code: def _Rforce(self,R,phi=0.,t=0.):
          
          return -R*(1.+R*numpy.sin(3.*phi))
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2017-10-16 - Written - Bovy (UofT)