Re: Closing speed vs relative speed for dummies

On 16 feb, 10:11, "harry" <harald.NOTTHISvanlin...@xxxxxxx> wrote:
Uncle Ben wrote:
On Feb 13, 2009, 9:51 am (EST), "Androcles"
<Headmas...@xxxxxxxxxxxxxxxx> wrote:

Snivelling relativistic lying *** trolls claim that c+v is not
the speed of anything or is a "closing speed", accusing
mathematicians
of not being able to distinguish a "closing speed" from a "relative
speed" (which, with all honesty, I readily own that I cannot -'

Poor Androcles, stuck in the 19th century, cannot understand how
Albert Einstein, writing in the 20th, can write an equation containing
the factors (c+v) and (c-v) and yet claim that the speed c is a
universal constant.

The Newspeak you refer to has been invented in or around the 50ties of the
twentieth century - Einstein did NOT take participate in that. However, you
are of course quite right that Andro got confused by the simple terms such
as c-v, but this appears to have happened WITHOUT the introduction of such
jargon. On the other hand, it is possible of course that he FIRST got
confused by that jargon, and next the reading of Einstein's paper was done
with confused eyes, and so the damage was irreversible...

Thus in his delicate and tactful manner,
Androcles suggests that Einstein violates the 2nd Postulate of SR,
when actually Einstein is merely calculating the time of closure
between a light wave with speed c and a target moving at speed v.  If
the target is moving away from the oncoming light beam, the distance
betweem them decreases at the rate c-v; if it is moving towards the
oncoming beam, the closing rate is c-v.

Me thinks you surely mean:
" If the target is moving away from the oncoming light beam, the distance
betweem them decreases at the rate c-v; if it is moving towards the oncoming
beam, the closing rate is c+v." ;-)

Moreover, if the target is moving away from a retreating lightbeam, the
distance betweem them increases at the rate c+v; if it is following the
retreating beam, the opening rate is c-v.

Some of us have used the term "closing speed" for the factors quoted
above, and this has mystified Androcles.  Closing speed is the rate of
change of the distance between two objects as they both move.  Closing
speed is certainly a rate of change of a distance, but in the given
frame of reference, there is no material object nor even a photon that
moves with that speed.

Relative speed, the speed of one object relative to another, is a
different matter.  In Galilean physics, there is no difference between
closing speed and relative speed, because distance and time are on the
same scale regardless of motion between the two frames of reference.
Relative speed, in short, is the speed of one object with respect to a
frame of reference in which the other object is at rest.

Not quite, see your own text just above; as you correctly stated initially,
*relative speed* is the speed of one object relative to another; and the
other object may be either a small object or a reference frame (even a
virtual one).
However, *speed* generally means the relative speed of one object wrt a
frame of reference.

Hello Harald. I am very interested in the way you handle the concept
of "(inertial) reference frame" (IRF) here. You identify an IRF with a
"small object"? If that were the case, you associate a mass with an
IRF? Can you clear to me what do you mean by "virtual IRF" and "non-
virtual IRF"? At the end, in what I am really interested, is to know
about if every specific IRF (for example, the ECI one of the GPS) can
be used or not to describe the movement with respect to it of ANY
object of the whole Universe (for example, the Sun). As maybe you can
remember, since many years I am claiming here that a specific IRF can
be only the centre of mass one corresponding to some determined body
set (for the ECI of the GPS, the Earth and its operating artificial
satellites), and I am also claiming that the UNIQUE bodies whose
movement can be described using some specific IRF are the ones
belonging to its associated body set, the unique ones taken into
account when determining the centre of mass.

RVHG (Rafael Valls Hidalgo-Gato)

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