15.1
Pressure
459
atter is normally classifed as being in one oF three states: solid, liquid, or
gas. ±rom everyday experience, we know that a solid has a defnite volume
and shape. A brick maintains its Familiar shape and size day in and day out.
We also know that a liquid has a defnite volume but no defnite shape. ±inally, we
know that an unconfned gas has neither a defnite volume nor a defnite shape.
These defnitions help us picture the states oF matter, but they are somewhat artif-
cial. ±or example, asphalt and plastics are normally considered solids, but over
long periods oF time they tend to ﬂow like liquids. Likewise, most substances can
be a solid, a liquid, or a gas (or a combination oF any oF these), depending on the
temperature and pressure. In general, the time it takes a particular substance to
change its shape in response to an external Force determines whether we treat the
substance as a solid, as a liquid, or as a gas.
A
ﬂuid
is a collection oF molecules that are randomly arranged and held to-
gether by weak cohesive Forces and by Forces exerted by the walls oF a container.
Both liquids and gases are ﬂuids.
In our treatment oF the mechanics oF ﬂuids, we shall see that we do not need
to learn any new physical principles to explain such eFFects as the buoyant Force
acting on a submerged object and the dynamic liFt acting on an airplane wing.
±irst, we consider the mechanics oF a ﬂuid at rest—that is,
ﬂuid statics
—and derive
an expression For the pressure exerted by a ﬂuid as a Function oF its density and
depth. We then treat the mechanics oF ﬂuids in motion—that is,
ﬂuid dynamics.
We can describe a ﬂuid in motion by using a model in which we make certain sim-
pliFying assumptions. We use this model to analyze some situations oF practical im-
portance. An analysis leading to
Bernoulli’s equation
enables us to determine rela-
tionships between the pressure, density, and velocity at every point in a ﬂuid.
PRESSURE
±luids do not sustain shearing stresses or tensile stresses; thus, the only stress that
can be exerted on an object submerged in a ﬂuid is one that tends to compress
the object. In other words, the Force exerted by a ﬂuid on an object is always per-
pendicular to the surFaces oF the object, as shown in ±igure 15.1.