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

In summary, the phrase "Two equations 'combined' don't give the desired result" suggests that simply merging two equations may not yield a correct or useful outcome. This highlights the importance of understanding the underlying principles and relationships between the equations rather than relying solely on mathematical operations to produce a valid 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
 

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

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Why don't the combined equations give the desired result?

There could be several reasons for this, including incorrect formulation of the equations, errors in the coefficients, or assumptions that do not hold true in the context of the problem. It's important to recheck each step of the derivation and the underlying assumptions.

How can I verify if the equations are correct?

To verify the correctness of the equations, you can check the dimensional consistency, validate against known special cases, or compare the results with experimental or empirical data. Additionally, peer review and consultation with colleagues can provide valuable feedback.

What steps should I take to troubleshoot the issue?

First, ensure that each equation individually gives the correct results. Then, check the process of combining the equations for any algebraic or logical errors. Revisiting the assumptions and constraints of the problem can also help identify the source of the issue.

Could numerical methods be causing inaccuracies?

Yes, numerical methods can introduce errors, especially if the equations are sensitive to initial conditions or if they involve iterative processes. Ensure that the numerical methods used are appropriate for the problem and consider refining the mesh or step size if applicable.

How can I improve the accuracy of the combined equations?

Improving accuracy might involve refining the model, using more precise data for coefficients, or employing more sophisticated mathematical techniques. Sensitivity analysis can also help identify which parameters have the most significant impact on the results, allowing for targeted improvements.

```
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