Can You Subtract Moments of Inertia to Find the MOI of Unknown Sections?

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SUMMARY

To determine the Moment of Inertia (MOI) of unknown sections in an assembly, such as an airplane, one cannot simply subtract the MOIs of known sections unless all parts share a common axis of rotation. The discussion emphasizes that the MOIs of different sections can only be summed when they are calculated about the same axis. Additionally, there are established rules for scaling the MOI when the axis of rotation is translated, which must be applied to accurately compute the MOI of the unknown sections.

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  • Understanding of Moment of Inertia (MOI) principles
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  • Knowledge of scaling rules for MOI
  • Basic mechanics of rigid body dynamics
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Jweck002
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It's been ages since I have done Moments of Inertia of complex objects! :( Can anyone help?? Say you have an assembly like an airplane and you want to just get the moment of inertia(MOI) of the tail section and you are given the Mass of whole plane,mass of tail section, moment of inertia of front section, and moment of whole plane(moments about each axis for both). Can you just subtract the MOIs of known sections to get the MOI of unknown sections? If not, how would you go about getting the unknown sections MOI from that info.
 
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Sorry about the delay.
Only when all parts share a common axis of rotation do the MsOI of the parts sum.
There are rules used to scale the MOI when the axis of rotation is translated.
https://en.wikipedia.org/wiki/Moment_of_inertia
 
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