code: def _phiforce(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 = phi
          dr_dphi = 0; dtheta_dphi = 0; dphi_dphi = 1
          return self._computeforceArray(dr_dphi, dtheta_dphi, dphi_dphi, R,z,phi)
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuth force at (R,z, phi)
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             azimuth force at (R,z, phi)
          HISTORY:
             2016-06-06 - Written - Aladdin