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M
atter is normally classified as being in one of three states: solid, liquid, or gas. From
everyday experience, we know that a solid has a definite volume and shape. A brick
maintains its familiar shape and size day in and day out. We also know that a liquid has
a definite volume but no definite shape. Finally, we know that an unconfined gas has
neither a definite volume nor a definite shape. These descriptions help us picture the
states of matter, but they are somewhat artificial. For 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 par-
ticular substance to change its shape in response to an external force determines
whether we treat the substance as a solid, a liquid, or a gas.
A
fluid is a collection of molecules that are randomly arranged and held together
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 do not need to learn any new physi-
cal principles to explain such effects as the buoyant force acting on a submerged ob-
ject and the dynamic lift acting on an airplane wing. First, we consider the mechanics
of a fluid at rest—that is, fluid statics. We then treat the mechanics of fluids in motion—
that is, fluid dynamics. We can describe a fluid in motion by using a model that is based
upon certain simplifying assumptions.
14.1 Pressure
Fluids do not sustain shearing stresses or tensile stresses; thus, the only stress that can
be exerted on an object submerged in a static fluid is one that tends to compress the
object from all sides. In other words, the force exerted by a static fluid on an object is
always perpendicular to the surfaces of the object, as shown in Figure 14.1.
The pressure in a fluid can be measured with the device pictured in Figure 14.2.
The device consists of an evacuated cylinder that encloses a light piston connected to a
spring. As the device is submerged in a fluid, the fluid presses on the top of the piston
and compresses the spring until the inward force exerted by the fluid is balanced by
the outward force exerted by the spring. The fluid pressure can be measured directly if
the spring is calibrated in advance. If F is the magnitude of the force exerted on the
piston and A is the surface area of the piston, then the
pressure P of the fluid at the
level to which the device has been submerged is defined as the ratio F/A:
(14.1)
Note that pressure is a scalar quantity because it is proportional to the magnitude of
the force on the piston.
If the pressure varies over an area, we can evaluate the infinitesimal force dF on an
infinitesimal surface element of area dA as
P
!
F
A
Figure 14.1 At any point on the
surface of a submerged object, the
force exerted by the fluid is per-
pendicular to the surface of the ob-
ject. The force exerted by the fluid
on the walls of the container is per-
pendicular to the walls at all points.
Definition of pressure
F
Vacuum
A
Figure 14.2 A simple device for
measuring the pressure exerted by
a fluid.