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Homework Help: Conservation of angular momentum

  1. Sep 13, 2012 #1
    1. The problem statement, all variables and given/known data
    When a block is dropped to a disc that is rotating with a constant angular velocity about its centre, at the end, we know that both of them will rotate with the same new angular velocity which is slower than the previous one.
    Question: What is the force that makes the block to start rotating?

    2. Relevant equations

    3. The attempt at a solution

    Is it friction? Can friction actually starts a motion of an object? I know that it is friction that slows down the disc but what force makes the object to start its rotational motion? I don't know.
  2. jcsd
  3. Sep 13, 2012 #2


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    Gold Member

    Assemble the disc on a table and create a little pulley on the edge of the table. Tie a bob of some mass around some string and arrange this over the pulley. Then wrap the other end of the string around the axle of the disc. When the string unwinds, the bob will descend to the floor and it's weight will create a downward force, [itex] F [/itex], at some distance [itex]r[/itex], thus creating a torque, which turns the disc.

    If it weren't for friction, we wouldn't be able to walk.
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