Beyond Statics: Area vs. Mass moment of inertia

[Dan Baker]

Hive mind,

I've been wanting to better differentiate between area moment of inertia (for Statics and Solids) and mass moment of inertia (for Dynamics). The textbooks I've used suggest the variable "I" for both. Do any of you use "J" for mass moment of inertia? I've also seen "J" used for the polar area moment of inertia which I believe is used in solids as well. Any other suggestions?

Thanks for your input!

Dan

____________________________________________________

Daniel W Baker Ph.D. PE, Teaching Assistant Professor
  Office:  ENGR B206                      Phone: 970-491-0261
  Email:  dan....@colostate.edu   Twitter: @drdanteaches

  YouTube: DrDanTeaches

[James Lord]

Hi Dan,

All of my books use I for both as well. My references are mixed on whether polar area moment of inertia should be J or Iz, or either. J is fairly commonly used though.

I suppose you could denote area moment of inertia with Ix, Iy, Iz, and mass moment of inertia with Jx, Jy, Jz which would be fine for just your notes but would potentially confuse students who reference other texts.

Perhaps a subscript for mass moment of inertia? IG maybe? I wish I could be more useful. I don't teach anything requiring mass moment of inertia so I've never needed to differentiate between them.

Best,

James

---

Dr. James Lord

ESM Undergraduate Program Chair

Biomedical Engineering and Mechanics

225-C Norris Hall

jkl...@vt.edu

(540) 231-6844

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[Dan Baker]

Thanks for the input James. I like the idea of using Iz for the polar area moment of inertia and J's for just the mass moments of inertia. In all cases, the subscripts would continue to reference the axis each term is about. Maybe I'll start the revolution!

Dan

____________________________________________________

Daniel W Baker Ph.D. PE, Teaching Assistant Professor
  Office:  ENGR B206                      Phone: 970-491-0261
  Email:  dan....@colostate.edu   Twitter: @drdanteaches

  YouTube: DrDanTeaches

To view this discussion on the web visit https://groups.google.com/d/msgid/engineering-statics-oer-text/CAJ%2BXkWJcHrS8nvgzaU5rg8qg7fH6iXZ-xcpri%3D4MMsOSh64eew%40mail.gmail.com.

[James Lord]

Hi Dan,

I think it makes sense. Reminds me of this though.

---

Dr. James Lord

ESM Undergraduate Program Chair

Biomedical Engineering and Mechanics

225-C Norris Hall

jkl...@vt.edu

(540) 231-6844

To view this discussion on the web visit https://groups.google.com/d/msgid/engineering-statics-oer-text/CAL1OSWx9SukNuewbLMHE%2BnVVWRP18yxeope3XN9sHyBG1d2grg%40mail.gmail.com.

[Dan Baker]

So true.

After more interaction with colleagues who teach Solids and Structures it seems that 'J' is the standard for polar moment of inertia used for torsion - which unfortunately isn't always around the z axis. Thus I guess I am stuck with 'I' for both area and mass MOI.

To view this discussion on the web visit https://groups.google.com/d/msgid/engineering-statics-oer-text/CAJ%2BXkWKdYUxUXwt-Fpdp8dFcki4YRtSmEVGOmuiWkhNeC_KXAg%40mail.gmail.com.