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Asked by Melissa on 27 Aug 2013

Good Morning,

I am trying to calculate the principal moment of inertia as well as the principal axis of inertia.

If I am given a tensor matrix of the rotational moment of inertia:

I=[Jx -Jxy -Jxz; -Jxy Jy -Jyz; -Jxz -Jyz Jz];

And wish to find the principal moments and principal axis of inertia then I would simply use matlab eig.

[Axes,Moments]=eig(I);

And then diagonal of the Moments matrix is the principal moments. Now say I want to find the cosine of the angle of the principal axis of inertia with the three axes X,Y,Z that are center at the CG...how would I go about this?

Is it cos(Axes(:,1)), cos(Axes(:,2)), cos(Axes(:,3)??

Thank you.

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Answer by Roger Stafford on 27 Aug 2013

Accepted answer

**"Is it cos(Axes(:,1)), cos(Axes(:,2)), cos(Axes(:,3)??"**

No, the components of the eigenvectors themselves, Axes(:,1)), Axes(:,2), Axes(:,3), are already the cosines of the angles between the three principal axes of inertia respectively and the x, y, and z axes, provided the eigenvectors are normalized.

Of course there is an ambiguity about the sense of each of these vectors depending on which of two opposite directions are to be considered as the positive directions of these axes of inertia.

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