Can anyone see my error in how I am using the RotationMatrix function?

Questions
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1

I am working with tensors for the first time and am trying to solve some simple problems with the inertia tensor in 3-D. All goes well until I try some simple coordinate system rotations using the RotationMatrix function. I have built a very simple notebook to illustrate my problem. I have a particle located at a point {a,a,0} in my basis coordinates. I then rotate the coordinate system by 30 degrees about the x-axis.

I expect the transformed x’ to be the same as x and it is, I expect y’ to be positive and > a and it is, BUT I expect z’ to be non-zero and negative but it isn’t.

This must be trivial but I cannot find a FAQ on this community and so I am looking for your help.

Thanks in advance, David

2

RotationMatrix rotates the vectors, not the coordinate system. Either change the sign of your angle or transpose your matrix if you want to rotate the coordinates.

3

Thank you John. I