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Ice skaters speed of rotation

  1. Jun 6, 2009 #1
    1. Two ice skaters, each with mass M = 72.5 kg, both moving with speed V = 9 m/s, approach each other along straight-line parallel paths that are separated by a distance of D = 3.6 m. When directly opposite each other, the skaters grab the ends of a light rod that is the same length as the distance between them. What is the initial speed of rotation of the joined skaters in rad/s?


    2. Equations
    maybe use conservation of momentum to find vfinal
    m1vo+m2Vo=m1vf+m2vf
    then use ac=v^2/r
    r comes from d/2

    3. this results in the incorrect answer. I'm unsure of how to solve this problem. Am I using the wrong equations?
     
  2. jcsd
  3. Jun 6, 2009 #2

    Doc Al

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    Staff: Mentor

    While conservation of linear momentum holds, that's not what you need for this problem. Hint: Translate their linear speed into angular speed about the center. (If you like, you can think of something else that is conserved.)
     
  4. Jun 6, 2009 #3
    so the v=9m/s which when converted into radians is 9*2pi= 56.55rad/sec,
    then w=v/r,
    so w= 56.55/3.6= 15.7
    centripetal acceleration = r*w--> 3.6*15.7= 56.52m/s^2
    then I'm not sure- are they an inelastic collision that is spinning? but I have acceleration so that doesn't help.
     
  5. Jun 6, 2009 #4

    Doc Al

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    This isn't correct. Don't try to "convert" a linear speed into rad/s.
    That's what you need. Hint: The radius ≠ 3.6 m. (Where's the center?)
     
  6. Jun 6, 2009 #5
    w=9/1.8=5
    then centripetal acceleration a=rw^2 = 5^2*1.8= 45m/s^2
    then could I use ac=v^2/r to get the velocity? No that can't be correct. I'm highly confused.
     
  7. Jun 6, 2009 #6

    Doc Al

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    Good.
    ω = v/r = (9 m/s)/(1.8 m) = 5 rad/s
    And you're done! No need to mess around with centripetal acceleration or any of that.
     
  8. Jun 6, 2009 #7
    w- is the average velocity. Thanks for your help! Good thing someone enjoys physics.
     
  9. Jun 6, 2009 #8

    Doc Al

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    Staff: Mentor

    ω is the rotational speed, not the average velocity.
    You're welcome.
     
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