# Inertia in 3d

1. Sep 17, 2015

### Isawyou0

2. Sep 17, 2015

### SteamKing

Staff Emeritus
The formula you attached appears to calculate the second moment of area of a general polygon, using the (x,y) coordinates of the vertices.

You'll have to be more specific about what you mean by "calculat(ing) inertia in 3d".

3. Sep 17, 2015

### Isawyou0

the moment of inertia in 3d is it the same for 2d?

4. Sep 17, 2015

### SteamKing

Staff Emeritus
It depends.

The second moment of area is sometimes referred to as the moment of inertia. This is a property of plane figures.

The mass moment of inertia for 3-D bodies determines how easy or how difficult it is to accelerate a body in rotation.

As I asked before, which of these two properties do you wish to calculate in 3-D?

5. Sep 18, 2015

### Isawyou0

so there is second moment of inertia and moment of inertia I didn't knew that :/
of course I'm talking about second moment of inertia in 3d, for different shapes...

Last edited: Sep 18, 2015
6. Sep 18, 2015

### SteamKing

Staff Emeritus
No, you have confused things.

1. There is a second moment of area for plane shapes, which is also referred to as a moment of inertia. The second moment of area has units of L4.

https://en.wikipedia.org/wiki/List_of_area_moments_of_inertia

2. There is a mass moment of inertia for 3-D bodies, which has units of ML2:

https://en.wikipedia.org/wiki/List_of_moments_of_inertia

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