138 Chapter 6 Fundamental Dimensions and Units

TABLE 6.6 Systems of Units and Conversion Factors

System of Units

Dimension SI BG U.S. Customary Conversion Factors

Length Meter ( m) Foot (ft) Foot (ft) 1 ft 0.3048 m

1 m 3.2808 ft

Time Second (s) Second (s) Second (s)

Mass Kilogram (kg) Slugs* Pound mass (lbm)1lbm 0.4536 kg

1 kg 2.2046 lbm

1 slug 32.2 lbm

Force Newton (N) 1 lbf** One pound mass*** 1 N 224.809E-3lbf

weighs one pound force 1 lbf 4.448N

at sea level

Temperature Degree Celsius (°C) Degree Fahrenheit (°F) or Degree Fahrenheit (°F) or

or Kelvin (K)**** degree Rankine (°R) degree Rankine (°R)

K °C 273.15 °R °F 459.67 °R °F 459.67

Work, Energy Joule ( J) (1N)(1m) lbfft (1 lbf)(1 ft) 1 J 0.7375 ft lbf

Commonly written lbfft (1 lbf)(1 ft) 1 ftlbf 1.3558 J

as ftlbf 1 Btu 778.17 ftlbf

Power

kW 1000 W

1 hp 0.7457 kW

*Derived or secondary dimension

**Fundamental dimension

***Note unlike SI and BG systems, the relationship between pound force and pound mass is not defined using Newton’s second law

****Note a Temperature value expressed in K reads Kelvin not degree Kelvin



1 hp 550

ft#lbf

s



1 W0.7375

ft#lbf

s

lbf#ft

second^ ^

1 1lbf 21 1 ft 2

1 second

lbf#ft

second^ ^

1 1lbf 21 1 ft 2

Watt 1 W 2  (^) 1 second
1 Joule
1 second


 

 #  
°R

9

5 K

K

5

   9 °R

°F

9

5 °C^32

°C

5

9 3 °F^324

1N 11 kg2a 1 

m

s^2

b

 1 1 slug2a 1 

ft

s^2

b











6 6..3 3 UUnniitt CCoonnvveerrssiioonn

Some of you may recall that not too long ago NASA lost a spacecraft calledMars Climate

Orbiterbecause two groups of engineers working on the project neglected to communicate

correctly their calculations with appropriate units. According to an internal review

conducted by NASA’s Jet Propulsion Laboratory, “a failure to recognize and correct an

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