code: def _phiforce(self,R,phi=0.,t=0.):
          
          #Calculate relevant time
          if not self._tform is None:
              if t < self._tform:
                  smooth= 0.
              elif t < self._tsteady:
                  deltat= t-self._tform
                  xi= 2.*deltat/(self._tsteady-self._tform)-1.
                  smooth= (3./16.*xi**5.-5./8*xi**3.+15./16.*xi+.5)
              else: #fully on
                  smooth= 1.
          else:
              smooth= 1.
          return smooth*self._twophio*R**self._p*m.sin(2.*(phi-self._phib))
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuthal force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             phi - azimuth
             t - time
          OUTPUT:
             the azimuthal force
          HISTORY:
             2011-10-19 - Written - Bovy (IAS)