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: Change in the precession angle

  1. Oct 29, 2006 #1
    Hi. I have: A 3 kg bicycle wheel rotating at a 2484 rev/min angular velocity has its shaft supported on one side, as show in the figure. When viewing from the left (from the positive x-axes), one sees that the wheel is rotating in a clockwise manner. The distance from the center of the wheel to the pivot point is 0.6 m. The wheel is a hoop of radius 0.4 m and its shaft is horizontal.

    Assume all the mass of the system is located at the rim of the bicycle wheel. The acceleration of gravity is 9.8 m/s^2.

    Find the change in the precession angle after a 1.5 s time interval.

    In my book I have the equation:

    [tex]d\phi = \frac{(M g h) dt}{L}[/tex]

    so [tex] L = I \omega = M R^2 \omega[/tex]

    therefore [tex]d\phi = \frac{(M g h) dt}{M R^2 \omega} = \frac{(g h) dt}{R^2 \omega}[/tex]

    Now [tex]\omega = \frac{2484 "rev"}{"min"} \frac{1 "min"}{60 "sec"} \frac{2 \pi "rad"}{"rev"} = \frac{260.124 "rad"}{"sec"}[/tex]

    and let [tex]h = 0.6, R = 0.4, g = 9.8, dt = 1.5[/tex]

    Using these I came up with .211918 but the answer was wrong. Any ideas where I went wrong? Thank you.

    [Edit]I tried -.211918 (the bicycle is rotating in a clockwise fashion) but that was wrong as well.
    Last edited: Oct 29, 2006
  2. jcsd
  3. Oct 29, 2006 #2
    I was supposed to answer in degrees but I was putting the answer in radians. This has been a humbling learning experience, but alas, those are the ones that seem to make such a difference. Thanks everyone! :-)
  4. Oct 29, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If by everyone, you mean yourself (since nobody actually did anything), then sure.

    Nice to know you got the answer though
  5. Oct 29, 2006 #4
    Thanks! I figured no one responded because they saw I was on the right path and assumed I would figure it out . Hence, by not responding they aided my development. It's happened before! :-)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook