1. Not finding help here? Sign up for a free 30min 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!

Moment of inertia for physical pendulum

  1. Dec 14, 2008 #1
    I have a rigid, massless strut of length 11m. A mass of 55kg is located on the free end, and another mass of 55kg is located at the midpoint. I'm having some trouble calculating the moment of inertia I for this physical pendulum.

    I = [tex]\int r^{2} dm[/tex] (for continuous objects)

    I = [tex]\sum m D[/tex] for all particles/objects composing the system.


    I'm a little confused. Should I calculate I for each 55kg mass and add them together?
     
  2. jcsd
  3. Dec 14, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Exactly.
     
  4. Dec 14, 2008 #3
    so:
    55*5.5 = 302.5
    55*11 = 605

    so I for the whole pendulum is 907.5 kg-m^2, correct?
     
  5. Dec 14, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    No, your formula for I of a point mass is incorrect. (I didn't notice that earlier.)

    For both continuous objects and point masses, the distance must be squared. (Imagine integrating the first formula to get the second. All of the mass is at the same distance from the axis, so the integral is trivial.)
     
  6. Dec 14, 2008 #5
    Ah, so then:

    55*5.5^2 = 1663.75
    55*11^2 = 6655

    1663.75 + 6655 = 8318.75 kg-m^2
     
  7. Dec 14, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    You got it.
     
  8. Dec 14, 2008 #7
    awesome! thanks!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?