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Homework Help: Rotational Mechanics Problem typical one

  1. Dec 6, 2012 #1
    Hi friends,

    Here is a typical problem


    It is an elastic collision. Here I am applying simply the conservation of linear momentum as,

    Momentum before collision = Momentum after collision

    Total translational K.E. of the block will be transferred to the disk. Hence disc's COM will have velocity v after collision. and hence the angular velocity be ω = v/r.

    But this is not the correct answer. The correct answer is option (b).

    Please friends help me in this problem.

    Thank you all in advance.
  2. jcsd
  3. Dec 6, 2012 #2


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    2017 Award

    Staff: Mentor

    Angular velocity around what?
    The surface is rough, so I think you should assume that the disk is rotating on the surface. This needs additional energy, and the disk will be slower than v.
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