The direct solution of the Boltzmann equation in six-dimensional phase space is not practicable.
Simplifications have to be made.
If the terms in Eq. 2.7 cancel for a given moment it means that the total moment before the collision is the same as after, i.e. that this moment is conserved during the scattering process. The assumptions made until this point are that elastic collisions are the only interaction process for the monoatomic gas particles, and that the viscous stress is only a linear function of the velocity shear. All the microphysics are included in the coefficients P , and .