What is the Rotation Tensor Matrix for Rotation About e1+e2 Axis?

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Discussion Overview

The discussion revolves around determining the rotation tensor matrix for a rotation by angle θ about an axis aligned with e1+e2. It includes aspects of mathematical reasoning and technical explanation related to rotation matrices in three-dimensional space.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the rotation would primarily affect the e3 axis, proposing that the matrix would have all zeroes except for a complicated value in the R33 component.
  • Another participant proposes a method of rotating e1+e2 onto e1, performing the rotation through θ, and then rotating back to e1+e2.
  • A different participant introduces the equation (R-I)u=0, indicating that the rotation tensor R could be represented by a specific matrix, but questions whether the entries should be in terms of sinθ and cosθ instead of 1.
  • A subsequent post notes that the determinant of the proposed matrix is -1, indicating a potential issue with the matrix representation.

Areas of Agreement / Disagreement

Participants express differing views on the correct form of the rotation tensor matrix, with no consensus reached on the appropriate representation or the implications of the determinant.

Contextual Notes

The discussion includes assumptions about the behavior of rotation matrices and the implications of specific matrix forms, but these assumptions remain unresolved.

samee
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Homework Statement



Write the matrix of a rotation tensor corresponding to the rotation by angle θ about an axis aligned with e1+e2

Homework Equations



I know that the matrix for a rotation tensor about e3 is;

cosθ -sinθ 0
sinθ cosθ 0
0 0 0

The Attempt at a Solution



I assume that the rotation would be changing only on the e3 axis because the axis are aligned with e1+e2, right? So the matrix will be all zero with the R33 component being some complicated rotated value?
 
Last edited:
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hi samee! :smile:

(try using the X2 button just above the Reply box :wink:)

how about rotating e1+e2 onto e1, then rotating through θ, then rotating e1 back again onto e1 +e2 ? :wink:
 
Okay! I had some new revelations.

I know that (R-I)u=0 where R is the rotation tensor, I is Identity and u is some vector, here it's e1+e3.

So, this equation will be true if the R matrix is;

0 1 0
1 0 0
0 0 1

BUT! I think it needs to be in sinθ and cosθ instead of 1... right?

Also- thanks for the tip on the X2 ^_^
 
samee said:
0 1 0
1 0 0
0 0 1

erm :redface:

the determinant of that is -1 :biggrin:
 

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