Finding the Lagrangian Matrix for Two-Spring Systems

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SUMMARY

The discussion focuses on deriving the Lagrangian matrix for a two-spring system, specifically addressing the kinetic energy matrix ##\mathbf T## and potential energy matrix ##\mathbf V##. The kinetic energy is expressed as ##T=\frac{1}{2} m(\dot x_1^2 + \dot x_2^2)##, which can be represented in matrix form. The participants clarify that the Lagrangian is defined as L=T-V, and emphasize the importance of correctly identifying the matrices involved in the calculations. Additionally, the discussion highlights the need to understand the equilibrium position and the eigenvalues and eigenvectors relevant to the system.

PREREQUISITES
  • Understanding of Lagrangian mechanics
  • Familiarity with matrix representation of kinetic and potential energy
  • Knowledge of eigenvalues and eigenvectors in linear algebra
  • Basic concepts of spring systems and equilibrium positions
NEXT STEPS
  • Study the derivation of the Lagrangian for multi-body systems
  • Learn about the construction of kinetic and potential energy matrices in mechanics
  • Explore eigenvalue problems in the context of mechanical systems
  • Investigate equilibrium positions in spring-mass systems
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Students and professionals in physics, particularly those studying classical mechanics, as well as engineers working with dynamic systems involving springs and mass. This discussion is also beneficial for anyone looking to deepen their understanding of Lagrangian formulations.

MattIverson
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Homework Statement


The problem is attached. I'm working on the second system with the masses on a linear spring (not the first system).
I think I solved part (a), but I'm not sure if I did what it was asking for. I'm not sure exactly what the question means by the... L=.5Tnn-.5Vnn. Namely, I'm not sure what the n's are and what they represent, so I'm worried that I'm missing something. I also am not sure about the equillibrium position...
Then I'm pretty sure I can do part (b) and (c), but I'm wondering if I just find the eigenvector and values like a normal linear algebra problem. Do I simply just take the matrix I have and operate on it?
Screen Shot 2018-10-27 at 10.41.39 PM.png

Homework Equations


L=T-V
e^-iwt
d/dt(dL/dqdot)-(dL/dq)

The Attempt at a Solution


Attached (for the second system). I'm pretty sure I can figure out (1) if I figure out (2).
Pt1.png
pt2.png
 

Attachments

  • Screen Shot 2018-10-27 at 10.41.39 PM.png
    Screen Shot 2018-10-27 at 10.41.39 PM.png
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  • Screen Shot 2018-10-27 at 10.41.39 PM.png
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  • Screen Shot 2018-10-27 at 10.41.39 PM.png
    Screen Shot 2018-10-27 at 10.41.39 PM.png
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  • Pt1.png
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  • pt2.png
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You didn't do what was asked for in part (a). It's asking you to find two matrices, ##\mathbf T## and ##\mathbf V##, for one thing.

In the Lagrangian, you have for the kinetic energy ##T=\frac 12 m(\dot x_1^2 + \dot x_2^2)##. You can write this as
$$T = \frac 12 \begin{pmatrix}\dot x_1 & \dot x_2 \end{pmatrix} \begin{pmatrix} m & 0 \\ 0 & m \end{pmatrix}\begin{pmatrix}\dot x_1 \\ \dot x_2 \end{pmatrix}.$$ The matrix of masses is the matrix ##\mathbf T## you're being asked to find. You can express the potential energy similarly in terms of a matrix.
 

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