# Question about moment of inertia of area. lecture note from malaysia uni given.

1. Sep 9, 2009

### aiklone1314

please download the lecture note to understand my question.

1. What is the difference between (I) and (I bar) ? is I = I bar?they are the same?
2.in the slide i uploaded, chapter5.8 pg 31, the equation for rectangle, (I bar x prime) = (1/12bh cube) and (Ix) = (1/3bh cube) ,as you can see,their equation are different.
I know (I bar x prime) = (1/12bh cube) is used in the equation (Ix)= (1/12bh cube+ Ady square) , but when to use this equation (Ix) = (1/3bh cube) ?

p/s:I am an engineering student from malaysia university.

File size:
1.1 MB
Views:
236
2. Sep 9, 2009

### minger

How about you go back to section 5.5, page 9 and reread the information regarding the Parallel-Axis theorem.

3. Sep 9, 2009

### aiklone1314

i know ,but the assignment question given by the lecturer doensnt means this.
if i use (I bar x)=(1/12bhcube) equation to solve the question in assignment 2(b) (Ibar x)and(Ibary), the answer is wrong.

File size:
411.1 KB
Views:
117
4. Sep 9, 2009

### minger

I with no bar isn't used on that assignment. In fact, the question tells you which axis to take the moment about. You need to include the parallel axis theorem, the answer will be:
$$I = \sum \frac{bh^3}{12} + Ad^2$$

5. Sep 9, 2009

### nvn

aiklone1314: It appears your teacher made a mistake in his nomenclature in question 2(b). He accidentally typed Ibarx and Ibary, but he meant to type Ix and Ibary. Therefore, for question 2(b), use the following parallel axis theorem formula, mentioned by minger, to compute Ix.

Ix = summation[(1/12)b*h^3 + A*d^2]

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook