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Rotational Motion HW Help

  1. Nov 26, 2007 #1
    A skater holds her arms outstretched as she spins at 120rpm. What is the speed of her hands if they are 130cm apart?
  2. jcsd
  3. Nov 26, 2007 #2


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    Staff: Mentor

  4. Nov 26, 2007 #3

    From what I understand in class

    Vt=Angular velocity * radius

    from my calculations

    120rpm = 2rps
    d=.130m, so C= .4082m

    And I multiplied

    2*.4082 to find the total distance in one revolution

    =.8164m in one sec

    Now I'm stuck
  5. Nov 26, 2007 #4
    I Got It!

    Thanks for your help!
  6. Nov 26, 2007 #5
    Nope didn't get it....I tried doing the equation v=w(pi)f wrong! Gerrrrr
  7. Nov 26, 2007 #6


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    Well one has the answer - =.8164m in one sec => 0.8164 m/s!

    rpm is revolutions per minute and rps is revolutions per second.

    Now in one revolution, a point on the circumference travels 2[itex]\pi[/itex] radians, which is a measure of angle.

    So 2 rps = 2 * 2[itex]\pi[/itex] rad/s = 12.57 rad/s, which is [itex]\omega[/itex], the angular velocity.

    Now apply the formula - Vt=Angular velocity * radius.
  8. Nov 26, 2007 #7
    The answer I get from that is 0.81705
  9. Nov 26, 2007 #8


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    Yes. There is round off accuracy involved in both calculations.

    If one uses 12.566 (4 pi) instead of 12.57 rad/s, then the answer becomes 0.8168 m/s.
  10. Nov 26, 2007 #9
    I understand that, but that is not the answer...I tried it already.
  11. Nov 26, 2007 #10
    For Future reference, the equation for this problem is V=[2(pi)r]/t

    t is found per second
    i.e. 60/120
  12. Nov 1, 2009 #11
    Hmm the answer of 0.82 doesn't seem to be correct according to Mastering Physics. I've followed the steps provided, and I still can't seem to figure out where we're going wrong. Any help would be appreciated.
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