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Moment of inertia

  1. Dec 10, 2014 #1
    Why the moment of inertia of a molecule about the bond axis is small enough to ignore ?
     
  2. jcsd
  3. Dec 10, 2014 #2

    Simon Bridge

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    Try calculating it and see.
     
  4. Dec 10, 2014 #3
    I really don't have any idea.Please guide me.
     
  5. Dec 10, 2014 #4

    Bystander

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    Where is the mass of an atom located?
     
  6. Dec 10, 2014 #5
    throughout the atom.But being more technical i think where center of mass is present.
     
  7. Dec 10, 2014 #6

    Simon Bridge

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    How do you normally calculate the moment of inertia?
    What would the distribution of mass in a diatomic molecule look like - roughly?
     
  8. Dec 10, 2014 #7

    Bystander

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    "????" Back up. Describe the structure of an atom.
     
  9. Dec 10, 2014 #8
    sorry i think the nucleus contain all mass as electrons have negligible mass.
     
  10. Dec 10, 2014 #9

    Bystander

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    Better. What's the diameter of a typical nucleus, or has that been covered for you at all?
     
  11. Dec 10, 2014 #10
    I normally calculate moment of inertia as total mass multiplied by distance square.
     
  12. Dec 10, 2014 #11
    about 1.75 fm.
     
  13. Dec 10, 2014 #12

    Bystander

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    Sounds about right. What's nuclear mass x 1/2 of that squared?
     
  14. Dec 10, 2014 #13
    Sorry I didn't understand.Nuclear mass would depend on atom of which element we are taking.
    squared of what?
     
  15. Dec 10, 2014 #14
    I normally calculate moment of inertia as total mass multiplied by Distance between the axis and rotation in m..
     
  16. Dec 10, 2014 #15

    Bystander

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    Pick an atom.

    "What?" The nuclear diameter. That's an end point of a bond axis.
     
  17. Dec 10, 2014 #16
    Sorry i am talking about a molecule.Here x axis is bond axis.
    upload_2014-12-10_11-51-26-png.76442.png
     
  18. Dec 10, 2014 #17

    Bystander

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    No, that is one of three rotational axes directed at right angles to one another through the center of mass of the molecule; the bond axis is the line connecting the nuclei of the two atoms, and is the one axis of the three that you're interested in for this problem.
     
  19. Dec 10, 2014 #18
    Bystander is correct, the axis you have as your x-axis is one with a non-negligible moment of inertia. There would also be one perpendicular to this and the axis along the bond, which also has a non-negligible moment of inertia, this is why diatomic gases have two rotational degrees of freedom. The bond axis is that which joins the two nuclei.
     
  20. Dec 10, 2014 #19
    [QUOTE="Bystander, post: 4940685, member: 50"the bond axis is the line connecting the nuclei of the two atoms,
    upload_2014-12-10_19-45-17.png
    Now is x axis bond axis?
     
  21. Dec 10, 2014 #20
    No, your black line (the bond) is (lies along) the bond axis.
     
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