Beginning with Eq. $2 / 18$, the expression for particle velocity in spherical coordinates, derive the acceleration components in Eq. $2 / 19 .$ (Note: Start by writing the unit vectors for the $R$ -, $\theta$ -, and $\phi$ -coordinates in terms of the fixed unit vectors $\mathbf{i}$
$\mathbf{j}, \text { and } \mathbf{k} .)$