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If we look the relations of Lorentz Boost; $$t'=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}} (t-\frac{vx}{c^2})$$ $$x'=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}} (x-vt)$$ $$y'=y$$ $$z'=z$$

we can concentrate on 2D case, and as $$v=\frac{ax}{bt}$$ and $$v'=\frac{ax'}{bt'}$$ Here $a$ is some certain numeric amount of length, and $b$ some certain numeric amount of time, so that $0<v=\frac{a}{b}<c$, ofcource the $x$ and $t$ could have been directly such amounts. By using $a$ and $b$ it can be prevented that something in the calculation becomes "Geometrized".

then we can write; $$t'=\frac{1}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (t-\frac{(\frac{ax}{bt})x}{c^2})$$ $$x'=\frac{1}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (x-(\frac{ax}{bt})t)$$

And these Reduce a bit to; $$t'=\frac{1}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (t-\frac{\frac{ax^2}{bt}}{c^2})$$ $$x'=\frac{1}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (x-\frac{ax}{b})$$

Wo when these are combined to $v'$ then $$v'=\frac{\frac{a}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (x-\frac{ax}{b})}{\frac{b}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} (t-\frac{\frac{ax^2}{bt}}{c^2})}$$

Which can be written; $$v'=\frac{a(x-\frac{ax}{b})}{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}} \frac{\sqrt{1-\frac{(\frac{ax}{bt})^2}{c^2}}}{b(t-\frac{\frac{ax^2}{bt}}{c^2}) }$$

And this reduces quickly just to; $$v'=\frac{a(x-\frac{ax}{b})}{1} \frac{1}{b(t-\frac{\frac{ax^2}{bt}}{c^2}) }=\frac{a(x-\frac{ax}{b})}{b(t-\frac{ax^2}{btc^2})}$$

And $$v'=\frac{ax-\frac{a^2x}{b}}{bt-\frac{bax^2}{btc^2}}=\frac{ax-\frac{a^2x}{b}}{bt-\frac{ax^2}{tc^2}}$$

So, this would be the Lorentz -Transformation for velocity; $$v'=\frac{ax-\frac{a^2x}{b}}{bt-\frac{ax^2}{tc^2}}$$

Question; Are these kind of higher level Lorentz Transformation been properly analysed before? (Source)
1. Velocity
2. Acceleration
3. Jerk
4. Momentum

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  • $\begingroup$ What are $a$ and $b$? $\endgroup$
    – Mark H
    Commented Jan 24, 2017 at 7:41
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    $\begingroup$ For velocity transformation (the correct formula), see for example this article. $\endgroup$
    – user130529
    Commented Jan 24, 2017 at 7:47
  • $\begingroup$ The relative velocity is the same in both frames (same magnitude opposite direction) $\endgroup$ Commented Jan 24, 2017 at 8:28
  • $\begingroup$ @MarkH a is some value for length, and b is some value for time. They could also be chosen in away that makes "c" -at claude chuber, thanks. But I wanted to maintain the velocity in a Length/Time -format, so that the true changes would be shown. - at John Rennie, Thanks, But are there any deeper meaning in ie. length variation which is revealed ($a^2$) when written as in question. $\endgroup$
    – Jokela
    Commented Jan 24, 2017 at 8:40
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    $\begingroup$ The error I found is that you is that used v, the speed between the two reference frames as if it were the same that the speed v observed for an object in that same reference frame. Under such assumption in the primed reference frame the observed speed will reduce to zero, which agrees with your equations, which reduce to v'=0 when a=b=1. So, I guess you equations might be correct. $\endgroup$
    – user126422
    Commented Jan 24, 2017 at 15:52

1 Answer 1

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The problem with your derivation starts before you attempt to generalize by defining speed as $$v = \frac{x}{t}.$$ This is incorrect because $x$ and $t$ are coordinates, not lengths or durations. In your final expression, the velocity is a function of time, which shouldn't be the case if the velocities are constant. The proper definition of velocity is $$v = \frac{x_2 - x_1}{t_2 - t_1}$$ or, for instantaneous velocity, $$v = \frac{dx}{dt}.$$

So, the proper procedure to find the velocity transformation (well, one procedure), is to define an object in the primed frame traveling at a constant velocity $v'$ with respect to that frame. $$v' = \frac{dx'}{dt'}.$$ Then, you can solve for the speed in the unprimed frame by solving the Lorentz boost equations for the unprimed $x$ and $t$ in terms of their primed coordinates and calculating $$v = \frac{dx}{dt}.$$ Note that, due to the chain rule, you'll need the expression for $$\frac{dt'}{dt},$$ which is the time dilation factor.

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  • $\begingroup$ Thanks for the answer; x is just a coordinate? I need to absorb this a bit, but I upvote cause this answer is useful. ... If x is just a coordinate, then it would mean that the position is depending form the velocity? $\endgroup$
    – Jokela
    Commented Jan 25, 2017 at 9:22
  • $\begingroup$ @JokelaTurbine Yes. x, t, x', and t' are all coordinates specifying the current position in space and time (spacetime if you want to be fancy) in different reference frames: x and t in one, x' and t' in the other. In special relativity, a reference frame is a coordinate system with inertial motion (not accelerating). For me, it helps to imagine an observer at the origin of the reference frame in order to imagine what she sees. (continued) $\endgroup$
    – Mark H
    Commented Jan 25, 2017 at 11:25
  • $\begingroup$ @JokelaTurbine The variable v is usually taken to be the velocity of one reference frame in terms of one defined to be at rest. The designation of "at rest" is arbitrary. You could just as easily say the "at rest" frame is really moving at -v velocity according to the "moving" frame (hence the name relativity). (continued) $\endgroup$
    – Mark H
    Commented Jan 25, 2017 at 11:29
  • $\begingroup$ @JokelaTurbine As to your question: "it would mean that the position is depending form the velocity?" the answer is yes. If an object has a non-zero velocity, then its position x must be changing with time t. However, an object that is moving in one frame may be at rest in another. You always have to specify what an object is moving with respect to, since every object is at rest in its own reference frame. $\endgroup$
    – Mark H
    Commented Jan 25, 2017 at 11:31

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