
How do particles in a fluid exert pressure on a
container?
  Answer 1:
A fluid is in fact a collection of molecules which
have certain mobility. You can think of a fluid as
millions of very tiny balls, indeed so tiny, that
they can move around in space, unaffected by
gravity (this is not exactly true, but for these
purposes, let's just assume it is). These
molecules bounce with each other and with the
container walls, and just like a ball bouncing on
your body pushes you back, these molecules exert
such push in their containers.
  Answer 2:
The explanation for your question lies in the
'Kinetic theory of fluids'. All fluids are, of
course, made up of molecules. At any temperature
above absolute zero (0 K or 273.16 C), these
molecules have a kinetic energy that is
proportional to the temperature. The kinetic
energy results in random motion of the molecules,
which undergo collisions with each other and with
the walls of a container. These collisions are
the cause of fluid 'pressure'. it is immediately
obvious that fluid pressure must be 'isotropic' 
it is the same in all directions  and that as the
wall area increases, so will the pressure
force,
i.e. Force = Pressure *
Area
this link has an excellent java applet
demonstrating the idea of fluid
pressure.
fluid_pressure
using
simple assumptions about the collisions of the
molecules (namely, that these collisions are
elastic) one can derive the pressure exerted by
the molecules of an ideal gas to
be
pressure = 1/3 * molar density *
c^{2}
where c, the average velocity
of the molecules, is proportional to temperature.
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