\begin{align*} & \text{ the position vector}\\\\ &\mathbf R= \left[ \begin {array}{c} r\cos \left( \theta \right) \sin \left( \phi \right) \\ r\sin \left( \theta \right) \sin \left( \phi \right) \\ r\cos \left( \phi \right) \end {array} \right]\\\\ &\text{from here you obtain the velocity}\\\\ &\mathbf v=\frac{\partial\mathbf{R} }{\partial r}\,\dot{r}+ \frac{\partial\mathbf{R} }{\partial \phi}\,\dot{\phi}+ \frac{\partial\mathbf{R} }{\partial \theta}\,\dot{\theta}\\ &\mathbf v=\mathbf e_r\,\dot{r}+\mathbf e_\phi\,r\,\dot{\phi}+\mathbf e_\theta\,r\,\sin(\phi)\,\dot{\theta}\\\\ &\text{where $~\mathbf{e}~$ are unit vectors} \end{align*} those
$p_r$ momenta towards $\mathbf{e}_r$
$p_\phi$ momenta towards $\mathbf{e}_\phi$
$p_\theta$ momenta towards $\mathbf{e}_\theta $