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:
           2010-07-10 - Written - Bovy (NYU)
        """
        if self.alpha == 0.:
            return -z/self.q2/(R**2.+z**2./self.q2+self.core2)
        else:
            m2= self.core2+R**2.+z**2./self.q2
            return -m2**(-self.alpha/2.-1.)*z/self.q2
def _zforce ( self , R , z , phi = 0. , t = 0. ) : if self . alpha == 0. : return - z / self . q2 / ( R ** 2. + z ** 2. / self . q2 + self . core2 ) else : m2 = self . core2 + R ** 2. + z ** 2. / self . q2 return - m2 ** ( - self . alpha / 2. - 1. ) * z / self . q2
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:
           2010-07-10 - Written - Bovy (NYU)