For mixtures of gases (e.g. air), the total force is simply the sum of the forces due to each component of the gas, noting with care that each component (species) will have a different n , a different m and a different c ¯ ¯ . Note that n m = ρ ρ where ρ ρ is the gas density, noting again, with care, that in the case of multiple species, there are multiple different such densities contributing to the overall force.