code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          return -self._denom(R, z)**-1.5 * R
docs: NAME:
              _Rforce
          PURPOSE:
              evaluate the radial force for this potential
          INPUT:
              R - Galactocentric cylindrical radius
              z - vertical height
              phi - azimuth
              t - time
          OUTPUT:
              the radial force = -dphi/dR
          HISTORY:
              2016-05-09 - Written - Aladdin