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Momentum Conservation of Turntable and Beetle System

  1. Nov 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A beetle with a mass of 30.0 g is initially at rest on the outer edge of a horizontal turntable that is also initially at rest. The turntable, which is free to rotate with no friction about an axis through its center, has a mass of 80.0 g and can be treated as a uniform disk. The beetle then starts to walk around the edge of the turntable, traveling at an angular velocity of 0.0600 rad/s clockwise with respect to the turntable.


    a. With respect to you, motionless as you watch the beetle and turntable, what is the angular velocity of the beetle? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

    b. What is the angular velocity of the turntable (with respect to you)? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

    c. If a mark is placed on the turntable at the beetle’s starting point, how long does it take the beetle to reach the mark again?


    2. Relevant equations

    L= I theta L= mvrsin

    3. The attempt at a solution

    I know in order to find the angular velocity of the beetle with respect to me, i need to know the angular velocity of the turntable, but without the radius, i cannot figure out how to find this.
     
  2. jcsd
  3. Nov 11, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi amb202! Welcome to PF! :smile:
    Just call the radius r … you'll find it drops out in the end. :wink:
     
  4. Nov 11, 2009 #3
    But wouldn't I need the angular velocity of the turntable to figure out the velocity of the beetle with respect to me?
     
  5. Nov 12, 2009 #4

    tiny-tim

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    Yes, and you can find that from the information given.

    This is physics, not geometry …

    you can't solve this just with geometry, you need a physical equation also …

    which one do you think it is? :smile:
     
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