Central Forces 2021
Show that the plane polar coordinates are equivalent
to spherical coordinates if we make the choices:

The direction of \(\theta=0\) in spherical coordinates is the same as the
direction of out of the plane in plane polar coordinates.

Given the correspondance above, then if we choose the \(\theta\) of spherical coordinates is to be \(\pi/2\), we restrict to the equatorial plane of spherical coordinates.