Talk:Relative velocity

message: Thank you for making the clarifications to some of the written portions of the article, they are certainly more clear than what I had originally wrote. On the other hand, this article was not flawed, as the difference between
a + b and a - ( - b ) is strictly a matter of convention and in fact they are completely equivalent statements. Also, merely switching signs does not change the physics, and again is a matter of convention that does not at all render the physics as flawed.
Robert A. Mitchell 21:05, 11 October 2006 (UTC)
thread_title: Comments

message: If a car (A) was travelling North at 50 m/sec and another car (B) was travelling East at 
50 m/sec what would car B's velocity be relative to car A? So would car B's velocity (relative to car A) be 70.7 m/sec South-East or 70.7 m/sec North-West?
--Phat Solja 09:38, 4 June 2007 (UTC)
thread_title:  Direction of relative velocity 

message: Hi the definition of "relative velocity" here is at odds with that as used by for example Einstein and the Physics text books that I know. According to those, "relative velocity" is defined in *one* reference frame, and is the vector subtraction of the velocities of the objects. 
That there are two competing definitions - one that has been standard and another one that some people try to replce it with - has also been discussed in a paper in the Aus.J.P., and that paper was highly critical of the definition that this Wikipedia article promotes.
Thus at the moment this article is WP:POV. Surprisingly, I now notice that this problem has already been addressed by Tayana, but without any reaction. Thus I'll mark this article POV so that people may notice.
References:
- Einstein's 1905 paper on SRT, paragraph 3
http://www.fourmilab.ch/etexts/einstein/specrel/www/
- Alonso&Finn, "Fundamental University Physics", volume 1
- G. Builder, "The constancy of the velocity of light", Aust.J.P. 1958 p.457-480
Harald88 22:58, 13 June 2007 (UTC)
thread_title:  Biased opinion without citing differently opinioned reliable sources 

message: I notice that one or two people continuously revert ot a version that is blatantly against NPOV policy; moreover, it is poorly sourced and I think that that version is of less good quality (less clear for occasional readers) than the version that was elaborated by some of us, as was discussed above. If only they added some parts of it to expand one meaning of relative velocity it probably wouldn't be too bad. I propose to ask for semi-protection of this page. Harald88 21:58, 2 October 2007 (UTC)
thread_title:  Protect the page? 

message: We overlooked that in fact there are not less than three meanings of "relative velocity": many older publications and some modern publications use the word "velocity" for "speed". Thus the relative speeds as in Doppler effect can also be described with the term "relative velocity". For example, the English translation of Einstein's 1905 relativity paper uses the term both for vectorial velocity in the first meaning of the term, as for the Doppler "velocity". What may also play a role is that in many other languages (such as German) there is only one word for both meanings. It will be good to mention that shortly as such inconsistencies can lead to quite a lot of confusion. And perhaps we should also add an example to stress how different the outcome is in some particular cases. BTW, I notice that the Doppler effect article needs some improvement on just this point! Harald88 12:54, 3 October 2007 (UTC)
thread_title:  Third meaning: relative speed 

message: I find the following example not so useful and doubtful; moreover it contains an error, so I moved it here for discussion:
Now, let us change the example. A rocket flies at high speed to the moon. It sends a laser beam sideways. When the rocket passes the space station at a very short distance, the laser beam hits a photo cell on the space station, which causes the station to send a short burst of laser light to the moon. At the same time a photo cell in the rocket gets hit by a laser beam that the space station has been emitting side ways and the rocket also sends a short burst of laser light to the moon. As calculated (naively) by an observer on the space station, the relative velocity (a+b) of the photons in the burst of laser light emitted by the rocket is the velocity (a) of the rocket relative to the space station plus the velocity (b) of the photons relative to the rocket. Because the photons in both bursts of light travel at the speed of light, the photons emitted by the rocket should hit the moon before the photons emitted by the space station. But it turns out that the photons in both laser bursts will hit the moon at the same time.
It's (too?) complicated, has at least one error and in classical mechanics as well as in relativity light is a wave and its speed is independent of the velocity of the source; thus not even naively one would think so! Also, the example was in the wrong section. But I do think that such an example, perhaps simpler and with a bullet, could be useful for the last part of the article. Suggestions? Harald88 23:12, 11 October 2007 (UTC)
thread_title:  doubtful example 

message: I found the existing explanation of usage 2 much too complicated. I simplified it as follows : 
:2. The velocity of one object as measured in another object's rest frame (that is, a coordinate system in which that other object is at rest at that point in time).
I also added a textbook reference for it and adapted the corresponding example. 
Moreover, I added an example for the third usage as well as a remark about the preferred use of inertial frames. Harald88 06:55, 5 November 2007 (UTC)
thread_title:  Usage 2: relative to rest frame 

message: The lead was much too long, making that readers might even not reach the introduction. For completness I copy the deleted part here below for consideration if it's useful to insert somewhere - some of it actually may fit in the article on velocity:
:Everything in your environment that you consider to be at rest (the room’s walls, floor, the table, your chair e.t.c.) constitutes your frame of reference.
:If you are standing on a moving walkway in the airport, you may be standing still, but you will be moving relative to the airport walls and floor. Relative to the walkway, however, you will be at rest. Similarly, say you were in a car and travelling at a speed of 60kmh-1 (relative to the road of course!), and the car in front of you was also travelling with the same speed in the same direction, the other car would appear to you as being at rest. If it were travelling at a lower speed, say 50 kmh-1, it would appear to you that the car is moving towards you (with a speed of 10kmh-1). Its relative velocity (or apparent velocity as it is sometimes called) to you will be 50kmh-1 – 60kmh-1 = -10kmh-1. Your velocity relative to the other car would be 60kmh-1 – 50kmh-1 = 10kmh-1.  Note the difference of sign between your velocity relative to the other car, and the other car’s velocity relative to you.
:From this we can deduce:
:VA/B = VA - VB
:Where VA/B is the velocity of A relative to B
:VA is the velocity of A*
:VB is the velocity of B*
:* The velocities VA and VB must be measured relative to the same object or frame of reference.
:Note that velocities are vectors, not scalars. In the above example of the two cars, we simply subtracted the magnitudes of the two velocities because they were in the same direction. If object A is moving in one direction and B in another, the velocity of A relative to the velocity of B must be calculated by subtracting the vectors :VA and VB. 
Harald88 13:08, 11 November 2007 (UTC)
thread_title:  Much too long lead 

message: This article is a candidate for for deletion, as it is in a mess and has had inconsistent and inaccurate edits with seeming destructive edit wars. Relative velocity is not an article of opinion but of basic fact!. A paragraph can be incorporated in Velocity.
Zubenzenubi (talk) 02:46, 21 January 2008 (UTC)
replies: text: No, you won't. There is a process for deletion. I will start that process now. Please don't "delete" (blank) the article. Thank you. ➔ REDVEЯS is standing in the dark 13:36, 21 January 2008 (UTC)
replies: text: OK my bad. Go ahead. Mushoo (talk) 16:53, 23 January 2008 (UTC)
thread_title:  Article for Deletion 

message: When presented correctly, relative velocity is a trivial concept, it doesn't require it's own article.  —Preceding unsigned comment added by NOrbeck (talk • contribs) 17:18, 16 July 2010 (UTC)
thread_title:  I strongly suggest reconsidering deleting this article. 

message: In the subsection now titled In two dimensions I propose to not use the Galilean transformation, but instead the concept of vector subtraction.  Referring to the same figure, I would begin with equations of motion for A and B:
:\vec r_A=r_{A0}+v_A t
:\vec r_B=r_{B0}+v_B t
\vec r_B-\vec r_A represents the location of B as seen from A.  (we need either a figure or a link describing vector subtraction).
:\vec r_B-\vec r_A= r_{B0}-r_{A0} + \underbrace{(v_B-v_A )}_{relative\;velocity} t
--guyvan52 (talk) 03:53, 20 January 2014 (UTC)
thread_title:  Propose alternate derivation of non-relativistic relative velocity. 

message: # I moved the section Special Relativity Theory one level higher because it is directly relevant to the Galilean transformation. Except for one minor edit, I left it untouched.
# Wikipedia seems to lack a simple derivation of the relativistic velocity transformation rule.  I like this derivation that uses 4-vectors.  But an alternative derivation that does not use 4-vectors would be helpful for some readers: Just take dx/dt and dx'/dt' directly from the Lorentz transformation, and after a 'bit-o-algebra' the result,v_\mathrm{B|A}=\frac{v_\mathrm{B}-v_\mathrm{A}}{1-\frac{v_\mathrm{A}v_\mathrm{B}}{c^2}}, should emerge.  But I won't write the derivation if it can be found elsewhere in Wikipedia (or Wikiversity).  Can anybody find this derivation?
# I am all for teaching readers Latin phrases, but only if they are either in common usage, OR if they are so apt that they should be.  I know nothing about inter alia.  Can somebody advise me on this phrase?
Yours truly, --guyvan52 (talk) 22:14, 20 January 2014 (UTC)
thread_title:  Before I edit "Special Relativity Theory" 

message: Is it just me or is the figure at the left too cluttered?  I propose one that resembles the figure to the right.  I would relabel the symbols so that the two large vectors (currently A and C) would become A and B (to represent the motion of the two particles) and the short vector (currently B) would be the difference.  I would include two orange balls at the at the tails, labeled Ai and Bi, that are nearly overlapping to represent the initial locations of object A and B.  Then use the subscripts at Af and Bf at the heads.  Somewhere (in the figure, caption, or text) it would be explained that the magnitude of A is vAt. In the final state (at the heads) the particles are |Af-Bf| apart, and they have been traveling for time t. See The River Needs a Cork
:One of my concerns is that the current figure focuses two much on the fact that the relative velocity as seen by the two observers are equal and opposite.  That they are equal and opposite is obvious.  Which takes which sign? There lies the difficulty, and I don't think the figure helps.--guyvan52 (talk) 00:52, 10 February 2014 (UTC)
thread_title:  Replace figure for two dimensional non-relativistic motion 

message: * See Relative_velocity#Special_Relativity.  I have trouble with the idea that two observers would see different velocities for their relative motion (unless they are using rotated coordinate systems.  I know that Thomas Precession rotates coordinates, this doesn't seem like Thomas precession.   This makes me suspicious of the entire section.--guyvan52 (talk) 13:13, 5 August 2014 (UTC)
thread_title:  Disputed section Special Relativity 

message: It is at present written on the page that dt=dt' is not valid at high speed (time dilation). This is so poorly written. It is not valid AT ALL since Relativity theory, but can be neglected in usual calculations, because of extremely small differences in results. Yet, there is ALWAYS a difference, even at low speed. This is a mistake that propagate everywhere the concept that Relativity is only a "high speed theory". It is not. It's a change in paradigm, whatever the speed you consider. Newton theory became just an approximation of Relativity theory. (talk) 13:53, 5 August 2014 (UTC)
thread_title:  Note 4