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Homework Help: Angular velocity on skater

  1. Oct 26, 2006 #1
    A skater is initially spinning at a rate of 15 rad/s with a rotational inertia of 2.78 kg·m2 when her arms are extended. What is her angular velocity after she pulls her arms in and reduces her rotational inertia to 1.65 kg·m2?

    I have so much trouble with these problems because I haven't a clue where to begin. I know that rotational inertia = http://library.thinkquest.org/16600/advanced/5-7.gif.[/URL] Angular velocity is [PLAIN]http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/avel3.gif.[/URL] [Broken] Knowing this, how can I derive the the new angular velocity?
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Oct 26, 2006 #2

    radou

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    Think about the physical meaning of that situation. What is conserved?

    P.S. This is probably the most typical example for the conservation of _______ ________. :smile:
     
  4. Oct 26, 2006 #3
    cool, I got it. 2.78 * 15 = 1.65w

    w=25.27 rad/s!

    I have one more conservation of momentum....

    The rotational inertia for a diver in a pike position is about 15.5 kg m2; it is only 8.0 kg m2 in a tucked position (the figure above).

    (a) If the diver gives himself an initial angular momentum of 106 kg m2/s as he jumps off the board, how many turns can he make when jumping off a 10.0-m platform in a tuck position? [Hint: Gravity exerts no torque on the person as he falls; assume he is rotating throughout the 10.0 m dive.]



    So, I know that

    P=106kg m^2
    d=10m
    I=8kg m^2


    I need to find out what w is so that I can convert to revolutions per second and then figure out how many times the guy rotates before he comes in contact with the water, only I don't know how to do this without his mass or radius???
     
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