The International System of units, abbreviated SI after the French Système International d’Unités, is a modern version of the metric system which has received worldwide recognition. As shown in Table 1, the system defines length in meters (m), time in seconds (s), and mass in kilograms (kg). The unit of force, called a newton (N), is derived from F = ma. Thus, 1 newton is equal to a force required to give 1 kilogram of mass an acceleration of 1 m/s2 (N = kg.m/s2). Think of this force as the weight of a small apple.
If the weight of a body located at the “standard location” is to be determined in newtons, then W = mg must be applied. Here measurements give g = 9.80665 m/s2; however, for calculations, the value g = 9.81 m/s2 will be used. Therefore, a body of mass 1 kg has a weight of 9.81 N, a 2-kg body weighs 19.62 N, and so on, Fig. 1.
Rules for Use: Here are a few of the important rules that describe the proper use of the various SI symbols:
• Quantities defined by several units which are multiples of one another are separated by a dot to avoid confusion with prefix notation, as indicated by N = kg.m/s2 = kg.m.s-2. Also, m.s (meter-second), whereas ms (milli-second).
• The exponential power on a unit having a prefix refers to both the unit and its prefix. For example, mN2= (mN)2 = mN.mN. Likewise, mm2 represents (mm)2 = mm.mm.
• With the exception of the base unit the kilogram, in general avoid the use of a prefix in the denominator of composite units. For example, do not write N/mm, but rather kN/m; also, m/mg should be written as Mm/kg.
• When performing calculations, represent the numbers in terms of their base or derived units by converting all prefixes to powers of 10. The final result should then be expressed using a single prefix.
Also, after calculation, it is best to keep numerical values between 0.1 and 1000; otherwise, a suitable prefix should be chosen. For example,
 |
| Table 1. International System of Units |
Prefixes: When a numerical quantity is either very large or very small, the SI units used to define its size may be modified by using a prefix. Some of these prefixes used are shown in Table 2. Each represents a multiple or submultiple of a unit which, if applied successively, moves the decimal point of a numerical quantity to every third place. For example, 4 000 000 N = 4 000 kN (kilo-newton) = 4 MN (mega-newton), or 0.005 m = 5 mm (milli-meter). Notice that the SI system does not include the multiple deca (10) or the submultiple centi (0.01), which form part of the metric system. Except for some volume and area measurements, the use of these prefixes is generally avoided in science and engineering.
 |
| Table 2. Prefixies |
• Quantities defined by several units which are multiples of one another are separated by a dot to avoid confusion with prefix notation, as indicated by N = kg.m/s2 = kg.m.s-2. Also, m.s (meter-second), whereas ms (milli-second).
• The exponential power on a unit having a prefix refers to both the unit and its prefix. For example, mN2= (mN)2 = mN.mN. Likewise, mm2 represents (mm)2 = mm.mm.
• With the exception of the base unit the kilogram, in general avoid the use of a prefix in the denominator of composite units. For example, do not write N/mm, but rather kN/m; also, m/mg should be written as Mm/kg.
• When performing calculations, represent the numbers in terms of their base or derived units by converting all prefixes to powers of 10. The final result should then be expressed using a single prefix.
Also, after calculation, it is best to keep numerical values between 0.1 and 1000; otherwise, a suitable prefix should be chosen. For example,
(50 kN).(60 nm) = [50.(103) N] [60.(10-9) m] = 3000.(10-6) N.m = 3.(10-3) N.m = 3 mN.m
Notes:
•Historically, the meter was defined as 1/10,000,000 the distance from the Equator to the North Pole, and the kilogram is 1/1000 of a cubic meter of water.
•The kilogram is the only base unit that is defined with a prefix.