Hi all,(adsbygoogle = window.adsbygoogle || []).push({});

I'm at uni starting off engineering, and I'm looking for a walk through in this algebra involved in the second moment of area. While I know it's probably pretty basic I'm undertaking a bridging course to try and keep up with my maths.

http://home.exetel.com.au/peleus/smoa.jpg

Here's a picture of the most relevant lecture slide discussing the problem.

I'll type out the steps they undertook to get the final formula for the second moment of area for a rectangle.

On the next page we take the integral of this, which I can do fine.

This gives

1. [tex]I = \frac{b}{3}[y^3][/tex] with limits +h/2 and -h/2

Taking it further we end up with

2. [tex]I = \frac{b}{3}[\frac{h^3}{8}-(-\frac{h^3}{8})][/tex]

and finally we take it to the step

[tex]I = \frac{bh^3}{12}[/tex]

Ok, I can understand a bit about this but here are my questions.

- Why are the limits h/2 and -h/2, isn't this simply the middle of the rectangle?

- Why is [tex]dI = y^2 dA[/tex], where does the [tex]y^2[/tex] come from?

Any help is greatly appreciated.

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# Second moment of area - Need walkthrough

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