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: Toy gyroscope problem

  1. Sep 10, 2007 #1
    1. The problem statement, all variables and given/known data

    The rotor (flywheel) of a toy gyroscope has mass 0.140 kilograms. Its moment of inertia about its axis is 1.20x10^{-4} kilogram meters squared. The mass of the frame is 0.0250 kilograms. The gyroscope is supported on a single pivot with its center of mass a horizontal distance 4.00 centimeters from the pivot. The gyroscope is precessing in a horizontal plane at the rate of one revolution in 2.20 seconds.


    Find the upward force exerted by the pivot.

    Find the angular speed omega at which the rotor is spinning about its axis, expressed in revolutions per minute.

    2. Relevant equations

    angular momentum and acceleration equations eg:
    w = O / t
    w= w0 + at
    O = w0t + ½ at^2
    w^2 = w0^2 + 2aO

    3. The attempt at a solution

    having a lot of trouble determining what information is relevant in this question...very little idea how to go about it sorry.

    i think it has to do with L = Iw, with I being 1.2x10^4, but i'm unsure of how to get the angular velocity, and after that the force they are asking for.
  2. jcsd
  3. Sep 10, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

  4. Sep 14, 2007 #3
    now guess why im here....
  5. Oct 22, 2007 #4
    You will have to change (Big Omega) O from rev/2.2s into rad/s : O = 2pi/2.2s = 2.856 rad/s.
    The upward force is equal to the weight because the gyroscope is not moving sideways, up or down so Fx = 0, and Fy = 0. Hence n = w = (m_rotor + m_frame)*9.8.

    We know that O = T/L (where Torque: T = radius * w and L = I * w (small omega))

    I is given with 1.2E^-4 kg*m^2.

    There's an equation that tells you that L = I*w here so you just have to find the L from the equations above and then isolate w(small omega) from L = I * w(small omega).

    then you'll get some number but you will have to answer in rev/min and remember that O is given in rev/2.2s but you changed it to 2.856rad/s

    I have to admit that I'm in trouble finding the answer in rev/min if somebody could point out the right way to do so.
  6. Oct 23, 2007 #5
    well evanQ, guess who had two attempts left when doing that. then i got the right answer!! but it wasnt it revolutions per minute. so i had to convert it....and i MULTIPLIED BY 60 INSTEAD OF DIVIDED!!! ahhh!!! soooooo annoyed.

    anywho, thats all in the past....now there another one to worry about. where we get marks taken off for getting multichoice wrong, nooooooo
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook