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Homework Help: Rotation of a rigid body

  1. Feb 14, 2004 #1
    a thin metallic disc pivoted at its centre, and there is a resultant torque act on it. moment of inertia about its centre is 1.50 kgm2

    if the metallic disc is replaced by another identical wooden disc, state qualitatively the effect on the angular momentum of wood.

    my tutor told me that there will be no change in angular momentum. is he right?
  2. jcsd
  3. Feb 14, 2004 #2


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    Science Advisor

    The only difference would be in the material,
    which implies mass. I can't say from that
    question if it changed, but it seems like that's
    what they're getting at. (Moment of inertia is in: kg*m^2/sec.)

    Live long and prosper.
  4. Feb 14, 2004 #3

    Doc Al

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    Staff: Mentor

    A net torque produces a rotational acceleration inversely proportional to the moment of inertia. Did the moment of inertia change when you switched from a metal disk to a wooden one?

    Moment of inertia depends on:
    Shape/mass distribution (Disk versus rod, etc.; uniform density)
    Axis of rotation

    Did any of these factors change?
  5. Feb 14, 2004 #4


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    If the two disks are identical in shape, then the angular acceleration produced by the identical torque on each will be inversely proportional to the mass. The angular momentum after the torque will be proportional to the mass. Yes, the angular momentum in each case will be the same.

    The heavier (presumably metal) disk will accelerate more slowly than the wood disk, reach a lower final angular velocity, but, because of its greater mass, have the same angular momentum.
  6. Feb 14, 2004 #5

    Doc Al

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    Staff: Mentor

    I just realized that the answer doesn't depend on the moment of inertia at all. The same torque for the same time produces the same angular momentum. (D'oh!)

    In this case, only the mass changes. But it doesn't matter what changes, as long as the applied torque is the same. So the fact that the disc is "identical in shape" is irrelevant.
  7. Feb 17, 2004 #6


    r x F =T

    As T=Ia
    Torue= Moment Of Inertia about axis of rotation x Angular acc.

    Torque is same in both the cases.

    But moment of inertia of both mentioned discs may be different.

    Torque is constant ...so Ia should remain constant.

    If I of new disc is more .......then angular acc. produced by same torque will be same and thus changes accordingly.....

    That sums it all up...

    and ur proff.. is true man
    Last edited: Feb 17, 2004
  8. Feb 22, 2004 #7
    thank you for all the explaination.
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