Lagrange Pendulum Equation of Motion

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Discussion Overview

The discussion revolves around the derivation and solution of the equation of motion for a Lagrange pendulum. Participants explore the correctness of the derived equation and the implications of different assumptions, particularly regarding amplitude.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a derived equation of motion for a single pendulum but expresses uncertainty about solving the associated differential equation.
  • Another participant questions the correctness of the derived equation, suggesting that a diagram with proper labels would aid in the discussion.
  • A different participant points out a potential error in the equation, noting that the terms on the right and left sides involve different quantities and raises the question of whether a small amplitude assumption is applicable.
  • In response, one participant asserts that a small amplitude assumption is not essential for formulating the problem, although it complicates the solution.
  • A later reply acknowledges a mistake in the derivation process and provides a corrected equation of motion.
  • Another participant mentions that the problem can be solved using elliptic integrals and notes that the small angle assumption is commonly treated at both undergraduate and graduate levels, with increasing complexity in the latter.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correctness of the initial equation of motion and the necessity of the small amplitude assumption. The discussion remains unresolved as participants continue to explore these points.

Contextual Notes

There are limitations regarding the assumptions made in the derivation, particularly concerning the dimensionality of variables and the treatment of amplitude. The discussion also highlights the complexity of solving the problem without the small amplitude assumption.

Andrew Deleonardis
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Hi, I've derived the equation of motion for a regular single pendulum, but do not know how to solve the differential equation.
I have the following:
r2θ''2=mg(cosθ-rsinθ)
 
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Before you worry too much about a solution, I suggest that you check your derivation. That equation does not appear to be correct. If you want to continue the discussion, please give us a diagram with proper labels.
 
unless r is dimensionless (probably not) your equation above cannot be correct. You are adding r sin theta to cosine theta for part of the term on the right. In general the right hand term (apparently) contains a mass, but the term on the left involves solely geometric quantities.

In addition, most of these type problems involve a small amplitude assumption. Is this the case here.
 
A small amplitude assumption is not essential. The large amplitude pendulum can be formulated just fine, although it is much more difficult to solve than with the small amplitude assumption.
 
Dr.D said:
Before you worry too much about a solution, I suggest that you check your derivation. That equation does not appear to be correct. If you want to continue the discussion, please give us a diagram with proper labels.

You're right, I completely messed up. My mistake was a mistake correcting a mistake. I had accidentally noticed that I wasn't supposed to take the derivative of the lagrangian with respect to the r because it was a constant, and mistakenly removed only half of it from my working out, thus the extra cos.

θ''=-g/r*sin(θ)
 

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Yes, the small angle assumption is not necessary, and the problem is solvable using elliptic integrals. Solving the problem with the small angle assumption is commonly treated at the undergraduate level. Even at the graduate level, the small angle assumption is common, and the problems become more sophisticated because they treat many degrees of freedom (e.g. coupled equations and normal modes).
 

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