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OT: speeds (physical, not computing) [was Re: 1-0.95]

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On Wed, 02 Jul 2014 21:06:52 -0700, Rustom Mody wrote:

> On Thursday, July 3, 2014 7:49:30 AM UTC+5:30, Steven D'Aprano wrote:
>> On Wed, 02 Jul 2014 23:00:15 +0300, Marko Rauhamaa wrote:
> 
>> > On the other hand, floating-point numbers are perfect whenever you
>> > deal with science and measurement.
> 
>> /head-desk
> 
> <wink>
> 
> Just as there are even some esteemed members of this list who think that
> c - a is a meaningful operation
>   where
>     c is speed of light
>     a is speed of an automobile
> 
> 
> </wink>


You seem to be having some sort of nervous tic.

Subtracting two numbers a and c *is* a meaningful operation, even if they 
are speeds, and even in special relativity.

Consider an observer O in an inertial frame of reference. A car x is 
driving towards the observer at v metres per second, while a photon p 
travels away from the observer at c m/s:


x --> v          O         p ----------> c


According to the observer, the difference in speeds between x and p is 
just (c - v), the same as in classic mechanics. The technical term for it 
is "closing speed" (or "opening speed" as the case may be) as seen by O.

Note that this is *not* the difference in speeds as observed by x, but I 
never said it was.


You don't have to believe me. You can believe the Physics FAQs, 
maintained by John Baez:

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html


The important part is the paragraph titled "How can that be right?" and 
ending "In this sense velocities add according to ordinary vector 
addition."

As I wanted to confirm my understanding of the situation:

https://groups.google.com/forum/#!topic/sci.physics/BqT0p_7tHYg




-- 
Steven

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1-0.95 Pedro Izecksohn <izecksohn@yahoo.com> - 2014-07-01 14:17 -0700
  Re: 1-0.95 pecore@pascolo.net - 2014-07-02 01:13 +0200
  Re: 1-0.95 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-02 15:27 +0000
    Re: 1-0.95 Marko Rauhamaa <marko@pacujo.net> - 2014-07-02 19:59 +0300
      Re: 1-0.95 Skip Montanaro <skip@pobox.com> - 2014-07-02 12:34 -0500
      Re: 1-0.95 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-02 18:34 +0000
        Re: 1-0.95 Marko Rauhamaa <marko@pacujo.net> - 2014-07-02 23:00 +0300
          Re: 1-0.95 Chris Angelico <rosuav@gmail.com> - 2014-07-03 10:16 +1000
          Re: 1-0.95 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-03 02:19 +0000
            Re: 1-0.95 Rustom Mody <rustompmody@gmail.com> - 2014-07-02 21:06 -0700
              Re: 1-0.95 Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-07-03 16:55 +1200
                Re: 1-0.95 Rustom Mody <rustompmody@gmail.com> - 2014-07-02 22:21 -0700
                Re: 1-0.95 Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-02 23:50 -0600
              OT: speeds (physical, not computing) [was Re: 1-0.95] Steven D'Aprano <steve@pearwood.info> - 2014-07-03 09:15 +0000
            Re: 1-0.95 Marko Rauhamaa <marko@pacujo.net> - 2014-07-03 09:51 +0300
              Re: 1-0.95 Steven D'Aprano <steve@pearwood.info> - 2014-07-03 09:36 +0000
                Re: 1-0.95 Marko Rauhamaa <marko@pacujo.net> - 2014-07-03 14:17 +0300

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