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What system does this lagrangian represent?

  1. Jun 26, 2005 #1
    Hey guys,

    I am struggling with this tutorial problem: If you have the following lagrangian:

    L = m/2(aX^2 + bXY + cY^2) - K/2(ax^2 + bxy + cy^2)

    (Where captial letters indicate first derivative with respect to time, a,b,c constants)

    What physical system does it represent?

    It looks like a simple coupled oscillator, but I dont know what the extra velocity term (bXY) could represent? Any help would be much appreciated.

  2. jcsd
  3. Jun 26, 2005 #2

    James R

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    Does it matter what physical system it represents? What does the question ask?

    It's really hard to work back to from the Lagrangian to a particular system, anyway, since the coordinates can be generalised coordinates rather than simple displacements, for example.

    It would be possible to transform to different generalised coordinates (U,V) which would completely remove the XY terms in the expression, and give an equivalent Lagrangian with no terms of the form kUV.

    For example, try:

    U = X + Y
    V = X - Y
  4. Jun 26, 2005 #3
    The question is as follows:
    1) Derive the Equations of motion for this sytem

    Examine particularly cases a=c=0, and b=0,c=-a (I am not sure whther this applies to part 1 or 2/3)

    2) What is the physical system described by the above Lagrangian
    3) Write the natural form of the Lagrangian for this system

    But thanks for the help so far.
    Last edited: Jun 26, 2005
  5. Jun 26, 2005 #4


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    You can probably write each term in terms of a symmetric matrix. Finding the eigenvectors and eigenvalues (i.e., "normal modes") should suggest a nicer choice of coordinates.
  6. Jun 26, 2005 #5
    for the first term, do you mean X or X dot?

    do you mean [itex] \frac {m}{2} (a\dot{x}^2 . . ..[/itex] ???
  7. Jun 26, 2005 #6
    JamesR and robphy, thanks. Using normal coordiantes seems to be the way to go.

    sniffer: yes, by X I meant x dot, I just havent worked out how to use the equation editor on forum.
  8. Jun 26, 2005 #7
    sorry, i didn't read carefully.
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