A commonly used special case is where there is a uniform density in the field of matter, with matter particles significantly lighter than the major particle under consideration i.e., M ≫ ≫ m and with a Maxwellian distribution for the velocity of matter particles i.e., f ( v ) = N ( 2 π π σ σ 2 ) 3 / 2 e − − v 2 2 σ σ 2 where N is the total number of stars and σ σ is the dispersion. In this case, the dynamical friction formula is as follows: