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Find the impluse momentum of the sphere

  1. Feb 7, 2016 #1
    1. The problem statement, all variables and given/known data
    A sphere with mass 0.4 kg circles around a circular trajectory with ray 2 m and angular speed 12 rad/S. Which is the momentum of the impulse of the sphere?

    2. Relevant equations
    L=Iw=2/5mr^2w
    w(angular speed)
    or
    L=mr^2w
    3. The attempt at a solution
    I think we should use the first formula becuase the I (inercy momentum) of sphere, but in the solution it is used L=mr^2w. Which is correct?
     
  2. jcsd
  3. Feb 7, 2016 #2

    mfb

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    The first would describe a sphere rotating around its center.
    The second is correct if you have a point-mass.

    For a sphere rotating around a point different from its center you need a sum of both, but keep in mind that the radius of the sphere is not the same as the radius of the rotation. If there is no radius given for the sphere itself, you cannot add this contribution. It can be negligible (if the sphere is small compared to the 2 meters).
     
  4. Feb 7, 2016 #3

    haruspex

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    You mean angular momentum, right? Are you told the radius of the sphere?
     
  5. Feb 7, 2016 #4
    I don't know if the radius (ray) given is of the sphere
     
  6. Feb 7, 2016 #5

    haruspex

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    You did not confirm whether you mean angular momentum.
    It is not stated whether the sphere is rotating on its own axis. If it is executing a circle of radius 2m but retaining a constant orientation then its being a sphere is irrelevant. You can treat it as a point mass.
     
  7. Feb 7, 2016 #6
    It says impulse momentum of sphere not angular. So we should take it s a point mass?
     
  8. Feb 8, 2016 #7

    ehild

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    In what language was the problem written?
    In my language, (Hungarian) the same word is used both for ray of light and radius of circle, and also the angular momentum is called impulse momentum. But in English, use radius and angular momentum:
    A sphere with mass 0.4 kg circles around a circular trajectory with radius of 2 m and angular speed 12 rad/S. What is the angular momentum of the sphere?
     
  9. Feb 8, 2016 #8

    haruspex

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    I see @ehild points out that "impulse momentum" might mean angular momentum (strange though that may seem). But that does not help because you would also need to know (a) the radius of the sphere and (b) the rotation rate of the sphere on its axis (which need not be related to fhe rate of rotation of the sphere about the given axis).
     
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