def _Rforce(self,R,phi=0.,t=0.):
        """
        NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           phi - azimuth
           t - time
        OUTPUT:
           the radial force
        HISTORY:
           2010-11-24 - Written - Bovy (NYU)
        """
        return self._A*math.exp(-(t-self._to)**2./2./self._sigma2)\
            /R*math.sin(self._alpha*math.log(R)
                        -self._m*(phi-self._omegas*t-self._gamma))