code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          dPhidrr= -(R**2.+z**2.+self._b2)**-1.5
          return dPhidrr*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
          HISTORY:
             2015-06-15 - Written - Bovy (IAS)