The thickness of the film can be calculated by the energy balance. Consider a small fluid volume element Δ Δ V which is located at a height h from the free surface. The potential energy due to the gravitational force acting on the fluid element is ρ ρ g h Δ Δ V , where ρ ρ is the total density and g is the gravitational acceleration. The quantum kinetic energy per particle is ℏ ℏ 2 / ( 2 m l 2 ) , where l is the thickness of the film and m is the mass of the particle. Therefore, the net kinetic energy is given by ℏ ℏ 2 f ρ ρ Δ Δ V / ( 2 m 2 l 2 ) , where f is the fraction of atoms which are Bose–Einstein condensate. Minimizing the total energy with respect to the thickness provides us the value of the thickness: