In reference [63] Swift et al. suggested that a force, which changes the momentum, can be introduced by changing equation (4.47) to
If this approach is taken, then the extra term must be included in the equations of motion. For example, the first-order density equation, equation ({4.54), will become
Thus again this method can only be used if the term 
is
negligible small. The form of the terms 
would also need to be considered.
