Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

10.5 Pressure and Stress —Force Acting Over an Area 273

Atmospheric Pressure

Earth’s atmospheric pressure is due to the weight of the air in the atmosphere above the surface

of the earth. It is the weight of the column of air (extending all the way to the outer edge of the

atmosphere) divided by a unit area at the base of the air column. Standard atmosphere at sea

level has a value of 101.325 kPa. From the definition of atmospheric pressure you should real-

ize that it is a function of altitude. The variation of standard atmospheric pressure and air den-

sity with altitude is given in Table 10.4. For example, the atmospheric pressure and air density

at the top of Mount Everest is approximately 31 kPa and 0.467 kg /m

3

, respectively. The ele-

vation of Mount Everest is approximately 9000 m (exactly 8848 m), and thus less air lies on

top of Mount Everest than there is at sea level. Moreover, the air pressure at the top of Mount

Everest is 30% of the value of atmospheric pressure at sea level, and the air density at its top is

only 38% of air density at sea level. New commercial planes have a cruising altitude capacity

of approximately 11,000 m. Referring to Table 10.4, you see the atmospheric pressure at that

altitude is approximately one-fifth of the sea-level value, so there is a need for pressurizing the

cabin. Also, because of the lower air density at that altitude, the power required to overcome air

resistance (to move the plane through atmosphere) is not as much as it would be at lower alti-

tudes. Atmospheric pressure is commonly expressed in one of the following units: pascals,

pound per square inch, millimeters of mercury, and inches of mercury. Their values are

Study Example 10.9 to see how these and other units of atmospheric pressure are related.

1 atm29.92 in#Hg

1 atm760 mm#Hg

1 atm14.69 lb/in

2

1 atm101325 Pa101.3 kPa

TABLE 10.3 The Relationship Between the Height of Water in a Water Tower

and the Pressure in a Pipeline at Its Base

Water Level in Water Pressure Water Level in Water Pressure

the Tower (ft) (lb/in

2

) the Tower (ft) (lb/in

2

)

10 4.3 130 56.4

20 8.7 140 60.7

30 13.0 150 65.0

40 17.3 160 69.4

50 21.7 170 73.7

60 26.0 180 78.1

70 30.4 190 82.4

80 34.7 200 86.8

90 39.0 210 91.1

100 43.4 220 95.4

110 47.7 230 99.8

120 52.0 240 104.1

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