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

AI Thread Summary
To determine the moment of inertia (MOI) of an unknown section, such as the tail of an airplane, you cannot simply subtract the MOIs of known sections unless all parts share a common axis of rotation. The MOI is additive only when the axes are aligned, and different axes require adjustments. The parallel axis theorem can be used to scale the MOI when translating the axis of rotation. Understanding these principles is crucial for accurately calculating the MOI of complex assemblies. Proper application of these rules will lead to the correct determination of the unknown section's MOI.
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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