Talk:Second moment of area

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The calculation of "y" the distance to the centroid is incorrect. It should be:

y=(h+h1)/4

John.g.taylor 04:30, 28 May 2007 (UTC)

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I think the second moment of area is different to the second moment of inertia.

John.g.taylor 04:29, 28 May 2007 (UTC)

Specifically:

Second moment of inertia = rather than dA. ..

The second moment of area is a more general concept. For example the static roll stability of a ship depends on the second moment of area of the waterline section - short fat ships are stable, long thin ones are not. Gordon Vigurs 08:10, 11 July 2006 (UTC)

[edit] reverting

I had several problems with the changes made by 69.251.146.46:

• The lead sentence shouldn't be about what it is confused with.
• Polar moment of area -- much less frequently used than polar moment of inertia... dead linkage

Nephron  T|C 02:49, 25 August 2006 (UTC)

'I' in the stress bending eqn should be for 2nd moment of area. I referred this from 'Mechanics of solids and structures', Benham, P.P. ISBN: 0273361910

[edit] Need diagrams

This article needs some diagrams to go with the formulas, to make the variables clear.

Some body needs to translate and lift the diagrams from the De page on this

Comment moved from article to talk (RJFJR 15:14, 2 May 2007 (UTC)) "I-beam" cross section

A suitably dimensioned and annotated I-beam diagram would greatly enhance the usefulness and applicability of this forula.

I added diagrams for the I-beam section. My lettering, which didn't look that bad in the original stands out now. I can redraw if there is interest. I could also add a variable for the the thickness of the top and bottom pieces. Before I do it, are there any other changes? (you can drop a comment on the diagrams on my talk page.) RJFJR 19:45, 13 May 2007 (UTC)

[edit] Product Moment of Area

This material on the "product moment of area" was int eh article but seems to lack context and explanation:

I_xy is also known as Product Moment of Area

Insertformulahere

What is this used for? Is it really related to the second moment because an object with both x-axis and y-axis symmetry would have a value of zero, whcih is certainly not true of the second moment (it more resembles a first moment). Can we clearify this before we put it back in teh article, if this is where it belongs. And why does this have a redlink? RJFJR 13:48, 6 May 2007 (UTC)

It is used in the bending formula, if for some reason you have a poor choice of coordinate system. The bending formula in the general form is relatively complicated:

The trick is to find a coordinate system, which has a product moment of area equal to zero. This is always possible, and as you have noticed, it is trivial with a symmetric cross section. Then the bending formula becomes a lot simpler:

The reason it has a redlink, is probably because all common cross sections have symmetry, and then there is no point in calculating it, so in real life it is only rarely used. Hemmingsen 19:22, 13 May 2007 (UTC)