code: def _Rforce(self, R, z, phi=0, t=0):
          
          if not self.isNonAxi and phi is None:
              phi= 0.
          r, theta, phi = bovy_coords.cyl_to_spher(R,z,phi)
          #x = R
          dr_dR = nu.divide(R,r); dtheta_dR = nu.divide(z,r**2); dphi_dR = 0
          return self._computeforceArray(dr_dR, dtheta_dR, dphi_dR, R,z,phi)
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force at (R,z, phi)
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             radial force at (R,z, phi)
          HISTORY:
             2016-06-06 - Written - Aladdin