Confusion in $\mathbb{R}^3$ Rotations derivation

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I just can’t understand how a rotation components can be written in terms of partial derivatives. Please throw some light on this, and also explain how we reached to the conclusion about orthogonality highlighted in the next para.

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Since $S$ rotates $x$ to $x'$ the transpose of $S$ rotates $x'$ back to $x$: $$ x_1=s_{11}x'_1+s_{21}x'_2+s_{31}x'_3 $$ and so on. It is easy to see that for example $s_{21}=\frac{\partial x_1}{\partial x'_2}$ and so on.