# Relative velocity video (review and rate)

1. Nov 4, 2012

### 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

Last edited by a moderator: Sep 25, 2014
2. Nov 4, 2012

### Bobbywhy

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

3. Nov 4, 2012

### harrylin

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: https://www.physicsforums.com/showpost.php?p=3992825&postcount=2

Oh and remove the typo in Refrence

Last edited by a moderator: Sep 25, 2014
4. Nov 4, 2012

### maheshshenoy

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 :)

5. Nov 4, 2012

### harrylin

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.
Exactly. However:
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.
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"?
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!

6. Nov 5, 2012

### maheshshenoy

"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??

7. Nov 5, 2012

### harrylin

I'm not sure if you now fully understand it... maybe you do!
However I feel handicapped for I wished I had a nice video like the one you made to explain this! :uhh:

So, let's have a look at your video together.

For some reason that escapes me, you make a difference between "with respect to" and "relative", although you don't really make use of that difference. Still your good word choice may be helpful for my explanation to you! I will consistently use "relative" for a difference in velocities (a vector subtraction), and "with respect to the ground" for the ground as reference system.

First you compare a velocity measurement of the bike by the ground with a velocity measurement of the ground by the bike.

That is called a "system transformation", which happens when you make a conversion of measurements of one reference system to those of another one. And the transformation that you use is called a "Galilean" transformation. Coincidentally, also in special relativity, v_bike-ground = -v_ground-bike; so you can indeed say "always". But that is really an exception.

I suppose that with v_bike-ground you do NOT intend the - as a minus sign, which would indicate a velocity difference (=relative velocity in some textbooks). You clearly say "velocity of the bike with respect to the ground", and "velocity of the bike as seen by the ground". Thus with that you take the ground as reference system. Perfect.

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!

Take for example the moon. From the earth you always see the same side of the moon. What is its motion with respect to the Earth? And what is the motion of the Earth with respect to the moon? An astronaut who lands on this side of the moon, will not see the Earth go down or rise.

Next you nicely introduce the concept of a "reference". Good!
My suggestion is that you say somewhere (after 4:00) that the rule that you are going to give is only true in classical mechanics. And if a student asks you why, I know that you will give the right answer.

Then, at 5:40 you introduce (but hardly noticeable!) a new concept: relative velocity.
You say that the relative velocity of the road and the limousine "cancel out". You probably mean that with respect to the car, the velocity of the ground relative to the car is the negative of the velocity of the limousine relative to the ground, so that the velocity of the limousine relative to the car is zero. That is a straightforward vector subtraction, although in one dimension; it is all calculated using a single reference system, the car.

If that is indeed what you did, then you used there "relative velocity" definition 1 of my explanation in the link of my first reply. Do you copy that?
The advantage of that definition is that it is equally valid in special relativity. The advantage of the other definition (no.2) is that it looks somewhat simpler, even though it is in fact a system transformation. However, I noticed in discussions that it can hinder the capability to arrive at a good understanding of special relativity (which is why I nickname it "Newspeak"; you will know what I mean if you read Orwell!).

Using definition 1, I would say that the relative velocity of the limousine and the car are zero with respect to the car. And in such a case the relative velocity of the limousine and the car with respect to the ground is of course also zero.

Last edited: Nov 5, 2012
8. Nov 7, 2012

### maheshshenoy

Oki .. wow.. i should be so careful when using words :O.. anyways thank you for that :) :) :)..

9. Nov 7, 2012

### harrylin

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. :tongue2:

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.

10. Nov 7, 2012

### 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??

11. Nov 7, 2012

### maheshshenoy

I had always thought RELATIVE VELOCITY only mean's VELOCITY with respect to anything BUT ground! :D

12. Nov 7, 2012

### harrylin

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.
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.

Last edited: Nov 7, 2012
13. Nov 7, 2012

### 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 :)

14. Nov 7, 2012

### 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

15. Nov 7, 2012

### harrylin

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).
I think that that is fine, if your students already know what that means.
OK! (tonight, and better remove that address from internet to keep SpAm limited - you can still edit it out and I have it now :tongue2:)

Last edited: Nov 7, 2012
16. Nov 7, 2012

### maheshshenoy

LOL THANK YOU :D for the heads up!!