These molecules made their last collision at y = ± ± l cos ⁡ ⁡ θ θ , where l is the mean free path. Each molecule will contribute a forward momentum of p x ± ± = m ( u 0 ± ± l cos ⁡ ⁡ θ θ d u d y ) , where plus sign applies to molecules from above, and minus sign below. Note that the forward velocity gradient d u / d y can be considered to be constant over a distance of mean free path.