# Polar moment of inertia

1. Nov 28, 2009

### R Power

Hi guys
Can anyone explain what is physical significance polar moment of inertia. Well i know it's formula e.g in case of shafts but not it's physical meaning.

2. Nov 28, 2009

### FredGarvin

It is the angular analogy to moment of inertia in that it describes a section's resistance to torsion. In the linear world, the MOI is the resistance to bending.

3. Nov 30, 2009

### minger

To further clarify on what Fred mentioned, especially because you mentioned shafts. Polar moment of inertia is NOT the same thing as mass moment of inertia.

Often (and I mean daily) I hear people here asking for polar moment of inertia, when they actually want a mass moment of inertia about an objects axis.

Make sure to always check your units. Area moment (which resists bending) and polar moment (which resists torsion) are in units of length^4.

4. Nov 30, 2009

### Mech_Engineer

5. Nov 30, 2009

### minger

There is where the problem lies. #1 and 2 do not agree with #3. Using #3 is very confusing and has been phased out in newer nomenclature. The proper term for an objects ability to resist angular acceleration is Mass moment of inertia, not polar moment of inertia.

Last edited by a moderator: May 4, 2017
6. Nov 30, 2009

### Mech_Engineer

It's just a computer program, it's can't differentiate between older and newer uses of the terminology. Sure it might give you multiple answers, but a 2-second search would have gotten him in the ballpark.

As with some engineering terminology, it may be contextually-based and require some knowledge of how the term was used in the sentence. For example, "Moment of Inertia" is used interchangeably in many places in structural analysis and dynamics. While it may be technically inaccurate in some usages, if you know something about the question being posed you can decide which one you're after.

7. Nov 30, 2009

### minger

"In the ballpark" has burned me more than once. I don't like assuming people know what they're talking about it. It has happened more than once where people have asked me for the polar moment of inertia of a shaft (for example). When I ask about what plane, they get confused.

While it may be OK to erroneously use certain terms in certain displicines, I think one should first learn the proper way describe things. So, while a 5-second Google search could have gotten him "in the ballpark", he also could have found the correct link, a link to Second Moment of Area, which is also "moment of inertia", and a link to Moment of inertia, which is close to polar moment of inertia, and a link to a Car and Driver page which incorrectly gives the term.

In five years, I might work with this fellow, and I'd be pretty peeved if he incorrectly gave me J instead of I, etc etc.

8. Nov 30, 2009

### Mech_Engineer

It's obvious that you feel strongly about this subject, and I'm not trying to argue with you about the proper use of the term. As I said before, context is more important than the name, because LOTS of people use different/old terminology to describe values such as this (it also occurs accross language barriers).

I run into this problem all the time in my daily work; rather than argue about the proper use of the term, I define and contextualize what I'm specifically talking about. Some battles just aren't worth fighting if you're going to burn bridges in the process.