One amazing thing about living on earth is that we are constantly walking around in a high-pressure fluid -- a substance with mass and no shape.
The air around us is composed of several different elements in a gaseous
state. In this gas, the atoms and molecules of the elements fly around
freely, bumping into each other and everything else. As these particles
collide against an object, each of them pushes with a tiny amount of energy.
Because there are so many particles in the air, this energy adds up to a
considerable pressure level (at sea level, about 14.7 pounds of pressure per
square inch (psi), or 1 kg per square centimeter (kg/cm2!).
The force of air pressure depends on two things:
* The rate of particle collision -- if more particles collide in a
period of time, then more energy is transferred to an object.
* The force of the impact -- if the particles hit with greater force,
more energy is transferred to an object.
These factors are determined by how many air particles there are in an area
and how fast they are moving. If there are more particles, or if they are
travelling more quickly, there will be more collisions, and so greater
pressure. Increasing particle speed also increases the force of the
particle's impact.
Most of the time we don't notice air pressure because there is air all
around us. All things being equal, air particles will disperse evenly in an
area so that there is equal air density at every point. Without any other
forces at work, this translates to the same air pressure at all points. We
aren't pushed around by this pressure because the forces on all sides of us
balance one another out. For example, 14.7 psi is certainly enough to knock
over a chair, or crush it from the top, but because the air applies roughly
the same pressure from the right, left, top, bottom and all other angles,
every force on the chair is balanced by an equal force going in the opposite
direction. The chair doesn't feel substantially greater pressure from any
particular angle.
So, with no other forces at work, everything would be completely balanced in
a mass of air, with equal pressure from all sides. But on Earth, there are
other forces to consider, chiefly gravity. While air particles are extremely
small, they do have mass, and so they are pulled toward the Earth. At any
particular level of the Earth's atmosphere, this pull is very slight -- the
air particles seem to move in straight lines, without noticeably falling
toward the ground. So, pressure is fairly balanced on the small scale.
Overall, however, gravity pulls particles down, which causes a gradual
increase in pressure as you move toward the earth's surface.
In the next section, we'll explore how this works.
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