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 (except in the US!). Conversion between different systems of units is, in principle, perfectly straightforward, but, in practice, a frequent source of error. Witness, for example, the recent loss of the Mars Climate Orbiter because the engineers who designed its rocket engine used fps units whereas the NASA mission controllers employed mks units. Table 2 specifies the various conversion factors between mks, cgs, and fps units. Note that, rather confusingly (unless you are an engineer in the US!), 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 US, $1 {\rm ton} = 2,000 {\rm lb}$; in the UK, $1 {\rm ton} = 2,240 {\rm lb}$), 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 ( $1 {\rm hr} = 3,600 {\rm s}$), the day ( $1 {\rm da} = 86,400 {\rm s}$), and the year ( $1 {\rm yr} = 365.26 {\rm da} = 31,558,464 {\rm s}$).

Table 2: Conversion factors

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