PRESSURE AND
FLUID STATICS
T
his chapter deals with forces applied by fluids at rest. The fluid prop
erty responsible for those forces is
pressure
, which is a normal force
exerted by a fluid per unit area. We start this chapter with a detailed
discussion of pressure, including
absolute
and
gage pressures
, the pressure
at a
point
, the
variation of pressure with depth
in a gravitational field, the
manometer
, the
barometer
, and other pressure measurement devices. This is
followed by a discussion of the
hydrostatic forces
applied on submerged
bodies with plane or curved surfaces. We then consider the
buoyant force
applied by fluids on submerged or floating bodies, and discuss the
stability
of such bodies. This chapter makes extensive use of force balances for bod
ies in static equilibrium, and it will be helpful if the relevant topics from
statics are first reviewed.
49
CHAPTER
3
OBJECTIVES
When you finish reading this chapter, you
should be able to
n
Determine the variation of
pressure in a fluid at rest
n
Calculate the forces exerted by a
fluid at rest on plane or curved
submerged surfaces
n
Analyze the stability of floating
and submerged bodies.
John Ninomiya Fying a cluster of 72balloon helium
±lled balloons over Temecula, California in April
of 2003. The helium balloons displace approximately
230 m
3
of air, providing the necessary buoyant force.
Photograph by Susan Dawson. Used by permission.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document3–1
n
PRESSURE
Pressure
is defined as
a normal force exerted by a fluid per unit area
. We
speak of pressure only when we deal with a gas or a liquid. The counterpart
of pressure in solids is
normal stress
. Since pressure is defined as force per
unit area, it has the unit of newtons per square meter (N/m
2
), which is called
a
pascal
(Pa). That is,
The pressure unit pascal is too small for pressures encountered in prac
tice. Therefore, its multiples
kilopascal
(1 kPa
5
10
3
Pa) and
megapascal
(1 MPa
5
10
6
Pa) are commonly used. Three other pressure units com
monly used in practice, especially in Europe, are
bar
,
standard atmosphere
,
and
kilogramforce per square centimeter
:
Note the pressure units bar, atm, and kgf/cm
2
are almost equivalent to each
other. In the English system, the pressure unit is
poundforce per square
inch
(lbf/in
2
, or psi), and 1 atm
5
14.696 psi. The pressure units kgf/cm
2
and lbf/in
2
are also denoted by kg/cm
2
and lb/in
2
, respectively, and they are
commonly used in tire gages. It can be shown that 1 kgf/cm
2
5
14.223 psi.
Pressure is also used for solids as synonymous to
normal stress
,wh
ich
±is
force acting perpendicular to the surface per unit area. For example, a 150
pound person with a total foot imprint area of 50 in
2
exerts a pressure of
150 lbf/50 in
2
5
3.0 psi on the floor (Fig. 3–1). If the person stands on one
foot, the pressure doubles. If the person gains excessive weight, he or she is
likely to encounter foot discomfort because of the increased pressure on the
foot (the size of the foot does not change with weight gain). This also
explains how a person can walk on fresh snow without sinking by wearing
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '10
 Dr.Ra’fatAlWaked
 Center of pressure, manometer, Patm

Click to edit the document details