1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    *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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook