The net force is due both to momentum transfer to the sphere due to species impinging on it, and momentum transfer due to species leaving, due either to reflection, evaporation, or some combination of the two. Additionally, the force due to reflection depends upon whether the reflection is purely specular or, by contrast, partly or fully diffuse, and the force also depends upon whether the reflection is purely elastic, or inelastic, or some other assumption regarding the velocity distribution of reflecting particles, since the particles are, after all, in thermal contact - albeit briefly - with the surface. All of these effects are combined in Epstein's work in an overall prefactor " δ δ ". Theoretically, δ δ = 1 for purely elastic specular reflection, but may be less than or greater than unity in other circumstances. For reference, note that kinetic theory gives c ¯ ¯ = 8 π π ⋅ ⋅ k B T m . For the specific cases considered by Epstein, δ δ ranges from a minimum value of 1 up to a maximum value of 1.444. For example, Epstein predicts δ δ = 1 + π π / 8 ≃ ≃ 1.39 for diffuse elastic collisions. One may sometimes encounter δ δ = 1 + α α π π 8 where α α is the accommodation coefficient, which appears in the Maxwell model for the interaction of gas species with surfaces, characterizing the fraction of reflection events that are diffuse (as opposed to specular). (There are other accommodation coefficients that describe thermal energy transfer as well, but are beyond the scope of this article.)