# Homework Help: Rotational motion

1. Oct 30, 2011

Okay so my mechanics teacher is taking a rather mathematical approach to CM and its really confusing me. Can someone explain to me what this actually means?

d hat{r}/dt=dot{theta} hat{theta} and

d hat{theta}/dt=-dot{theta} hat{r}

I mean I know that anything with a hat on top is a unit vector and i also know that theta dot represents angular speed. Thankyou.

2. Oct 30, 2011

### I like Serena

Yes, $\hat r$ is the local unit vector in the r direction at some point (r, theta).
We could also write $\hat r(r, \theta)$, since it is a function of r and theta.
However, if theta increases a little bit (by $d\theta$), that unit vector changes.
To be precise its angle changes by $d\theta$.
Perhaps you can make a drawing of it and consider which vector represent the change in $\hat r$?