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

In summary, the conversation discusses the topic of Moments of Inertia for complex objects, specifically in the context of an airplane assembly. The question is posed about whether the MOI of a specific section (the tail) can be calculated by subtracting the MOIs of known sections. It is mentioned that this is only possible when all parts share a common axis of rotation, and there are rules for scaling the MOI when the axis is translated. A helpful resource for understanding Moments of Inertia is also provided.
  • #1
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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  • #2
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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FAQ: Can You Subtract Moments of Inertia to Find the MOI of Unknown Sections?

1. What is the purpose of subtracting an assembly's MOI?

The purpose of subtracting an assembly's MOI (Moment of Inertia) is to accurately calculate the overall MOI of the system by removing the contribution of specific parts or components.

2. How is an assembly's MOI calculated?

An assembly's MOI is calculated by summing the individual MOIs of each component within the assembly. This can be done using mathematical formulas or computer-aided design (CAD) software.

3. What factors affect an assembly's MOI?

The main factors that affect an assembly's MOI are the mass and distribution of the components within the assembly. Components with greater mass and/or located further from the axis of rotation will contribute more to the overall MOI.

4. How does subtracting an assembly's MOI impact its performance?

Subtracting an assembly's MOI can have a significant impact on its performance, particularly in terms of its rotational motion. A lower MOI means the assembly will be easier to rotate and change direction, resulting in faster acceleration and smoother movements.

5. What are some common methods for subtracting an assembly's MOI?

There are a few common methods for subtracting an assembly's MOI, including using the parallel axis theorem, integrating the MOI of individual components, and using software programs specifically designed for calculating MOI. It is important to carefully consider the method used to ensure accurate results.

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