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Angular velocity?

  1. Aug 9, 2011 #1
    A disk of radius R and mass M is mounted with its axis vertical.It is inially at rest.A bullet of mass m and velocity v is fired horzentally and tengential to disk .It enters the disc at its perimeter.What angular velocity will be acquired by disk?
    1=mv/(m+M)R
    2=mv/(2M+m)R
    3=Mv/mR
    4=2mv/(MR+2mR)
    5=(m+2M)v/MR

    plz explain how u compute the answer
     
  2. jcsd
  3. Aug 9, 2011 #2

    cepheid

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    Have you considered applying the conservation of angular momentum?

    Hint: what is the angular momentum of the bullet around the disc's centre just prior to impact? What is the angular momentum due to the combined rotation of the disc+bullet around the disc's centre after the collision?
     
  4. Aug 9, 2011 #3
    ok...i see the approach
    the angular momentum of bullet at tangent to disk will be mvR
    now how to calculate velocity?
     
    Last edited by a moderator: Aug 9, 2011
  5. Aug 9, 2011 #4

    Doc Al

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    Good.
    First answer cepheid's second question.
     
  6. Aug 9, 2011 #5
    well as i see it the combined angular momentum after collision will be (m+M)Rv'
    correct??
     
  7. Aug 9, 2011 #6

    Doc Al

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    No, not quite. How would you express the angular momentum of a rotating disk? How does the presence of the bullet stuck onto the disk change that?
     
  8. Aug 9, 2011 #7
    maybe the bullet changes mass distibution
    i am not quite getting it
     
  9. Aug 9, 2011 #8

    Doc Al

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    Sure it does.

    But before worrying about disk + bullet, how would you express the angular momentum of a rotating disk alone?
     
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