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I'll start with the simple situation of a gas in a one-dimensional
container (so the particles only travel along that one dimension),
made up of particles of identical speeds under no attraction and
totally elastic collisions. That way the speeds remain identical.
I then add a small attraction so that if I remove the "top" of the
container, the fluid doesn't necessarily spit out the top. This can
then be considered a liquid. The speeds won't always be identical at
any given time but on average they are.
At any given time, only the particle at the top is exposed to the
vacuum above and thus only that particle can "leave" the liquid state
and fly away as a free particle. That particular particle need not
be the "fastest" of them all, does it? Does it even need to be going
faster than the "average" of them in order to leave?
I'd argue it doesn't. It just needs to be going fast enough to
overcome the attraction. And, while undergoing the escape process,
it slows down AND the particle it was attracted to slows down. This
leads to a cooling of both, independent of whether it was initially
going faster than those it "left behind" or not.
In other words, the real physics is on the cooling that results from
the "breaking" of the bonds (so to speak). Focusing on the speed of
those left behind (what JD calls "cancelling the sixes") allows
students to ignore the real physics.