DYNAMICAL FRICTION
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Caused by the tendency towards the equipartition.

Let a massive object with the mass    and velocity
move through a "sea" of particles with masses
and velocities    , distributed as

If one neglects the self-gravity of the "sea",
then Chandrasekhar's dynamical friction formula
applies:



where



and        

is the Coulomb logarithm:
~ 6 for an open cluster, 
~ 11 for a globular cluster, 
~ 18 for the solar neigborhood (gal. disk), 
~ 22 for an E galaxy,
~ 7 for a cluster of galaxies.


Since                      ~ number density,
for small     approximately:

and for large     :