Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Some SI and English Units

In SI, the units of mass, length, and time are the kilogram (kg), meter (m),

and second (s), respectively. The respective units in the English system are

the pound-mass (lbm), foot (ft), and second (s). The pound symbol lbis

actually the abbreviation of libra, which was the ancient Roman unit of

weight. The English retained this symbol even after the end of the Roman

occupation of Britain in 410. The mass and length units in the two systems

are related to each other by

In the English system, force is usually considered to be one of the pri-

mary dimensions and is assigned a nonderived unit. This is a source of con-

fusion and error that necessitates the use of a dimensional constant (gc) in

many formulas. To avoid this nuisance, we consider force to be a secondary

dimension whose unit is derived from Newton’s second law, that is,

or

(1–1)

In SI, the force unit is the newton (N), and it is defined as the force required

to accelerate a mass of 1 kg at a rate of 1 m/s^2. In the English system, the

force unit is the pound-force(lbf) and is defined as the force required to

accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s^2 (Fig. 1–7). That

is,

A force of 1 N is roughly equivalent to the weight of a small apple (m

102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four

medium apples (mtotal454 g), as shown in Fig. 1–8. Another force unit in

common use in many European countries is the kilogram-force(kgf), which

is the weight of 1 kg mass at sea level (1 kgf 9.807 N).

The term weightis often incorrectly used to express mass, particularly by

the “weight watchers.” Unlike mass, weight Wis a force.It is the gravita-

tional force applied to a body, and its magnitude is determined from New-

ton’s second law,

Wmg¬ 1 N 2 (1–2)

1 lbf32.174 lbm#ft>s^2

1 N1 kg#m>s^2

Fma

Force 1 Mass 21 Acceleration 2

1 ft0.3048 m

1 lbm0.45359 kg

6 | Thermodynamics

200 mL

(0.2 L)

1 kg

(10^3 g)

1 M

(10^6 )

FIGURE 1–6

The SI unit prefixes are used in all
branches of engineering.

m = 1 kg

m = 32.174 lbm

a = 1 m/s^2

a = 1 ft/s^2

F = 1 lbf

F = 1 N

FIGURE 1–7

The definition of the force units.

10 apples

m = 1 kg

4 apples

1 apple m = 1 lbm

m = 102 g

1 kgf

1 N 1 lbf

FIGURE 1–8

The relative magnitudes of the force
units newton (N), kilogram-force
(kgf ), and pound-force (lbf ).