When I try to find out how to compute work for rotational motion. I found an equation from a book online with a figure and equation as follows:
$ \vec{s}\ =\vec{\theta}\ \times\vec{r} $
$Thus,$
$d\vec{s}\ =d\left(\vec{\theta}\ \times\vec{r}\ \right)=d\vec{\theta}\ \times\vec{r}\ +d\vec{r}\ \times\vec{\theta}\ =d\vec{\theta}\ \times\vec{r}\ $
$Note\ that\ d\vec{r}\ is\ zero\ because\ \vec{r}\ is\ fixed\ on\ the\ rigid\ body\ from\ the\ origin\ O\ to\ point\ P. $
What I don't understand is that it says $ \vec{s}\ =\vec{\theta}\ \times\vec{r}$, it means the direction of $ \vec{\theta}\ $ is perpendicular to the direction of $ \vec{s}\ $. However, $ d\vec{s}\ $ and $\hat{\theta}$ are in the same direction in the figure.
In addition, from what I learned in my other physics class: $ d\vec{s}\ = r·d\theta·\hat{\theta} $ and it makes sense to me since $ d\vec{s}\ $ is the infinitesimal arc length in $\hat{\theta}$ direction(⊥ to $ \hat{r} $).
Therefore, I am not comfortable with $ \vec{s}\ =\vec{\theta}\ \times\vec{r} $ which is claiming that $ \vec{\theta}\ $ is perpendicular to $ \vec{s}\ $. I was wondering if someone could please give me some insights on this?
Reference Book: Physics book