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Homework Help: Angular momentum

  1. Jan 28, 2017 #1
    1. The problem statement, all variables and given/known data
    There is a man walking on a disk with mass 70 kg and speed 4 m/s. He walks on a circle with radius 1,5 m. How fast does the disk (mass 200 kg and radius 2 m) under him rotates (need to calculate angular velocity)


    2. Relevant equations
    angular momentum = J * w (J-moment of inertia, w, angular velocity)

    3. The attempt at a solution
    (J_disk) * (w_disk) = (J_man) * (w_man)
    1/2 MR^2 *(w_disk) = (mr^2* ( v_man)) / r
    (w_disk) = mr(v_man) / MR^2
    (w_disk) =1,05 /s

    What am I doing wrong?
  2. jcsd
  3. Jan 28, 2017 #2
    I have already solved it. It is so so easy (I knew that before, that is why I was asking here, as I was sure that my answer was correct). I did not see that it was written that the velocity (4m/s) meant the velocity of a man towards the disk, not towards the observer somewhere out of the disk.
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