def flattening(self,R,z):
        """
        
        NAME:
        
           flattening
        
        PURPOSE:
        
           calculate the potential flattening, defined as sqrt(fabs(z/R F_R/F_z))
        
        INPUT:
        
           R - Galactocentric radius (can be Quantity)

           z - height (can be Quantity)
        
        OUTPUT:
        
           flattening
        
        HISTORY:
        
           2012-09-13 - Written - Bovy (IAS)
        
        """
        return nu.sqrt(nu.fabs(z/R*self.Rforce(R,z,use_physical=False)\
                                   /self.zforce(R,z,use_physical=False)))