Talk:Relative velocity

Comments
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)

The physics on this page is incorrect.

The relative velocity of person A (moving with velocity a) with respect to person B (moving with velocity b)

is the vector a - b

The entire article is flawed accordingly.

Tayana 16:32, 20 September 2006 (UTC)

Your original article stated that the relative velocity is the vector sum (presumably a + b) while the cited equation show this as vector difference (a - b), which is the correct answer, but not in accord with your statement as summation of velocities [it is accepted that (a + b) and (a - b) are sumations, but getting the sign of velocity wrong fails to understand the basic physics of the situation.]. The relative velocity  a - b applies always which resolves to a + b in a subset of  cases Tayana 20:29, 27 October 2006 (UTC)
 * The relative velocity of A with velocity a relative to B with velocity b is not the vector sum but is the vector difference a - b.

Direction of relative velocity
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)

Biased opinion without citing differently opinioned reliable sources
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)

This article is a mess
The definitions are wrong, the derivations are wrong, the best thing is to wipe it out and start from scratch, there is no way to correct this junk. WWStone 01:06, 14 June 2007 (UTC)


 * I think that it's now quite OK. If so, we can remove the tag. Harald88 20:36, 31 August 2007 (UTC)


 * Harald88, thank you balancing the article by pointing out that "relative velocity" is used in two different ways. Good edit! I agree it's now quite OK. Rijkbenik 12:15, 1 September 2007 (UTC)


 * It's a pleasure - regretfully some people continue to mess with this article, so let's keep an eye on it! And let's hopethat also the unsourced view will be properly sourced (I only have reliable sources for the first meaning, but I assume that also the second meaning can be properly sourced).
 * Regards, Harald88 21:20, 4 September 2007 (UTC)


 * This article is thoroughly messed up since I last contributed to it. Currently (2007-09-07@00:22UTC), it is totally un-encyclopedic, confusing and does not address accurately the basic physics of one observer in motion relative to another also in motion and the perception of one in relation to another. There seems to be confusion in relation to one individual relative to another and the motion of two individuals relative to an third observer. References cited to relativity are fatuous.
 * The basic physics of relative motion as observed by one in motion with respect to another has been lost by dragging in Einstein's relativity and reversion to basic principles of physics is needed.

Tayana 00:26, 7 September 2007 (UTC)

Reverted to an accurate description to replace a poorly expressed special case, non encyclopedic and obviously a straight lift from a text book, while passing from the page of the text book to the Wikipedia page, without passing through the brain of the writer. 22:31, 29 September 2007 (UTC)


 * Not clear who's brain the article and textbooks have not passed through! As now the article was messed up to push only one POV, I reverted to the latest NPOV version by wwoods. See also the discussion above, it should be pretty easy to understand by now. Of course, clarifications and improvements of each meaning are OK. Harald88 19:06, 30 September 2007 (UTC)
 * I did notice a defect in the formulation of meaning 1 and corrected it (anyone else could have done the same). Hope it is clear enough like this. Harald88 23:02, 2 October 2007 (UTC)

Protect the page?
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)

Third meaning: relative speed
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)

Revert by Tayana
Tayana 23:57, 7 October 2007 (UTC)
 * The discussion of Harald88, confusing velocity and speed shows minimal understanding of basic principles. Harald88's contribution is inaccurate between his first and subsequent paragraphs and is propagating confusion and error, so a reversion is warranted.


 * The discussion that I reverted to was the result of the improvements to a clear and WP:NPOV version by several editors including myself and (surprise!) Tayana herself - see the above comments. The topic of adding the third meaning that at this time also happens to be used in Doppler effect (which I advice not to use, see my comments above as well as there!) is entirely a different topic.
 * Tayana is kindly requested to explain in which way the current article is "inaccuate" and/or to make appropriate minor corrections, as long they are in accordance with the cited references and not according to her own singleminded (and confused?) opinion.
 * Tayana, please note that I do think that the version to which you reverted has good stuff that may be included to refine explanations; but we must stop the POV pushing of one definition against the other. This is Wikipedia you know!
 * Now that I look more carefully to the version you preferred, it actually confuses all three different definitions in such a way that consecutive statements are incompatible with each other! Was that perhaps the reason that the article has been revised? Harald88 20:00, 11 October 2007 (UTC)


 * There is total confusion in this article as reverted by Harald88 at 20:00, 11 October 2007 (UTC).

In the first part of the reversion, the relative velocity of one body with respect to another body is defined correctly as a vector difference, yet in a follow up paragraph relative velocity is shown as a sum of the vectors. There is no indication of why. Harald88 is confusing the topic and does not appear to understand the basic principles. The reference to NPOV is somewhat naive. The version of Tayana is correct while Harald88 is incosistent and flawed.

194.46.251.87 20:51, 11 October 2007 (UTC)


 * I did write the abovementioned correct paragraph but I did not write a paragraph in which "relative velocity is shown as a sum of the vectors". To be fair, I had not noticed it as it's too big an error to notice - I only saw that someone took the effort to make a sketch to illustrate vectors. Thanks for pointing it out! I'll correct it (you could also correct such an error when you see it!). BTW, the reference to NPOV is warranted as it is based on a serious disagreement in the literature - it's actually naive to think that narrowminded POV is NPOV! Anyway, let's do this the Wikipedia way and take the good parts of both versions! (but I won't be able to finish that today). Harald88 22:09, 11 October 2007 (UTC)

doubtful example
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)

Merged parts of both versions
Done! This should be a better starting point for further improvements - I'm a bit tired, so don't make a fuss if something is still not quite OK but just improve more!

Cheers, Harald88 00:00, 12 October 2007 (UTC)


 * This article is still a mess as of 19:48, 18 October 2007 (UTC)

The paragraph: ''For example, let's assume that the objects are two cars that move towards each other, and the reference frame is the road. The velocity of the one car (moving with velocity a) relative to the velocity of the other car (moving with velocity b) is the vector (a - b). '

Harald88 is propagating heresy here. The relative velocity in the case of the two cars approaching each other as presented in above paragraph is (a+b), why; because the two cars are moving in opposite directions so the relative velocity is (a-(-b)) Reading through his contributions suggest a lack of basic understanding of elementary physics. A reference to a university website that explained the basic physics was removed by him. Please Harald88 desist from expressing opinions and reverting on items that you don't properly understand.

194.46.253.155 19:52, 18 October 2007 (UTC)


 * User 194.46.253.155 does not understand that heresy belongs to religion: it is anti-scientific to be anti-heretical. :-)
 * Of course - and as I stated - there may still be some glitches, and we encourage corrections by people to the sections of their expertise. The above paragraph gives an example for relative velocity according to meaning 1. I learned "relative velocity" according to the cited physics book (in agreement with some others such as Einstein and Builder); the notation is straight from that book and is certainly correct: the sum of two equally long and oppositely pointing vectors is 0.
 * Also, anyone can write anything on websites so that they are rarely admitted in Wikipedia - please refer to quality physics textbooks and peer-reviewed papers. Harald88 16:56, 4 November 2007 (UTC)

Usage 2: relative to rest frame
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)

This Page is Awful
Excuse me for being blunt, but this page is in terrible shape. Even if we accept the need for a separate article on "relative velocity" (which is questionable), the various definitions presented in the current version of this article (5-Nov-2007) are mostly just plain wrong. The whole section that talks about the rate of change of distance with time (ds/dt) and calls that the "scalar velocity" is flat out wrong. The scalar velocity is simply the magnitude of the velocity, which is to say, the square root of the sum of squares of the components. For example, if a particle is moving in a circle around the origin, at a particular moment when it crosses the x axis its velocity has components vx=0, vy=1. Now, it's distance from the origin isn't changing, so ds/dt = 0, but the scalar velocity of the particle with respect to the rest frame of a particle at the origin is not zero, it is sqrt(0^2 +1^2) = 1. This is just one example. The entire article needs to be re-written. Lumpy27 19:59, 5 November 2007 (UTC)


 * Lumpy, thanks for your suggestions; as you may understand from the discussions above, this article bears the scars of disagreements while no good version has existed before. What matters is to improve the sections that explain the views you understand, based on cited quality references.
 * In view of the mistaken idea of the most recent editor who misunderstood the different meanings, confused two of them and deleted the references, I see no other option than to revert to the properly referenced text; however, I'll include obvious text improvements by him, as long as they didn't introduce new, unreferenced meanings. For example, it was me who had added "citation needed" to "scalar velocity". The correct word for "scalar velocity" is "speed", as also one of the references makes clear. Moreover, I had made the same mistake as you to think that "relative velocity" only is used with inertial frames. However, neither of the two textbooks I consulted (and added to te text) shares that opinion, and in a co-rotating reference frame the article is not moving relative to the origin. That's why the remark about inertial frames is certainly very useful there for the readers.
 * Note: the confusions that exist between the different uses attests to the necessity for an acurate article about the topic that collects the differening opinions. NPOV is one of the points in which Wikipedia excels. Harald88 20:59, 6 November 2007 (UTC)

Merging with velocity
I believe we should merge this article with the velcity article. the reason is that having separate articles for velocity and relative velocity suggests that they are two distinct things while actually they are two names of exactly the same thing. all velocity is relative. all velocity is relative velocity, there exists no velocity which is not relative velocity. the word velocity always means relative velocity, and there is no such thing as absolute velocity. Mushoo 10:19, 9 November 2007 (UTC)


 * There certainly are those who have the convention and POV that "relative velocity" simply means "velocity" (but then why do they use that term?!). However, that is just one POV, and there are others (Einstein, Builder, Alonso&Finn) with a different POV or convention, which happens to be the one I was taught and which I appreciate. Thus your starting assumption is erroneous, as even explained in this article. It's the policy of Wikipdia to inform the readers of the different viewpoints; and proper mention of the different POV's results in stable articles (see for example centrifugal force.
 * It may be better however, to inverse the numbering in the intro; then the meaning of "relative velocity = velocity" can be simply stated as such, with a link to the article on velocity.
 * Apart of that, it is still open for discussion to merge the two articles, depending on the length that this article will obtain. I propose to wait half a year or so, and then decide if the combination of the two artilces is better or if they are of such length that it's better to keep them apart. Articles do have a tendency of getting longer, and it would be a waste of time to merge what later will be split up again.
 * Harald88 13:36, 11 November 2007 (UTC)


 * The article on centrifugal force is terrible... one of the worst I've seen in Wikipedia (which is saying something). Wikipedia policy is that if an idea is held by just one or just a very small number of people, it doesn't belong in Wikipedia. Only material that comes from reputable sources is allowed. I don't find any confusion as to the meaning of "relative velocity" in any reputable sources, so I don't think there is justification for filling up this article with confusion that exists only in the mind(s) of a very small number of people.
 * As to why the term "relative velocity" is used, I would point out that people often use the phrase 'velocity of A relative to B', so they don't even use the literal expression "relative velocity". It is simply two English words, "velocity", and "relative". Together they have a specific meaning, just like 'large' and 'dog'. Do we really need a separate Wiki article defining "large dog", considering that both the words "large" and "dog" are already adequately defined? Where would this end? Do we need a Wiki article on every possible combination of words?Lumpy27 21:44, 11 November 2007 (UTC)


 * Good to see how far our opinions differ. :-)
 * Already the above expressed opinion by Mushoo ("relative velocity" == "velocity") to which you responded is incompatible with your intro below, and on which we agree - it happens to also correspond to my preferred references.
 * If large dog could mean "large dog" but many people think it means "labrador", that would be worth at least a section and perhaps an article, depending on how much materal there is about it. As it stands, the existing article is already of considerable length eventhough I have removed some stuff; there clearly is a lot to say about the topic. We can also include mention of physics review articles about the subtle differences in definitions.
 * I hold that NPOV articles are much better than articles that are in a continuous state of edit warring, even if it makes them a little less slick. Anyway, our discussion above is now bypassed by your proposal just below which is certainly slick and possibly sufficiently NPOV.
 * However,I disagree that there would be a difference between "relative velocity" and "velocity [...] relative to". That's just a matter of syntax, depending on how one turns the phrase. Harald88 22:22, 11 November 2007 (UTC)

Merge with velocity, or else revise completely
I tend to agree that this article could be merged with the article on "velocity", because once the word "velocity" is understood, there is not much more to understanding the phrase "relative velocity". The only reason I can see for having a separate article on "relative velocity" is to explain the way in which this phrase is used. If we keep this as a separate article, I propose that the article should read as follows:

Relative Velocity

The expression “relative velocity” signifies the difference between the velocities of two objects, with the understanding that the individual velocities are each evaluated in terms of a single system of coordinates, usually an inertial coordinate system unless specifically stated otherwise. (See velocity)

For example, if the velocities of particles A and B are vA and vB respectively in terms of a given inertial coordinate system, then the relative velocity of A with respect to B (also called the velocity of A relative to B) is vA – vB. Conversely the velocity of B relative to A is vB – vA.

If one of the particles is at rest with respect to the coordinate system in terms of which the individual velocities are evaluated, then the relative velocity is simply the velocity of the other particle. If no other system of coordinates is specified, the expression “velocity of A relative to B” is usually understood as shorthand for “the velocity of A in terms of an inertial coordinates system with respect to which B is at rest”.

In Galilean kinematics (i.e., not accounting for the effects of special relativity), the relative velocity between two particles is the same with respect to any system of inertial coordinates. This is because changing from one inertial coordinate system to another (according to Galilean kinematics) simply adds a common increment vector to each velocity vector, so the difference between any pair of velocities is unaffected.

However, taking the effects of special relativity into account, the relative velocity between two particles actually depends on the inertial coordinate system in terms of which the individual velocities are evaluated. This is because the effect of changing from one system of inertial coordinates to another is more complicated than simply adding a common increment vector to each velocity vector. See special relativity.

References:
 * (1) Greenwood, Donald T, “Principles of Dynamics”.
 * (2) Goodman and Warner, “Dynamics”.
 * (3) Beer and Johnston, “Statics and Dynamics”.
 * (4) McGraw Hill Dictionary of Physics and Mathematics.
 * (5) Rindler, W., “Essential Relativity”.

Either it should be revised as above, or else merged into the "velocity" article. Do I hear any objections?


 * Yes I object but not in principle. The objection is that the above is too simple, too much a stub. It misses some of the currently provided references and it is lacking quite some precisions.
 * Also, your light-hearted comment above that 'once the word "velocity" is understood, there is not much more to understanding the phrase "relative velocity" suggests that you have no idea of the many confusions that exist - you risk that some editors will claim that your explanation is "totally wrong".
 * However, I rather like the phrasings and I think that a revision along the lines you sketch will certainly increase the article a lot. We should only try to include most of the existing text, in order to avoid the old situation of full reverts. Harald88 13:57, 11 November 2007 (UTC)
 * Harald88 13:57, 11 November 2007 (UTC)


 * Your objections don't amount to anything. You say the proposed wording is "lacking quite some precisions", but you don't indicate what "precisions" it is lacking. Also, you say the proposed text doesn't reflect the many confusions that exist, but surely the purpose of the article is not to convey confusions. You say some editors will claim the explanation is totally wrong, but why not let them speak for themselves? We can then explain to these hypothetical editors that they are mistaken, and that it isn't "my" explanation, it is the explanation presented in the numerous reputable references that have been provided, which substantiate each and every statement in the article.


 * As for the proposed article being "too simple" or "too much of a stub", well, we have already discussed here the fact that there is very little content to the idea of "relative velocity", and it could very well be merged with the article on velocity. There's no excuse for padding out an article to make it long just for the sake of being long. If there's not much to say about a subject, then there's not much to say about it.


 * If you have some specific objections to the proposed article, I'd be happy to hear them. I have a TON of objections to the current article... so many that it would be tedious for all concerned to go through them. Basically, the only suggestion I can make is to completely revert the current article and replace it with the proposed article. Many people on this discussion page have said the current article is totally crumby, so I think in this case a total revert to a better baseline is quite reasonable. Lumpy27 17:44, 11 November 2007 (UTC)


 * The precisions are most of the existing article, thus there is no need to repeat it here. I hope that you can see its length, which I reduced after another editor increased its for no apparent reason (see above). Some of the precisions are mine, many other precisions are by other editors who started this article and who insist on their inclusion. The current status is the follow-up of an edit war that consisted of complete reverts between incompatible versions by other editors; let's hope that others will agree with a more beautiful presentation!
 * Of course you could do like them and revert to their own versions, so that instead of two alternating poor versions we'll end up with an edit war between three versions. Needless to say, that is against Wikipedia rules and not productive.
 * As for objections, the article to discuss is in the article space. Generally what works in Wikipedia is to slowly improve, adding improvements step by step that replace well-meant but poorly presented material. I just started with that and welcome your help. For example, obviously the illustration of vector subtraction should be replaced by a special drawing that for this article, with proper indication of velocities.
 * Harald88 22:41, 11 November 2007 (UTC)

Much too long lead
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)

Proposal for New Baseline Article
I've made an attempt to further improve the proposed new baseline for this article. I think these words would be suitable, either for this article on "relative velocity", or as a new section in the main article on "velocity".

Relative Velocity

The expression “relative velocity” signifies the difference between the velocities of two objects, with the understanding that the individual velocities are each evaluated in terms of a single system of coordinates, usually an inertial coordinate system unless specifically stated otherwise. (See velocity)

For example, if the velocities of particles A and B are vA and vB respectively in terms of a given inertial coordinate system, then the relative velocity of A with respect to B (also called the velocity of A relative to B) is vA – vB. Conversely the velocity of B relative to A is vB – vA. If no other system of coordinates is specified, the expression “velocity of A relative to B” is usually understood as shorthand for “the velocity of A in terms of an inertial coordinates system with respect to which B is at rest”.

In Galilean kinematics (i.e., not accounting for the effects of special relativity), the relative velocity between two particles is the same with respect to any system of inertial coordinates. This is because changing from one inertial coordinate system to another (according to Galilean kinematics) simply adds a common increment vector to each velocity vector, so the difference between any pair of velocities is unaffected. However, taking the effects of special relativity into account, the relative velocity between two particles actually depends on the inertial coordinate system in terms of which the individual velocities are evaluated. This is because the effect of changing from one system of inertial coordinates to another is more complicated than simply adding a common increment vector to each velocity vector. See special relativity.

References:
 * (1) Greenwood, Donald T, “Principles of Dynamics”.
 * (2) Goodman and Warner, “Dynamics”.
 * (3) Beer and Johnston, “Statics and Dynamics”.
 * (4) McGraw Hill Dictionary of Physics and Mathematics.
 * (5) Rindler, W., “Essential Relativity”.

Does anyone have any specific and rational objections to this wording?Lumpy27 17:50, 11 November 2007 (UTC)


 * It's roughly the same as your earlier version, and as before, I welcome the clarity of expression. There is lack of in-text references but that can be fixed later. If other editors agree, I propose to replace the current introduction by Lumpy's text and continue to improve the article from that basis. Harald88 22:58, 11 November 2007 (UTC)


 * The article as current, 21 November 2007 @01:38 (UTC), is non encyclopedic and is somewhat obtuse and long winded in its' third paragraph and far removed from what is basic physics material. Reference to Galileo and his kinematics is certainly novel. The article as presented by Tayna had the basic physics correct and steered away from relativity and featured basics.
 * The references to physics textbooks is unsatisfactory, as not listing their publisher, edition  and ISBN numbers and indicating the page number cited. There was as reference to a basic physics educational website, in previous edits which explained the position clearly and was more accessible to users of the article. This should be reinstated as web based references are preferable to paper textbooks in an online encyclopedia.
 * Finally I would tend to agree with an editor who stated that Harald88's edits were misinformed and trust that his future edits will be reliable.

194.46.187.17 (talk) 01:46, 21 November 2007 (UTC)


 * My edits were mostly based on textbooks, and therefore the above remark by 194.46.187.17 is misguided. Moreover, as stated before I largely agree with current version of the article, as it is (again) based on the standard definition of textbooks.
 * However, contrary to another editor I recognize that other definitions do exist, and in view of NPOV mention should be made of all relevant definitions and with proper sourcing. That should end the edit warring. Possibly just a simple acknowledgment may suffice, such as: "some textbooks even define relative velocity as the velocity of A as measured in an inertial reference system in which B is at rest".
 * I now reinserted the external references that were lost by edits of others. Harald88 (talk) 14:18, 24 March 2008 (UTC)


 * As at least several users (such as here above) and myself find the external links to vector addition and relative velocity useful for this article, thus reverted the deletion again. Anon motivated: Remove spurious external links. One was just a web page on vectors, and the other a web page on special relativity. Both those topics have their own wiki articles. My comment: there is nothing spurious about these pages and many readers will benefit from direct links to the most tricky aspects of relative velocity - which are vector addition and special relativistic calculations. Of course, there is nothing against finding better external links. Harald88 (talk) 18:22, 29 March 2008 (UTC)

Article for Deletion
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)

I agree with you. This was my suggestion originally too. Anyone who objects to deleting this article and including a paragraph in the velocity article please object here before 23rd Jan 2008. I will delete this article by then if no one else does so. Mushoo (talk) 08:43, 21 January 2008 (UTC)


 * 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)


 * OK my bad. Go ahead. Mushoo (talk) 16:53, 23 January 2008 (UTC)

I strongly suggest reconsidering deleting this article.
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)

I respectfully disagree. As the category statement states below, this is a Start-Class article of high importance. A good grounding in Galilean notions of relative velocity is (1) difficult for students to acquire and (2) essential for grasping Special Relativity-guyvan52 (talk) 21:41, 19 January 2014 (UTC)

Propose alternate derivation of non-relativistic relative velocity.
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)

Before I edit "Special Relativity Theory"

 * 1) 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.
 * 2) 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?
 * 3) 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)

Replace figure for two dimensional non-relativistic motion
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)

Disputed section Special Relativity

 * See Relative_velocity. 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)

I just looked at Velocity-addition_formula and now believe your equations are correct. But that does not necessarily imply that they belong in this article. I propose that we instead reference the aforementioned Wikipedia article, where the equations are fully derived and stated in two different notations. The way I see it, Velocity-addition formula is the advanced article, and this article is for beginners (i.e. most college freshmen). I hate to do it to you because you worked so hard (and so carefully as far as I can tell). But a Wikipedia article needs consistency in its level. This article started with a picture showing a person walking on a train, which in my opinion, establishes it as an elementary article.

Since your equations appear to be correct, I will remove the harsh template questioning its correctness. But we still have a problem with the level of this article that needs to be resolved.--guyvan52 (talk) 13:51, 5 August 2014 (UTC)

Note 4
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) please correct this.