Two equations "combined" don't give the desired result

  • #1
nomadreid
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Homework Statement
From only (A) x'=g(x-vt) and (B) x^2-(ct)^2=x'^2-(ct')^2 derive
(C) t'=g(t-(vx/c^2)),
Relevant Equations
g= gamma = 1/sqrt(1-(v/c)^2) and the equations in the Statement
In https://phys.libretexts.org/Bookshe...__Relativity/5.06:_The_Lorentz_Transformation

First, the equation (5.6.7) apparently has a typo: the x' should not be in the denominator, as one can see by comparing it with the equation just above it from which it was derived. The corrected equation is Equation (A) in the Statement (standard Lorentz transformation).

Then two equations down (unnumbered), the author states the equation (B) in the Statement,
"We combine this with Equation 5.6.7 that relates x and x' to obtain the relation between t and t′:"
and then states the equation (C) in the statement.

How he means to "combine" them is what I don't successfully get. I tried substitution of x' from (A) into (B), and got a mess; I then tried solving (B) for x', and substituting this solution into (A), and got the same mess, that is,
(A) into (B)

first mess.PNG

which doesn't simplify to (C). Either: (a) my algebraic manipulation is wrong; (b) the author is including some other equation in the derivation.
Any indications where this is going wrong would be greatly appreciated. (Yes, there are other ways to derive the relation (C), but I am interested in this author's derivation.)
 
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  • #2
nomadreid said:
which doesn't simplify to (C)
Yes it does (up to the sign of the square root).
 
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  • #3
Thanks, Orodruin. OK, I will try again, now with the assurance that I just made some minor mistake made in simplifying. That fully answers my question! Super!
 
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  • #4
update: found the error. It all comes out. Thanks again, Orodruin
 

1. Why do two equations combined not give the desired result?

There are several reasons why combining two equations may not give the desired result. One possibility is that the equations are not compatible with each other, meaning they cannot be solved simultaneously. Another possibility is that there may be errors in one or both of the equations. Additionally, the equations may be missing important variables or have incorrect coefficients, leading to an incorrect result when combined.

2. How can I ensure that two equations combined will give the desired result?

To ensure that two equations combined will give the desired result, it is important to first check that the equations are compatible. This means that they must have the same number of variables and the same form (e.g. both linear or both quadratic). Additionally, it is important to double check the coefficients and variables in each equation to make sure they are correct. If necessary, you can also use a graphing calculator or software to graph the equations and visually confirm that they intersect at the desired solution.

3. What should I do if two equations combined give an incorrect result?

If two equations combined give an incorrect result, the first step is to carefully check the equations for any errors. Make sure all variables and coefficients are correct and that the equations are compatible. If everything appears to be correct, you can try using a different method to solve the equations, such as substitution or elimination. If the incorrect result persists, it may be necessary to seek assistance from a tutor or teacher.

4. Can combining two equations ever give more than one solution?

Yes, it is possible for two equations to have more than one solution when combined. This occurs when the equations are not independent, meaning they share a common solution. In other words, the equations represent the same line or curve. In this case, the equations will have infinitely many solutions that satisfy both equations simultaneously.

5. Is there a specific method for combining two equations?

There is no one specific method for combining two equations. The method used will depend on the form of the equations and the desired solution. Some common methods include substitution, elimination, and graphing. It is important to choose a method that is appropriate for the equations and will lead to the desired result.

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