## Relative velocity video (review and rate)

Hey Guys I am making a series of physics tutorials.. with an aim to solve problems and understand concepts without making using of any formulae by making use of animations and some real life examples

I started with conceps of relative velocity.. Please tell me if its useful..

I am sorry if this is against the forum policy .. but its an educational video! Cheers

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 Recognitions: Gold Member Physics is well done. One suggestion: Please remove the background music. Its noise adds nothing and only distracts.

 Quote by maheshshenoy Hey Guys I am making a series of physics tutorials.. with an aim to solve problems and understand concepts without making using of any formulae by making use of animations and some real life examples I started with conceps of relative velocity.. Please tell me if its useful.. I am sorry if this is against the forum policy .. but its an educational video! Cheers http://www.youtube.com/watch?v=JtPoqE_z2Jk
Hi welcome to physicsforums!
That's a nice instructive video but with an important issue.

A problem that regularly occurs is confusion due to mix-up between reference system transformation and velocity difference calculation. That mix-up has led to disagreement about "relative velocity" in textbooks (many textbooks mean with "relative velocity", simply "velocity", so that "relative" has been made meaningless). Regretfully this mix-up also happens in your presentation.

It is of no practical consequence in classical physics but goes wrong in relativity. The mixed-up understanding that is engrained in some people's head makes them think that basic rules of mathematics magically don't work in relativistic mechanics.

So, it would be better if you either do not make a stealth system transformation in your discussion, or explicitly mention that you do so, and that this makes no difference in classical mechanics.

Also you have to decide which definition to adopt, see my explanation here: http://physicsforums.com/showpost.ph...25&postcount=2

Oh and remove the typo in Refrence

## Relative velocity video (review and rate)

Thank you for your replies.. Bobby why .. I ll make sure i won't add any music anymore :D.

Harrylin.. so you saying when students would wanna study relativity they would get confused why they had learnt something before (simple vector addition) doesn't work at relativistic speeds??

But i thought.. relativity is usually taught by giving them examples of non relativistic speeds and telling that in those examples we make use of one important assumption , that the time and length do not change when we move (and even mass for that matter).. so they actually DO change but at normal speeds insignificant and hence simple vector addition works.. However at relativistic speeds we need to take into account the lorentz transformation (or something like that :D) for length contraction and time dialization which would make them realize why things "seem" to go weird at such speeds..

can you elaborate the meaning of "stealth system transformation" ?? Cause I didn't really get it :(.. thanks for the analysis though :)

 Quote by maheshshenoy [..] Harrylin.. so you saying when students would wanna study relativity they would get confused why they had learnt something before (simple vector addition) doesn't work at relativistic speeds??
To the contrary: vector addition always works. A theory of physics cannot change the mathematical fact that 2x+2x=4x, whatever the x. Einstein even used vector addition to re-derive the Lorentz Transformations. Regretfully I have discussed with several people who thought that the laws of mathematics don't always work, because they were confused by mixed-up explanations early on. Thus the clarity with which you explain this can be important.
 But i thought.. relativity is usually taught by giving them examples of non relativistic speeds and telling that in those examples we make use of one important assumption , that the time and length do not change when we move (and even mass for that matter).. so they actually DO change but at normal speeds insignificant
Exactly. However:
 and hence simple vector addition works..
Vector addition of the same things always works. Therefore, a velocity difference in a single reference system is a vector subtraction that always works. That is different from a system transformation.
 However at relativistic speeds we need to take into account the lorentz transformation (or something like that :D) for length contraction and time dialization which would make them realize why things "seem" to go weird at such speeds..
Right - if you make a transformation. So, what definition of "relative velocity" do you use, and why? What is the difference according to your definitions between "velocity" and "relative velocity"?
 can you elaborate the meaning of "stealth system transformation" ?? Cause I didn't really get it :(.. thanks for the analysis though :)
When you say "from the viewpoint of the car", I suppose that you mean wrt the car - thus as measured by the car - there you probably made a transformation from the road reference system to the car reference system!
 "Right - if you make a transformation. So, what definition of "relative velocity" do you use, and why? What is the difference according to your definitions between "velocity" and "relative velocity"? " Hmmmmm.. well I thought .. when we say velocity.. it actually means relative velocity WITH RESPECT TO earth.. and when we usually say RELATIVE VELOCITY.. it means velocity with respect to something else when i mean't simple vector addition, i mean't without considering the effects of time dilatation :D.. ok now i get the stealth system transformation thing that you mention i think.. so you saying people get confused whether to use VELOCITIES (wrto earth) and ADD THEM (vectorially) or do system transformation OHHH I GET IT NOWWW>> why thats the same in CLASSICAL PHYSICS and ITS NOT IN RELATIVITY :O :O .. i just got it while composing this reply lol :D.. But i think system transformation works in relativity right??
 Oki .. wow.. i should be so careful when using words :O.. anyways thank you for that :) :) :)..

 Quote by maheshshenoy Oki .. wow.. i should be so careful when using words :O.. anyways thank you for that :) :) :)..
Hi it's not too bad, I think that you are better than most! But it is always possible to improve, and I had in mind to go over your video with you from start to end - it can become one of the best on internet on that topic.

However, that is only feasible if you give more precise feedback.
I'm even still not sure what exact definition of "relative velocity" you use and want to explain, despite the fact that it is the title of your video.
 "But then (2:20) you arrive at slippery ground. You claim that this is true for the velocity between any two objects: that velocity of B wrt A is always the negative of the velocity of A wrt B. That is not necessarily true. It is where many textbooks go wrong; an object is not always a reference system. In fact it usually is not!" what if i said .. this is strictly restricted to ONE DIMENSION motion?? then??
 I had always thought RELATIVE VELOCITY only mean's VELOCITY with respect to anything BUT ground! :D

 Quote by maheshshenoy "But then (2:20) you arrive at slippery ground. You claim that this is true for the velocity between any two objects: that velocity of B wrt A is always the negative of the velocity of A wrt B. That is not necessarily true. It is where many textbooks go wrong; an object is not always a reference system. In fact it usually is not!" what if i said .. this is strictly restricted to ONE DIMENSION motion?? then??
I considered that, but it's perhaps too vague. For example you can have "one-dimensional motion" of a ball that is rolling over the street. It doesn't work for that, if you use the ball as reference - imagine how the street passes by as seen from a camera that is stuck inside the ball! The simplest and easiest fix is perhaps to leave "always" out or to replace "always" with something like "for such cases". No need (and not useful) to explain everything at once; that can come later.
 Quote by maheshshenoy I had always thought RELATIVE VELOCITY only mean's VELOCITY with respect to anything BUT ground! :D
In mechanics, the ground is just another object that can be used as reference.

PS. If you like I can send you a PDF copy of a chapter of a university textbook that explains it rather well in technical terms; just email me.
 "In mechanics, the ground is just another object that can be used as reference." exactly and when we say only velocity.. for example i am going at 10 km per hour.. i mean its with respect to ground.. so usually with respect to ground we say just velocity and when we want to take anything ELSE as the refrence.. we would explicitly mention saying velocity with RESPECT to something (other than ground) and we name it as RELATIVE VELOCITY.. (not that normal velocity is NOT relative velocity with respect to ground.. but since we most of the times use ground as the refrence we would drop RELATIVE.. and put it back only when we want to use something else as the reference.. ).. thats what I HAD thought :)
 "I considered that, but it's perhaps too vague. For example you can have "one-dimensional motion" of a ball that is rolling over the street. It doesn't work for that. The simplest and easiest fix is perhaps to leave "always" out or to replace "always" with something like "for such cases". No need (and not useful) to explain everything at once; that can come later. " how about one dimensional and PURELY TRANSLATIONAL motion?? :D

 Quote by maheshshenoy "In mechanics, the ground is just another object that can be used as reference." exactly and when we say only velocity.. for example i am going at 10 km per hour.. i mean its with respect to ground.. so usually with respect to ground we say just velocity and when we want to take anything ELSE as the refrence.. we would explicitly mention saying velocity with RESPECT to something (other than ground) and we name it as RELATIVE VELOCITY.. (not that normal velocity is NOT relative velocity with respect to ground.. but since we most of the times use ground as the refrence we would drop RELATIVE.. and put it back only when we want to use something else as the reference.. ).. thats what I HAD thought :)
I see what you mean, but I'm afraid that your definition does not correspond to either of the two definitions that I summarized. I don't think that it is of any use for students to introduce a third definition - it's bad enough to have two definitions! In physics the ground is only used as reference for laws of physics when its motion can be neglected (or alternatively all kinds of corrections must be made).
 Quote by maheshshenoy "I considered that, but it's perhaps too vague. For example you can have "one-dimensional motion" of a ball that is rolling over the street. It doesn't work for that. The simplest and easiest fix is perhaps to leave "always" out or to replace "always" with something like "for such cases". No need (and not useful) to explain everything at once; that can come later. " how about one dimensional and PURELY TRANSLATIONAL motion?? :D
I think that that is fine, if your students already know what that means.
 Quote by maheshshenoy yea man i would love to .. my email id is xxxxx thanks in advance :) :)
OK! (tonight, and better remove that address from internet to keep SpAm limited - you can still edit it out and I have it now )
 LOL THANK YOU :D for the heads up!!

 Tags animation, real examples, relative velocity, video