code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          self._compute_xyzforces(R,z,phi,t)
          return np.cos(phi)*self._cached_Fx+np.sin(phi)*self._cached_Fy
docs: NAME:
             _Rforce
          PURPOSE:T
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the radial force