip)
> > It's not so bad as that. For example, the Air Force
> > found out in the 1950s, building ejection seats for
> > jets, that, under high-jerk conditions (where "jerk"
> > is rate-of-change-of-acceleration), stress is actually
> > NOT proportional to strain. The proportionality
>
> Hmm--so are you saying that Young's modulus is not a
> constant in the situation you describe? 'Sounds possible
> --I will have to look at the derivation in the book to
> see if minimal-jerk assumption was entered into derivation.
I should probably say that the "minimal jerks" I mentioned
are of the more-or-less ignorable kind. For the moment,
instead of a battering ram that might be 2 kilometers long,
think about an ordinary wooden baseball bat. If I pretend
THIS is my battering ram, and apply a Force (as in F=m*a) at
one end of the bat, then just as in the long battering ram,
that Force cannot affect the opposite end of the bat until
it travels through the length of the bat at the speed of
sound in its wood. AFTER that Force has done that, we see
it accelerate in accordance with the equation. But WHILE
that Force is traversing the bat, it is only experiencing
jerk. However, since that "WHILE" only lasts less than one
thousandth of a second, we are able to completely ignore
the jerk of the bat in this situation, and focus on the
standard equation instead. Simply because we don't notice
the jerk in progress, it is so short and minor, and because
it has no adverse effects.
Non-ignorable jerks are easy to find, though. Just watch
a demonstration by masters of karate!
> BTW... if Young's modulus is not constant, then is it
> still required that the force (by the Superman) travel
> at the speed of sound? Speed of sound (equation for
> which I have given before) depends on Young's modulus,
> and for "sound" minimal-jerk assumption certainly holds,
> but for the force Superman is exerting, that assumption
> does not hold, and Young's modulus not being equal (that
> is, it's a function of da/dt, rather than a constant
> function), that force might be able to travel faster
> than normal speed of sound, as Young's modulus for that
> force (er... or jerk) might be greater than normal.
I will not object to that in principle. In detail
though, you may not be able to use it as freely as you
might at first think. As you know, Superman's applied
force causes a longitudinal compression wave -- a sound
wave, that is! -- to begin moving through the battering
ram. The ram as a whole is not compressed evenly; only
the region occupied by the moving wave is compressed.
If you could cause ANOTHER sound wave to start moving
just as the compressed zone went by, then I suppose the
second wave would find that the speed of sound in that
compressed zone would be greater than in the speed in
the normal state of the battering ram's material. But
I don't see how this can help you. PERHAPS the main
sound wave is able to "bunch up" because the compressed
material lets the trailing end of the sound wave go
faster than the leading edge. Note that that is the
realm where "solitary waves", or "solitons" begin to
be described; the formation of solitary waves is often
associated with significant jerks!
> > only holds for steady-acceleration conditions and
> > minimal jerks. The proof was that EVERY TIME they
> > designed certain parts that were required to withstand
> > loads that were computed using the proportionality,
> > the parts broke. Keep in mind that the Air Force had
> > a vested interest in light-weight parts that were only
> > as strong as they needed to be. Those ejection-seat
> > parts had to be tougher and heavier, to withstand the
> > ACTUAL loads. --Of course THAT explains why I wanted
> > a tough battering ram! Because I know that Super
> > jerks are going to be happening to it!
> (snip)
> > > But... as soon as we start considering relativistic
> > > effect, your whole scenario breaks apart--even if
> > > you put speakers everywhere possible, the fastest
> > > speed sound signal can travel through the wires is
> > > the speed of light--so, in that approximation
> > > limit, your man will not be pushing the rod in
> > > unison (and you should know, that, in theory of
> > > relativity, "unison" has not much meaning... as
> > > there is nothing that is "simultaneous").
> >
> > I am aware of the problem of simultaneouty.
> > Sometimes I think it is overhyped. Consider a single
> > microphone that leads to a powerful amplifier. From
> > that amplifier are HUNDREDS of equal-length wires,
> > each of which feeds one of the speakers along the
> > 2Km battering ram. Yes, I know this means a lot of
> > loose wire on the ground, where the speakers are
> > physically close to the amplifier. The point is,
> > the signals WILL travel in parallel over equal
> > distances from the amplifier, so all the speakers
> > should blare "simultaneously". I think the problem
> > that Relativity has with this is that it wants to
> > use a single point as the origin of a descriptive
> > reference frame, and a single point is inadequate
> > to deal with multiple parallel signals -- only
> > ONE of which can pass through that Origin. On the
> > other hand, there is still room for nitpickiness,
> > since how accurately can we cut those hundreds of
> > wires all exactly the same length? But then, this
> > is a THOUGHT-experiment, and something like that
> > should be allowed! Finally, if one wants to be
> > nitpicky about the varying distances from each
> > speaker to however-many men will hear it, then I
> > could resort to specifying two thousand headphones
> > instead of a lesser number of speakers, and that
> > eliminates the speed-of-sound-in-air reason for
> > nitpickiness. In conclusion, for PRACTICAL
> > purposes, we can get "close enough" to "unison",
> > that this thought-experiment is not invalidated.
>
> So, with all those little guys working in unison
> being possible, may I propose the following
> clarification:
> Think of the guys as little devils attached to the
> battering ram, in equal intervals (at least
> initially, as the ram might stretch and bend as it
> moves back and forth). So, the little devils will
> move with the ram, providing the force when they
> are asked to. (This deals with the problem...are
> the guys stationary with respect to the ground or
> the ram? What if the ram gets out of their reach?
>--Or did you want them to be stationary with respect
> to the ground?)
Heh heh heh. Please note that the thought experiment
as written in this Thread some messages ago was edited
from the form written in the document at
http://www.nemitz.net/vernon/artrr.rtf
In that description, I indicate the possibility that
the ram might experience some overall motion toward
Superman, and that he is not expected to remain in
place. You may take that as implication that the
2000 men don't have to stay in place, either, to
keep up with the ram's overall motion. **IF** there
is any such unidirectional motion, of course.
Encouraging the experts here to determine that (rather
than spout dogma) is the purpose of posting the
thought-experiment, of course. So, I don't think the
little devils are needed.
> I could have used a charged rod and constant electric
> field instead of little devils, but then we also run
> into problem of special relativity if we are going to
> turn to field on and off--the force exerted will not
> be in unison, unless we agree to ignore this small
> difference (but then, who knows what problem might
> occur due to that another assumption).
Well, you could replace all the men with electromagnetic
repulsors, and the 2000 parallel headphones could be
parallel power wiring instead. But then there is
the problem of the actuators keeping up with the
to-be-determined unidirectional motion of the ram.
> (snip)
>
> > Again, I do thank you for your analysis. Perhaps,
> > if the clarifications I've tried to provide are
> > adequate, you might consider trying again?
>
> I have a Greek final coming up tomorrow--so I will
> need to get to that first, and then see how this
> one works out... if I can work it out at all.
No hurry! Thanks again!
> Oh, BTW, then I suppose, for high jerks, it is more
> appropriate to think of the battering ram as super-
> elastic rubber rod? As it will most certainly be
> contracting (this one for sure, when Superman exerts
> his force), bending and stretching (not quite sure
> when this might happen....).
I'm not sure that it is necessary to worry about
the exact substance of which the battering ram is
made. Pretend it's wood if you must pick something
-- just remember to also pretend it won't break. :)
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