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

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.

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

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

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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:
$$I = \sum \frac{bh^3}{12} + Ad^2$$

nvn