CHAPTER 2
FORCES IN LIQUIDS
The study of liquids is divided into two main
parts: liquids at rest (hydrostatics) and liquids in
motion (hydraulics).
The effects of liquids at rest can often
be expressed by simple formulas. The effects
of liquids in motion are more difficult to
express due to frictional and other factors
whose actions cannot be expressed by simple
mathematics.
In chapter 1 we learned that liquids have a
definite volume but take the shape of their
containing vessel. There are two additional
characteristics we must explore prior to pro-
ceeding.
Liquids are almost incompressible. For
example, if a pressure of 100 pounds per square
inch (psi) is applied to a given volume of water
that is at atmospheric pressure, the volume will
decrease by only 0.03 percent. It would take a
force of approximately 32 tons to reduce its
volume by 10 percent; however, when this force
is removed, the water immediately returns to its
original volume. Other liquids behave in about
the same manner as water.
Another characteristic of a liquid is the
tendency to keep its free surface level. If the
surface is not level, liquids will flow in the
direction which will tend to make the surface
level.
LIQUIDS AT REST
In studying fluids at rest, we are con-
cerned with the transmission of force and
the factors which affect the forces in liquids.
Additionally, pressure in and on liquids and
factors affecting pressure are of great im-
portance.
PRESSURE AND FORCE
The terms force and pressure are used
extensively in the study of fluid power. It
is essential that we distinguish between the
terms. Force means a total push or pull.
It is the push or pull exerted against the
total area of a particular surface and is expressed
in pounds or grams. Pressure means the amount
of push or pull (force) applied to each unit area
of the surface and is expressed in pounds per
square inch (lb/in2) or grams per square
centimeter (gm/cm2). Pressure maybe exerted in
one direction, in several directions, or in all
directions.
Computing Force, Pressure, and Area
A formula is used in computing force,
pressure, and area in fluid power systems. In this
formula, P refers to pressure, F indicates force,
and A represents area.
Force equals pressure times area. Thus, the
formula is written
Equation 2-1.
Pressure equals force divided by area. By
rearranging the formula, this statement may be
condensed into
Equation 2-2.
Since area equals force divided by pressure,
the formula is written
Equation 2-3.
2-1
