Answer check please - Moment of Inertia Calculations

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Homework Help Overview

The discussion revolves around calculations related to the moment of inertia, specifically addressing two questions from a physics lab. The original poster seeks confirmation of their calculations for the moment of inertia of a system involving a pulley and various components, including washers and a CD.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster presents their calculations and requests verification. Some participants confirm the first calculation while questioning the interpretation of the second problem, particularly regarding the diagram and the use of the parallel axis theorem. There is discussion about the implications of the components having holes in the middle and the appropriateness of using hollow cylinder formulas for the moment of inertia.

Discussion Status

The discussion includes confirmations of certain calculations, along with clarifications about the assumptions made regarding the components involved. Participants are actively engaging with the original poster's reasoning and exploring different interpretations of the problem setup.

Contextual Notes

There appears to be some ambiguity regarding the diagram for the second question and the specific requirements for calculating the moment of inertia, particularly concerning the parallel axis theorem and the characteristics of the components involved.

ktvphysics
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Answer check please -- Moment of Inertia Calculations

For question 1 I got T=mnut * r pulley * gravity

For question 2 I got Isystem = 1/2 M(R23 + R24 + m (R21 + R22)

First day of physics lab and I just wanted to double check that these are correct.

Thanks.
 

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hi ktvphysics! welcome to pf! :smile:

(1 is fine)

i think you're misunderstanding the diagram for 2 …

the CD and the washers both have holes in the middle :wink:

also (although the question doesn't say), it must be asking for the moment of inertia about the centre, so you'll have to use the parallel axis theorem for the two washers!
 
tiny-tim said:
hi ktvphysics! welcome to pf! :smile:

(1 is fine)

i think you're misunderstanding the diagram for 2 …

the CD and the washers both have holes in the middle :wink:

also (although the question doesn't say), it must be asking for the moment of inertia about the centre, so you'll have to use the parallel axis theorem for the two washers!

So the answer to 2 would be:

Isystem = 1/2 M(R23 + R24) + 2[1/2m(R21 + R22) + md2]

That corrects the problem about the Parallel Axis Theorem, but I'm confused about what you were saying about the holes in the middle. I used the moments of inertia for hollow cylinders for the CD and Washers, which is essentially what each of them is.
 
Last edited:
hi ktvphysics! :smile:
ktvphysics said:
That corrects the problem about the Parallel Axis Theorem, but I'm confused about what you were saying about the holes in the middle. I used the moments of inertia for hollow cylinders for the CD and Washers, which is essentially what each of them is.

sorry, i was in a hurry to go out and i misremembered the formula! :redface:

yes, your answer is fine now :smile:​
 

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