- #1
Happiness
- 679
- 30
Suppose a position vector v is rotated anticlockwise at an angle ##\theta## about an arbitrary axis pointing in the direction of a position vector p, what is the rotation matrix R such that Rv gives the position vector after the rotation?
Suppose p = ##\begin{pmatrix}1\\1\\1\end{pmatrix}## and ##\theta## = -120##^\circ##.
My approach is as follows.
First, perform transformation ##T_1##: rotate the xy plane 45##^\circ## anticlockwise about the z axis. (I treat this as keeping v fixed but expressing v in terms of the new coordinate system CS1.)
Next, perform transformation ##T_2##: rotate the xz plane ##\tan^{-1}\frac{1}{\sqrt2}## clockwise about the y-axis (clockwise when the y-axis points into your eye) so that the x-axis is now pointing in the direction of p. (Again, I treat this as keeping v fixed but expressing v in terms of the new coordinate system CS2.)
Then, perform transformation ##T_3##: rotate the yz plane 120##^\circ## anticlockwise about the x axis, since rotating v by -120##^\circ## has the same effect as rotating the yz plane 120##^\circ## (turning the coordinate system about the rotation axis by 120##^\circ## while keeping v fixed).
We have ##T_1=\begin{pmatrix}\frac{1}{\sqrt2}&\frac{1}{\sqrt2}&0\\-\frac{1}{\sqrt2}&\frac{1}{\sqrt2}&0\\0&0&1\end{pmatrix}## , ##T_2=\begin{pmatrix}\frac{\sqrt2}{\sqrt3}&0&-\frac{1}{\sqrt3}\\0&1&0\\\frac{1}{\sqrt3}&0&\frac{\sqrt2}{\sqrt3}\end{pmatrix}## , ##T_3=\begin{pmatrix}1&0&0\\0&-\frac{1}{2}&\frac{\sqrt3}{2}\\0&-\frac{\sqrt3}{2}&-\frac{1}{2}\end{pmatrix}##
Next, perform the inverse of transformation ##T_2##. (Changing the expression of v from coordinate system CS2 back to CS1.)
Lastly, perform the inverse of transformation ##T_1##. (Changing the expression of v from coordinate system CS1 back to CS0, the original coordinate system.)
Thus, R = ##T_1^{-1}T_2^{-1}T_3T_2T_1 = \begin{pmatrix}0&0&-1\\1&0&0\\0&-1&0\end{pmatrix}##
But R should be ##\begin{pmatrix}0&1&0\\0&0&1\\1&0&0\end{pmatrix}## since rotating v about p by -120##^\circ## has the effect of turning the x-axis to the y axis, and the y-axis to the z axis, and the z axis to the x-axis (turning the coordinate system about the rotation axis p by 120##^\circ## while keeping v fixed).
Why do I get different answers? What's wrong with my approach?
Suppose p = ##\begin{pmatrix}1\\1\\1\end{pmatrix}## and ##\theta## = -120##^\circ##.
My approach is as follows.
First, perform transformation ##T_1##: rotate the xy plane 45##^\circ## anticlockwise about the z axis. (I treat this as keeping v fixed but expressing v in terms of the new coordinate system CS1.)
Next, perform transformation ##T_2##: rotate the xz plane ##\tan^{-1}\frac{1}{\sqrt2}## clockwise about the y-axis (clockwise when the y-axis points into your eye) so that the x-axis is now pointing in the direction of p. (Again, I treat this as keeping v fixed but expressing v in terms of the new coordinate system CS2.)
Then, perform transformation ##T_3##: rotate the yz plane 120##^\circ## anticlockwise about the x axis, since rotating v by -120##^\circ## has the same effect as rotating the yz plane 120##^\circ## (turning the coordinate system about the rotation axis by 120##^\circ## while keeping v fixed).
We have ##T_1=\begin{pmatrix}\frac{1}{\sqrt2}&\frac{1}{\sqrt2}&0\\-\frac{1}{\sqrt2}&\frac{1}{\sqrt2}&0\\0&0&1\end{pmatrix}## , ##T_2=\begin{pmatrix}\frac{\sqrt2}{\sqrt3}&0&-\frac{1}{\sqrt3}\\0&1&0\\\frac{1}{\sqrt3}&0&\frac{\sqrt2}{\sqrt3}\end{pmatrix}## , ##T_3=\begin{pmatrix}1&0&0\\0&-\frac{1}{2}&\frac{\sqrt3}{2}\\0&-\frac{\sqrt3}{2}&-\frac{1}{2}\end{pmatrix}##
Next, perform the inverse of transformation ##T_2##. (Changing the expression of v from coordinate system CS2 back to CS1.)
Lastly, perform the inverse of transformation ##T_1##. (Changing the expression of v from coordinate system CS1 back to CS0, the original coordinate system.)
Thus, R = ##T_1^{-1}T_2^{-1}T_3T_2T_1 = \begin{pmatrix}0&0&-1\\1&0&0\\0&-1&0\end{pmatrix}##
But R should be ##\begin{pmatrix}0&1&0\\0&0&1\\1&0&0\end{pmatrix}## since rotating v about p by -120##^\circ## has the effect of turning the x-axis to the y axis, and the y-axis to the z axis, and the z axis to the x-axis (turning the coordinate system about the rotation axis p by 120##^\circ## while keeping v fixed).
Why do I get different answers? What's wrong with my approach?
Last edited: