Other Units

The mks system is not the only system of units in existence. Unfortunately, the obsolete cgs (centimeter-gram-second) system, and the even more obsolete fps (foot-pound-second) system, are still in use today, although their continued employment is now strongly discouraged in science and engineering. Conversion between different systems of units is, in principle, perfectly straightforward, but, in practice, a frequent source of error. Witness, for example, the loss of the Mars Climate Orbiter in 1999 (CE) because the Lockheed Martin engineers who designed and built its rocket engine used fps units whereas the NASA mission controllers employed mks units. Table 1.3 specifies the various conversion factors between mks, cgs, and fps units. Note that a pound is a unit of force, rather than mass. Additional non-standard units of length include the inch ( $1\,{\rm ft} = 12\,{\rm in}$), the yard ( $1\,{\rm ya} = 3\,{\rm ft}$), and the mile ( $1\,{\rm mi} = 5\,280\,{\rm ft}$). Additional non-standard units of mass include the ton (in the U.S., $1\,{\rm ton} = 2\,000\,{\rm lb}$; in the U.K., $1\,{\rm ton} = 2\,240 \,{\rm lb}$), and the metric ton ( $1\,{\rm tonne} = 1\,000\, {\rm kg}$). Finally, additional non-standard units of time include the minute ( $1\,{\rm min} = 60\,{\rm s}$), the hour ( $1\,{\rm hr} = 60\,{\rm min}$), the (solar) day ( $1\,{\rm da} = 24\,{\rm hr}$), and the (Julian) year ( $1\,{\rm yr} = 365.25\,{\rm da}$).

Table 1.3: Conversion factors between the mks, cgs, and fps systems of units. Here, g, ft, and lb are the abbreviations for gram, foot, and pound, respectively.

1 cm	$=$	$10^{-2}$ m
1 g	$=$	$10^{-3}$ kg
1 ft	$=$	$0.3048$ m
1 lb	$=$	$4.448\,{\rm kg\,m\,s}^{-2}$
1 slug	$=$	$14.59$ kg