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Ball rotating on axle which is rotating itself

  1. May 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A ball of mass m is attached via a rod of length x to an axle that rotates with angular velocity ω. You can consider the ball to be a point mass.
    m = 5 kg, x = 0.3 m, y = 0.4 m, ω= 30 rad/s

    (a) What is the linear momentum (direction and magnitude) of the ball?
    (b) What is the angular momentum (direction and magnitude) of the ball about point A?

    I've included a diagram I made.The top ball has mass m = 5 kg and the bottom ball A is rotating with angular velocity ω. Hopefully that diagram makes sense...

    2. Relevant equations
    I = mr2
    L=Iω
    p=mv
    v=rω


    3. The attempt at a solution
    (a) p=mv
    v=rω=0.3m*30rad/s=9m/s
    p=5kg*9m/s=45kgm/s clockwise
    (b) L=Iω
    L=mr2ω
    This is where I am confused. It would be easy to calculate if A wasn't rotating but how do I . calculate the angular momentum of the ball about point A given the fact that A is rotating?
     

    Attached Files:

  2. jcsd
  3. May 1, 2013 #2

    SammyS

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    As I read this problem, there is only one ball. No ball at point A.

    attachment.php?attachmentid=58379&d=1367386096.jpg
     
  4. May 1, 2013 #3
    Ah, you again. Thanks for the help! :)

    I wish I could show you the diagram on my paper... it demonstrates the problem a lot better. The axle itself is rotating with angular velocity ω. There might not necessarily be a ball at the end of the axle but the point at the end of the axle is labeled A.
     
  5. May 1, 2013 #4

    SammyS

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    Do this mean you still haven't solved the problem ?
     
  6. May 1, 2013 #5

    haruspex

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    A is just a point in space taken as a reference. The axle rotates about there, but points don't have rotation.
     
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