Discrimination of impact approximations for velocity and velocity sign autocorrelation functions Original Research Article

The velocity and velocity sign autocorrelation functions as well as the distribution of time intervals between velocity sign changes of a particle in a gas or fluid are studied within the framework of impact theory. The results of the uncorrelated successive velocity reversal model (USVRM), for which this approach provides a limiting case of infinitesimal collision time, are checked. It is shown with this simple model that the two approaches are equivalent in two special cases: either when there is no correlation between pre- and post-collision velocities, or when the only possible result of a collision is elastic back scattering. Otherwise, the USVRM leaves out the scope of its view the important correlations of velocity in course of collisions.