$$% Arcus cosine. \def\acos{\cos^{-1}} % Vector projection. \def\projection#1#2{{proj_{#1}\left(#2\right)}} % Vector rejection. \def\rejection#1#2{{rej_{#1}\left(#2\right)}} % Norm. \def\norm#1{{\left\|#1\right\|}} % Cross product. \def\cross#1#2{\mathit{cross}\left(#1,#2\right)} % Dot product. \def\dot#1#2{{#1 \cdot #2}} % Magnitude. \def\mag#1{{\left|#1\right}} \def\group#1{\left(#1\right)}} \def\sbgrp#1{\left\{#1\right\}}$$

## Rotation around an arbitrary axis

Let $$\vec{p}$$ be a point vector, $$\alpha$$ an angle, $$\hat{r}$$ a unit axis vector. The expression $\vec{p}' = \vec{p}\cos\left(\alpha\right) + \cross{\hat{r}}{\vec{p}} \sin\left(\alpha\right) + \hat{r} \left(\dot{\hat{r}}{\vec{p}}\right)\left(1-\cos\left(\alpha\right)\right)$ is called the Rodrigues' Rotation Formula and representation a rotation by the angle $$\alpha$$ around the axis $$\hat{r}$$.

We derive an alternative form $\vec{p}' = \rejection{\vec{p}}{\hat{r}}\cos\left(\alpha\right) + \cross{\hat{r}}{\vec{p}} \sin\left(\alpha\right) + \projection{\vec{p}}{\hat{r}}$ of the Rodrigues' Rotation Formula.

### Proof

\begin{aligned} &\vec{p}'\\ &=\vec{p}\cos\left(\alpha\right) \cross{\hat{r}}{\vec{p}} \sin\left(\alpha\right) \hat{r} (\dot{\hat{r}}{\vec{p}})\left(1-\cos\left(\alpha\right)\right)\\ &=\vec{p}\cos(\alpha) + \cross{\hat{r}}{\vec{p}} \sin(\alpha) + u (\dot{\hat{r}}{\vec{p}}) - \hat{r} (\dot{\hat{r}}{\vec{p}}) \cos(\alpha)\\ &=[\vec{p} - \hat{r} (\dot{\hat{r}}{\vec{p}})]\cos(\alpha) + (\cross{\hat{r}}{\vec{p}}) \sin(\alpha) + u (\dot{u}{\vec{p}})\\ &=[v - u (\dot{u}{\vec{p}})]\cos(\alpha) + \cross{\hat{r}}{\vec{p}} \sin(\alpha) + u (\dot{u}{\vec{p}}) \end{aligned}
By the definition of the vector projection and rejection it follows the projection and rejection of $$\vec{p}$$ on $$\hat{r}$$ is defined as \begin{aligned} &\projection{\vec{p}}{\hat{r}} = \frac{\dot{\vec{p}}{\hat{r}}}{\norm{\hat{r}}^2}\hat{r} = \hat{r} (\dot{\hat{r}}{\vec{p}})\\ &\rejection{\vec{p}}{\hat{r}} = \vec{p} - \projection{\vec{p}}{\hat{r}} \end{aligned}
\begin{aligned} &=\rejection{\vec{p}}{\hat{r}}\cos(\alpha) + \cross{\hat{r}}{\vec{p}} \sin(\alpha) + \projection{\vec{p}}{\hat{r}} \end{aligned}