= Finite-test-mass dynamical friction coefficient
{title2=$\dot V=-16\pi^2G^2m(M+m)\ln\Lambda\,V^{-2}\int_0^V f(v)v^2\,dv$}
For a test <mass> $M$ and isotropic background particles of <mass> $m$, the hyperbolic <two-body problem> uses the relative deflection scale $b_{90}=G(M+m)/w^2$. The test body's longitudinal <velocity> change in one encounter is $-2mw/[(M+m)(1+b^2/b_{90}^2)]$. Integrating the encounter rate over <impact parameter> gives a logarithm; replacing its velocity-dependent value by a common <Coulomb logarithm in stellar dynamics> gives the displayed systematic drag. The approximation $M+m\simeq M$ requires $M\gg m$, not merely $M>m$. <Velocity> diffusion and exact cutoff dependence are distinct effects omitted by this leading-log drag model.
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