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: Conservation of Angular Momentum Experiment: Moment of Inertia

  1. Jul 29, 2012 #1
    1. The problem statement, all variables and given/known data
    I did a lab where there was a rotating solid disk with mass= 0.915kg and diameter=0.253m.
    This was rotating horizontally with an initial angular velocity of 3 different values ω radians/second. After recording the initial angular velocity, I dropped a thin-walled hollow cylinder with mass=0.708kg and diameter=0.125m in the center and measured the final angular velocity, testing the conservation of angular momentum.

    Issue: If I placed the ring off center of the disk by say, 1cm (0.01m), how will that affect my moment of inertia?

    2. Relevant equations
    Ihoop/hollow cylinder=(Mass)(radius)2
    Li=Idiskωdisk initial
    Lf=(Idisk+Ihoop/hollow cylindercombined final

    3. The attempt at a solution
    First, I calculated the moments of inertia-

    Ihoop/hollow cylinder=(Mass)(radius)2=(0.708kg)(0.125m/2)2=0.00277kgm2


    The Icombined is for the ideal situation of the ring being completely centered, but I have no idea what I would do to get the experimentally flawed moment of inertia. Would I just change the radius of the hoop/cylinder by 1cm? If so, would I add or subtract? I'm really not sure how I'd calculate it. I understand this all generally pretty well, but executing this has me a little stumped. I need a way to get the new final moment of inertia instead of the ideal (Idisk+Ihoop/hollow cylinder) to calculate a percent error.
  2. jcsd
  3. Jul 29, 2012 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Hi, chrismoon. Have you studied the "parallel axis theorem"? You can use it to calculate the moment of inertia of the hollow cylinder when it is placed off-center on the disk.
  4. Jul 29, 2012 #3
    have you used the parallel axis theorem?

    ok then...

    late again, sorry. Its getting strange how I am right behind Tnsy.

    I will now bow out again.
    Last edited: Jul 29, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook