Find the Moment Inertia of a Cross Section: y = 30 & 35

[ricky_fusion]

Hi..
I've got a problem about moment inertia. I don't understand about looking the distance of centroid (y). I am still confucing about the formula of "y" in Moment Inertia. Can you shows me about the formula of y or if any thread for this before, I am glad to see it too.Thanx's for your help..
I still don't know how to find put y =30 and 35

Homework Statement

Known : You can see the dimension and the picture in my attachment
Asking: How is to calculate y ?? Is the formula same to all geometry??

Homework Equations

I = y dA

The Attempt at a Solution

Cross section
Total Area: - A =>40*60=2400
- y => 30 Why is it 30?
- Ay =>72000
Inside Area:-A=> -20*30=-600
-y =>35 Why is it 35?
- Ay => -21000

 

[Delphi51]

Ricky, we can't see the picture until one of the tutors comes on line to release it.
If you are in a hurry, you could post the picture to photobucket.com and give us a link.

 

[ricky_fusion]

Hmm..

Maybe this enough..http://s588.photobucket.com/albums/ss326/ricky_fusion/?action=view&current=Moment.jpg

 

[LowlyPion]

Apparently you want the moment about the center of mass.

In which case you need to determine the center of mass (centroid point) and then calculate the I on the basis of the double integral over the distribution of mass about that point.

You can also exploit the symmetry that the objects present, if you consider that they can be broken up into 4 rectangles, and use the parallel axis theorem to determine the sum of the system. (Since you have some asymmetry you may need to do it in steps.)

 

[ricky_fusion]

Hmm..
I knows about it, but I just don't knows how to calculate the distance from center of mass to the model. I meant from this formula "I = y dA." How is to calculate "y"?? Becauses I don't have some information about it. How to find out the distance in I,T,U or not simetric model?? I knows that to find out "I", I should divide the model into 2-3 section, but after that how to calculate the "y" before I apply it into the moment Inertia formula (I = y dA)??