code: def _phiforce(self,R,phi=0.,t=0.):
          
          return self._Pot.phiforce(R,0.,phi=phi,t=t,use_physical=False)
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuthal force
          INPUT:
             R
             phi
             t
          OUTPUT:
            F_phi(R(,\phi,t))
          HISTORY:
             2016-06-02 - Written - Bovy (UofT)