As opposed to a solid, for a fluid it is possible to immerse a foreign
object, say a sphere, into it. The sphere makes room for itself by
displacing the liquid. The liquid responds by
pushing against the sphere as it attempts to re-occupy the space
occupied by the sphere. The force exerted by the fluid
is normal to the surface
of the immersed object. The magnitude of the force per square unit of surface area is called the pressure in the fluid:
The units for pressure are thus Newton/meter
which is also
called Pascal:
It is interesting to not that
and thus pressure has the dimensions of energy per volume unit.
Surprisingly perhaps our atmosphere is at
a rather high pressure. On average the pressure at sea level is
For example as I hold out my arm horizontally
the force on the upper side of the arm coming from
the atmospheric pressure is approximately
Which equals the force of gravity on a mass of 255 kg. Luckily
that the bottom side of our arm is subject to the same but opposite
force such that we in a sense do not directly experience this large
force. In fact our bodies are built to rely on the atmosphere as a compressive force. Without it we would blow up and pop as balloons and this is why astronauts and deep sea divers need special
pressurized suits to operate at pressures much lower and much higher than the atmospheric pressure that they were built for.