Mathematica Mathematica NDSolve different solutions same eq's

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The discussion revolves around a user's issue with Mathematica's NDSolve function while working with Lagrangians and second-order differential equations. The user encounters discrepancies in solutions when solving the equations directly versus transforming them using angular momentum. Initially, they believed the equations were equivalent, but later realized that the initial conditions set for each approach were not the same. This led to confusion regarding why different forms of what appeared to be the same equations yielded different initial conditions. Ultimately, the user seeks clarification on the relationship between the equations and the implications of the differing initial conditions.
Vrbic
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Hello, I have peculiar trouble with mathematica. I'm playing with lagrangians of some systems and I hit on "central problem". If I use my procedure in mathematica it solve 2 eq. of second order. But if you solve it by hand, you know, ##\phi## coord is "cyclic". And second eq. is reduced to ## \delta_t (mr^2 \phi)=0 ## and we introduce constant of angular momentum ##L##.
And the problem: If I let solve mathematica general equation without introduction ##L## I get another solution than if I transform second equation by introduction ##L##
(different eq.'s: ##2mr\phi'+mr^2\phi''=0## and ##mr^2\phi'=L##)
I used same initial condition.
Could you somebody advise where is the problem or how to resolve it?
 
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My bad, I found out that, the initial conditions aren't same. But it is for me still mystery. Why for this two same eq.'s in different forms, the initial conditions setted in same way aren't same?
 
As far as I can tell they are not the same equation.
 

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