First, you're very close, but saying that pressure is caused by the
speed of molecules bouncing off a surface is not quite right. The force
on the surface due to a single molecule is actually the result of the
mass of the molecule times the acceleration (or change in velocity) of
the molecule. When a molecule hits the surface, it bounces off at the
same speed, but in the opposite direction. So the change in velocity
(which depends on both speed and direction) is twice the speed of the
molecule. The force of one molecule on a surface isn't very much, but
molecules are constantly hitting it. So pressure on a surface is the
average force acting on the surface over the area of the surface.
Now when we talk about the pressure of the entire atmosphere, we're
talking about a lot more than one molecule. And all of the different
molecules are moving in different directions so that the average
velocity of the air is zero, assuming there's no wind causing many
particles to move in the same direction. So even though the individual
molecules are moving, the atmosphere as a whole actually is just
sitting there exerting a downward force. And since gravity tends to
hold the molecules down, there are more molecules near to the surface
of the Earth than higher up.
So the force on a surface is caused by the average force exerted on
it by molecules bouncing off, and the average force exerted by the air
molecules depends on the pressure of the surrounding air. For example,
if you go high up into the mountains, the atmospheric pressure is less,
so fewer air molecules hit you and you feel less air pressure.
Hope this answers you question,
Tim
(published on 10/22/2007)
