[24] An object is rotated about its Xˆ axis by an amount φ, and then it is rotated about its new Yˆ axis by an amount ψ. From our study of Euler angles, we know that the resulting orientation is given by
Rx (φ)Ry(ψ),
whereas, if the two rotations had occurred about axes of the fixed reference frame, the result would have been
Ry (ψ)Rx (φ).
It appears that the order of multiplication depends upon whether rotations are described relative to fixed axes or those of the frame being moved. It is more appropriate, however, to realize that, in the case of specifying a rotation about an axis of the frame being moved, we are specifying a rotation in the fixed system given by (for this example)
Rx (φ)Ry (ψ)R−1x (φ).
This similarity transform [1], multiplying the original Rx (φ) on the left, reduces to the resulting expression in which it looks as if the order of matrix multiplication has been reversed. Taking this viewpoint, give a derivation for the form of the rotation matrix that is equivalent to the Z–Y–Z Euler-angle set (α, β, γ). (The result is given by (2.72).)
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