# Tricks for memorizing moments of area/inertias?

**URL:** <https://boards.straightdope.com/t/tricks-for-memorizing-moments-of-area-inertias/437878>\
**Category:** Factual Questions\
**Created:** [February 16, 2008, 1:33pm UTC](https://boards.straightdope.com/t/tricks-for-memorizing-moments-of-area-inertias/437878 "2008-02-16T13:33:26Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![BrandonR](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@BrandonR](https://boards.straightdope.com/u/BrandonR)\
**Post date:** [February 16, 2008, 1:33pm UTC](https://boards.straightdope.com/t/tricks-for-memorizing-moments-of-area-inertias/437878/1 "2008-02-16T13:33:26Z")

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I’m an engineering student (mechanical) and if one thing has continuously eluded my memory, it’s the ability to memorize the differences between all the moment of inertia/area formulas (and when to use what). You have moment of inertia, polar moment of inertia, second moment of area, and first moment of area (did I miss any?). The terrible naming convention for all the formulas doesn’t exactly help either.

Has anyone here come up with any tricks to help memorize the differences? My latest frustration stems from a fluid mechanics test where a problem asked to calculate the resultant force on a submerged gate and the formula involved a I(XX). I just had to guess it was the standby bh^3/12 (hopefully getting it right) since the book conveniently forgoes explaining anything about it to the equation-overloaded engineering student. Anyway, any help would be appreciated. 🙂

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**Author:** ![BrandonR](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@BrandonR](https://boards.straightdope.com/u/BrandonR)\
**Post date:** [February 18, 2008, 3:21am UTC](https://boards.straightdope.com/t/tricks-for-memorizing-moments-of-area-inertias/437878/2 "2008-02-18T03:21:41Z")

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Well, it would appear I’m not the only confused one… 🙂

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**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\
**Post date:** [February 18, 2008, 4:20am UTC](https://boards.straightdope.com/t/tricks-for-memorizing-moments-of-area-inertias/437878/3 "2008-02-18T04:20:16Z")

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It’s been years since I was in school, but there are two things you could try. First, look at the units in each formula and try to determine the one that is going to give you units that make sense for the solution. Second, know how to derive the formula mathematically. When you can do that, you can figure out which one is the one you want to use.

In both cases though, it no trick. It’s simply knowing your subject.