code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          self._compute_xyzforces(R,z,phi,t)
          return numpy.cos(phi)*self._cached_Fx+numpy.sin(phi)*self._cached_Fy
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:
             2016-11-02 - Written - Bovy (UofT)