# Moment of Inertia

1. Nov 18, 2007

### mnandlall

Hello,

I was wondering if the moment of inertia of a 2-D triangle about its center of mass depends on the actual mass of the triangle. Formulas that I have found say that I = (bh^3)/36. Does this mean that the moment of inertia of triangles with the same dimensions will be the same about their center of mass regardless of their actual mass? I know that if you were taking the moment of Inertia about an arbitrary axis then the mass would come into play, according to the parallel axis theorem, but I'm not so sure about it when the inertia is about the center of mass.

Any help clearing this matter up for me would be appreciated.

Last edited: Nov 18, 2007
2. Nov 18, 2007

### Staff: Mentor

different "moments of inertia"

The moment of inertia (a.k.a., mass moment of inertia) of any object most certainly depends on its total mass, regardless of chosen axis. That formula looks like an area moment of inertia (a.k.a, second moment of inertia), which is something quite different from "ordinary" moment of inertia and is used to predict resistance to bending. See: http://en.wikipedia.org/wiki/Second_moment_of_area" [Broken]

Last edited by a moderator: May 3, 2017
3. Nov 18, 2007

### mnandlall

Alright, this makes a lot of sense. Thanks for clearing that up.

4. Nov 22, 2007

### sekar507

Please dont get confused with the area M.I & mass M.I. Both are different. area M.I is normally used when u calculate the bending moments and bending stresses in design of beams and structures. where as mass M.I is used in case of problems related to rotational dynamics. In Mass M.I the mass of the rotating object defenitely plays a role. Its value is calculated by multiplying the mass of individual elements and the square of distance between the element and the axis of rotation. Ie I=Mr^2... Hope its clear now....