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Re: Ballistic Theory and the Sagnac Experiment

Subject: Re: Ballistic Theory and the Sagnac Experiment
From: ""
Date: 29 Mar 2006 19:50:26 -0800
Newsgroups: sci.astro, sci.physics.relativity, sci.physics
George Dishman wrote:
> "[email protected]" <[email protected]> wrote in message
> news:[email protected]
> >
> > Jeff Root wrote:
> >> George Dishman wrote:
> >>
> >>
> >> Actually they're scenarios, which were described in order
> >> to pose and answer the questions.
> That's more accurate but Jim didn't seem to be
> listening so I needed a more direct opposite.
> The scenarios were used as "thought experiments"
> with the intent of making Jim question his
> explanation so "questions" isn't too far off.
> > Agree. George is running interference
> No, I was just trying to get you to pause and
> rethink. Jeff seems to have accomplished that.
> >> Of course, a scenario doesn't cause Doppler shift, any
> >> more than a question does.
> >>
> >> The question is:
> >> What causes the Doppler shift in scenario (3)?
> >
> > Make that question (a)
> > Answer: Change in position at instant of firing
> Excellent, we can all agree on that. Since the
> change of position is the only difference between
> (1) and (3) we can say the change of position must
> be the cause of the Doppler shift in this case, but
> we cannot yet say it is the _only_ cause of Doppler
> shifts, there might be other effects that also have
> the same effect.
> > Question (b)
> > What causes the Doppler shift in scenario (4)?
> There are two _possible_ cause comparing (1) and
> (4), first the position has changed, which we know
> will cause a Doppler change, but also the bullet
> speed has changed and that might also have an effect.
> We need a way to test that.
> To find out if a change of bullet speed produces a
> Doppler shift, we would like a scenario in which it
> is the only possible cause and in particular change
> of position has been eliminated. We have that by
> comparing (1) with (2). In (2) the rifleman is
> moving forward when he fires so the bullet speed is
> increased but he runs back between shots so the
> firing position is constant. From Jeff's animation,
> the target hit rate is identical to the firing rate
> in (2) so there is _no_ Doppler shift in that case.
> We can therefore conclude that a change of bullet
> speed does _not_ cause a Doppler shift.

Why do you now find it necessary to retrieve from the rubbish bin, the
(2) red herring which you agreed had NO bearing?
_IF_  (2) scenario was considered, it is obvious that the kE imparted
to the bullet
by way of the gun's motion (as separate from the charge) is cancelled
by hte reversal of the gun's direction (as it returns to the firing
So (2) is OUT, and (3) as well   (original scenario was an aircraft
mounted gun-
which does NOT stop to fire    :-)
> > Answer: Increased bullet velocity
> > .........(for an extra 1/2 mark, the position change is incidental)
> Sorry Jim, the correct answer is that the Doppler
> shift is again caused by the change of position
> between shots. The rifleman's motion at the time
> of firing does also produce the change of impact
> energy which is therefore the "incidental" aspect.

Blythely ignoring conservation of energy. The increase KE
MUST be due to increased velocity.
> > Question (c)
> > Which of the two ACTUALLY caused the Doppler shift, and where is the
> > evidence?
> > Answer: (4), because it demonstrated increased kinetic energy delivered
> > to the target, whereas (3) did not.
> As Jeff said, (4) is a scenario, not a cause.

FYI, in that "scenario", the "cause" of the doppler is increased bullet
> The correct answer is:
> > Answer: Change in position at instant of firing
> and the evidence is in two parts:
>   i) Comparing (1) with (3), the position at time
>      of firing is the only factor that changes so
>      must be _a_ cause.
>  ii) Comparing (1) with (2), the speed of the
>      bullet is changed but no Doppler shift
>      results hence bullet speed does _not_ cause
>      Doppler on its own.
> For an extra half mark you could note that when
> both the position and the bullet speed are changed
> the Doppler shift is not the same as when only the
> position changes and for the full extra mark you
> explain as follows:
> If the rifleman moves forward a distance L between
> shots then the time taken by the bullet to reach the
> target is reduced by L/v where v is the speed of the
> bullet, assumed to be the same for all bullets. That
> is then the difference between the firing and impact
> periods, Pf and Pi respectively hence we can write:
>   Pi = Pf - L/v
> Clearly if L=0 then Pi = Pf and there is no Doppler.
> In cases where L is not zero, the amount of Doppler
> shift depends on the speed v as well as the movement
> between shots L.

Thank you!
"the amount of doppler shift depends on the SPEED"
(as well as the movement between shots L.)
George D

The REAL reason for the doppler being tagged by that evidence
which is seen at the target--(4) produced the observed energy gain,
 whereas (3) DID NOT

Jim G

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