#### Section 1-10 THE INTERNATIONAL SYSTEM OF UNITS

The official name of the International System of Units is Le Systeme International d'Unites, abbreviated SI. The basic units of SI are the metre, kilogram-mass and second. The unit of force is derived and is called the Newton. The units of the Newton are:

```
F = M*L/T^2 = (kilogram)*(metre)/(second)^2 = kg*m/s^2

```

The abbreviation for the Newton is N. The seven base units, with their symbols, are shown in Table 1-2. These are dimensionally independent. Lower case letters are used for the symbols unless they are derived from a proper name, then a capital is used for the first letter of the symbol. Note that the unit of mass uses the prefix kilo; this is the only base unit having a prefix.

Table 1-2 shows that the SI unit of temperature is the kelvin. The Celsius temperature scale (once called Centigrade) is not a part of SI but a difference of one degree on the Celsius scale equals one kelvin. A second class of SI units comproses the derived units, many of which have special names. Table 1-3 is a list of those we shall find most useful in our work in this ebook.

The radian (symbol rad) is a supplemental unit in SI for plane angle. A series of names and symbols to form multiples and submultiples of SI units have been established to provide an alternative to the writing of powers of 10. Appendix Table A-1 includes these prefixes and symbols.

Table 1-2
##### SI BASE UNITS
```Quantity                   Name      Symbol
-------------------------  --------  ------
Length                     metre     m
Mass                       kilogram  kg
Time                       second    s
Electric Current           ampere    A
Thermodynamic Temperature  kelvin    K
Amount of Matter           mole      mol
Luminous Intensity         candela   cd
-------------------------  --------  ------
```
Table 1-3
##### EXAMPLES OF SI DERIVED UNITS
```
Quantity                   Unit                      Symbol  Formula
-------------------------  ------------------------  ------  ---------
Acceleration               metre per second squared          m*s^-2
Area                       square metre                      m^2
Density                    kilogram per cubic metre          kg*m^-3
Energy                     joule                      J      N*m
Force                      newton                     N      kg*m*s^-2
Fource couple              newton metre                      N*m
Frequency                  hertz                      Hz     s^-1
Power                      watt                       W      J*s^-1
Pressure                   pascal                     Pa     N*m^-2
Quantity of heat           joule                      J      N*m
Speed                      revolution per second             s^-1
Stress                     pascal                     Pa     N*m^-2
Torque                     newton metre                      N*m
Velocity                   metre per second                  m*s^-1
Volume                     cubic metre                       m^3
Work                       joule                      J      N*m
-------------------------  ------------------------  ------  ---------
```