Solving 2nd Order ODE: r\ddot\theta-g\sin\theta=0

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Discussion Overview

The discussion revolves around finding a solution to the second-order ordinary differential equation (ODE) given by r\ddot\theta-g\sin\theta=0, where r and g are constants. Participants explore various methods for solving this equation, including numerical and analytical approaches, and discuss the implications of small angle approximations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about methods to solve the differential equation.
  • Another suggests using numerical methods, specifically the Runge-Kutta method.
  • A participant expresses disappointment at the suggestion of numerical methods, indicating a preference for analytical solutions.
  • There is a question regarding the sign of the second term in the equation, with a later confirmation that it should indeed be a plus sign.
  • One participant mentions that the solution can be expressed in terms of elliptic integrals, which another participant appreciates as a potential direction for their solution.
  • Another participant suggests that for small displacements, the sine and cosine functions can be approximated, leading to a quasi-linear form of the equation.
  • One participant asserts that the solution should involve Jacobi elliptic functions.
  • A participant references a specific text on elliptic functions and applications as a valuable resource for understanding the solution to the simple pendulum ODE.

Areas of Agreement / Disagreement

Participants express a variety of approaches to solving the ODE, with some advocating for numerical methods while others prefer analytical solutions. There is no consensus on the best method, and multiple competing views remain regarding the approach to take.

Contextual Notes

Participants mention assumptions related to small angle approximations, which may limit the applicability of certain methods discussed. The discussion includes references to elliptic integrals and Jacobi elliptic functions, indicating a range of mathematical techniques that may be relevant.

NeutronStar
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How would I go about finding a solution to this differential equation?

[tex]r\ddot\theta-g\sin\theta=0[/tex]

Where r and g are constants.
 
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Oh pooh,...

That's what I didn't want to hear! :cry:
 
Did you intend for the second term to have a plus sign?

IIRC, you should be able to reduce the solution to quadratures expressed in terms of elliptic integrals.
 
Tide said:
Did you intend for the second term to have a plus sign?

IIRC, you should be able to reduce the solution to quadratures expressed in terms of elliptic integrals.
Yes, it should be plus. Sorry about that.

Hey, thanks for the tip about the elliptic integrals! That may be just what I'm looking for! :approve:
 
NeutronStar said:
How would I go about finding a solution to this differential equation?

[tex]r\ddot\theta-g\sin\theta=0[/tex]

Where r and g are constants.

If you want to solve the equations of your Lagrange Dynamics problems, you could also post it ¡n that thread you wrote. I didn't mention it to you, but the next step after writing the equations is solving them analytically. The usual assumptions made here by phsicists and engineers are to consider small displacements (i.e [tex]\theta\rightarrow 0[/tex]). Then you could remove [tex]sen\theta[/tex] and [tex]cos\theta[/tex] of your equations and made it quasi-linears. Try to go about that, because it is the usual estrategy in Lagrange Dynamic courses.
 
Such an equation usually appears for oscillating motions.
You can get a reasonably good approximation for small angles, where:

[tex]\sin \theta \approx \theta[/tex]

and

[tex]\cos \theta \approx 1-\frac{1}{2}\theta^2[/tex]
 
NeutronStar said:
How would I go about finding a solution to this differential equation?

[tex]r\ddot\theta-g\sin\theta=0[/tex]

Where r and g are constants.

The solution should be sinus amplitudinis,the Jacobi elliptic function.I'm sure of it.
 
I was curious as well to learn the solution of the simple pendulum ODE.The best approach i came across is the one in
Derek F.Lawden:"Elliptic Functions and Applications",Springer Verlag,1989,p.114 pp.117.
But the chapters 1 pp.3 (p.1 pp.94) are essential to understanding properly what he's doing when speaking of the simple pendulum.
 

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