  We use numerical N-body simulations in order to test whether the kinematics
of stars and brown dwarfs at birth depend on mass. In particular we examine how
initial variations in velocity dispersion can affect the spatial distribution
of stellar and substellar objects in clusters. We use 'toy' N-body models of a
Pleiades-like cluster in which brown dwarfs have their own velocity dispersion
sigma_(V_BD) which is k times larger than the stellar one.
  We find that in order to match the broad agreement between the brown dwarf
fraction in the field and in the Pleiades, the velocity dispersion of brown
dwarfs at birth has to be less than twice the stellar velocity dispersion, i.e.
cannot exceed a few km/s in the Pleiades cluster. In order to discern more
subtle differences between the kinematics of brown dwarfs and stars at birth,
our simulations show that we need to look at clusters that are much less
dynamically evolved than the Pleiades. One might especially seek evidence of
high velocity brown dwarfs at birth by examining spatial distribution of stars
and brown darfs in clusters that are about a crossing timescale old.
