Lagrangian of the system of two masses

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Discussion Overview

The discussion revolves around the formulation of the Lagrangian for a system consisting of two masses. Participants explore the appropriate representation of the Lagrangian, the choice of generalized coordinates, and the relationships between the coordinates in the context of the system's dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes a specific form for the Lagrangian, including kinetic and potential energy terms, and introduces relationships for the coordinates y' and x' in terms of the angle α.
  • Another participant notes that the relationship x² + y² = l² can be used to express the coordinates and angular velocity, suggesting that the initial formulation is correct.
  • A different participant suggests that only one generalized coordinate may be necessary to define the system completely, proposing the use of the y-coordinate of mass m1.
  • Further discussion indicates that while the proposed formulation is correct, there may be a more efficient way to express the system using a single generalized coordinate, potentially involving an angle and length.

Areas of Agreement / Disagreement

Participants express varying opinions on the best approach to define the system's dynamics, with some agreeing on the correctness of the initial Lagrangian while others suggest alternative formulations. The discussion remains unresolved regarding the optimal choice of generalized coordinates.

Contextual Notes

Participants have not fully explored the implications of their proposed formulations, and there may be assumptions regarding the relationships between the coordinates that are not explicitly stated. The discussion also reflects uncertainty about the most efficient representation of the system.

rouge89
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I am wondering, how does lagrangian of such system look like?

24n46zn.jpg


Will it be:

[tex]L=\frac{m_{1} \cdot \dot{y}^2}{2} + \frac{m_{2} \cdot \dot{x}^2}{2} +\frac{m_{3} \cdot (\dot{y'}^2+\dot{x'}^2)}{2} + \frac{I \cdot \dot{ \alpha }^2}{2} - mgy - mgy'[/tex]

where:

[tex]y'=\frac{l}{2}sin(\alpha)[/tex]
[tex]x'=\frac{l}{2}cos(\alpha)[/tex] ?
 
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x^2+y^2=l^2,so x,y,ω(angular velocity)can be obtained.by the way that is correct
 
I'm going to sit down and do this when I get a chance. But for now, it looks like you are only going to need one generalized coordinate to completely define the system. I would use the y-coord of m1.
 
Hmm so guys, this is correct (with this additional equation as, andrien wrote: x^2+y^2=l^2), but there is way to do that better using one generalized coordinate, right? Hmm very often this one generalize is an angle + length... but here maybe it could be done with y,x etc.
 

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