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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