In the kinetic theory of gases, the "mean free path" of a particle, such as a molecule, is the average distance the particle travels between collisions with other moving particles. The derivation above assumed the target particles to be at rest, therefore in reality, the formula formula_9 holds for a beam particle with a high speed formula_10 relative to the velocities of an ensemble of identical particles with random locations. In that case, the motions of target particles are comparatively negligible, hence the relative velocity formula_11.