def _zforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _zforce
        PURPOSE:
           evaluate the vertical force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the vertical force
        HISTORY:
           2010-07-09 - Written - Bovy (NYU)
        """
        sqrtbz= nu.sqrt(self._b2+z**2.)
        asqrtbz= self._a+sqrtbz
        if isinstance(R,float) and sqrtbz == asqrtbz:
            return (-z/
                     (R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)**(3./2.))
        else:
            return (-z*asqrtbz/sqrtbz/
                     (R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)**(3./2.))