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 of a hoop

  1. Sep 11, 2008 #1
    Hi all
    i am really confused about this, why does a thin cylindrical shell has the same moment of inertia of a hoop?
    i understand the I for a thin hoop is mr(square), and i know how to do this. but i just get confused why a cylindrical shell has the same result? and i dont know how to show the work.
    i asked my physics teacher but he didnt explain very clearly.

    Moderation note: Duplicate threads merged since both threads had been answered.
    Last edited by a moderator: Sep 11, 2008
  2. jcsd
  3. Sep 11, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Confusion in moment of inertia

    Consider a thin hoop and a cylindrical shell with an axis of rotation through the centre of both circular faces (i.e. looking down the tube). In both cases, where is all the mass located? Can you see any difference between the hoop and the cylinder if viewed end-on?
  4. Sep 11, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    In both cases, the object's entire mass is concentrated at the same distance (r) from the axis of rotation. So I=mr2

    What formula are you given to calculate I?
  5. Sep 11, 2008 #4


    User Avatar
    Homework Helper

    Because the cross section of a cylinder is the same as a hoop. You could combine a bunch of hoops together to create a cylinder. The distribution of mass versus radius stays the same, only the amount of mass varies (the "m" in m r^2).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?