1 INTRODUCTION 1.5 Other units
Factor Prefix Symbol Factor Prefix Symbol
1018 exa- E 10 −^1 deci- d
1015 peta-^ P^10 −^2 centi-^ c^
1012 tera-^ T^10 −^3 milli-^ m^
109 giga- G 10 − 6 micro- μ
106 mega- M 10 − 9 nano- n
103 kilo- k 10 − 12 pico- p
102 hecto- h 10 − 15 femto- f
101 deka- da 10 − 18 atto- aTable 1: Standard prefixes1.5 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 em-
ployment 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 ex-
ample, 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 ft = 12 in), the yard ( 1 ya = 3 ft), and the
mile ( 1 mi = 5, 280 ft). Additional non-standard units of mass include the ton
(in the US, 1 ton = 2, 000 lb; in the UK, 1 ton = 2, 240 lb), and the metric ton
( 1 tonne = 1, 000 kg). Finally, additional non-standard units of time include the
minute ( 1 min = 60 s), the hour ( 1 hr = 3, 600 s), the day ( 1 da = 86, 400 s), and
the year ( 1 yr = 365.26 da = 31, 558, 464 s).