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Rate of rotation and inertia (prevent homicide and help me, please)

  1. Apr 22, 2005 #1
    i understand 4 of the 6 problems on this week's online physics quiz, but this one is driving me completely NUTS!

    a diver rotates at 1 revolution per second in the stretched out position. when the diver tucks her head in and bends her legs, assume her length is shortened by 1/2. what is her rate of rotation in this position?
    my choices are 4 Hz or 5 Hz, you'd think having only 2 choices would make this easier, but alas, i am dumbfounded.

    i have tried thinking of the diver in the stretched out position as a rod, with rotational inertia 1/12ML^2
    the other position would be like a solid sphere, with rotational inertia 2/5MR^2

    1 revolution per second is 1 Hz, or 2pi r/s

    the angular momentum (which may be irrelevant) of the stretched out position is
    L= 1/12M(2r)^2 * 6.28 radians/1 revolution * 1 revolution/1second
    this leaves me with L=2.093Mr^2 rad/sec

    L of ball position is
    2/5Mr^2 * 6.28 radians/?sec= 2.51Mr^2 radians/?sec
    ?=0.39Mr^2 rad/sec

    slowly going insane, i see that rotational accleration = net torque/rotational inertia
    this also leads me nowhere

    any help, hints, or a slap across the face would be greatly appreciated

    (p.s.- anyone else think online physics is the STUPIDEST idea EVER?)
  2. jcsd
  3. Apr 22, 2005 #2


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    *Slaps NoHeart in the face*, "wake up man!"

    Lol...just kidding. :tongue2:

    Keep in mind these quizzes never have super complex questions, they are always just testing the basic concepts. In this case, angular momentum, L, can be expressed as:

    [tex] L = I\omega [/tex]

    Where I is the moment of inertia. Keep it simple, model the diver as having the same overall shape, whether she's fully extended or not. After all, they only give you length, [itex] l [/itex], to work with. So if:

    [tex] I \propto l^2 [/tex]

    then when [itex] l [/itex] is halved, by what factor does I decrease? _________

    But if I decreases by that factor, and by conservation of angular momentum, the product [itex] L = I\omega [/itex] must remain constant, then by what factor does [itex] \omega [/itex] increase to compensate?

    Yes, I also feel that online physics quizzes are a stupid idea.
  4. Apr 22, 2005 #3
    1/4! i love you!
    and i wish it were only the quizzes that are online, but it's the whole class- the online "lectures" are simply outlines of the chapters in the book, and the book is completely useless when it comes to the quizzes and tests. i have been teaching myself all of the concepts involved in the class so far, with the help of many physics sites and this here amazing message board. if i had found this board at the beginning of the quarter, i'd probably have an A instead of a B.
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