code: def _zforce(self,R,z,phi=0.,t=0.):
          
          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.))
docs: 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)