![]()
Section II
Mechanical applications
![]()
Chapter 14
SI units and density
Why it is important to understand: SI units and density
In engineering there are many different quantities to get used to, and hence many units to become familiar with. For example, force is measured in newtons, electric current is measured in amperes and pressure is measured in pascals. Sometimes the units of these quantities are either very large or very small and hence prefixes are used. For example, 1,000,000 newtons may be written as 106 N which is written as 1 MN in prefix form, the M being accepted as a symbol to represent 1,000,000 or 106. Studying, or working, in an engineering discipline, you very quickly become familiar with the standard units of measurement, the prefixes used and engineering notation. An electronic calculator is extremely helpful with engineering notation. Knowledge about density inspired engineers to build ships, surfboards and buoys; density is also an important property that engineers consider in the engines they design for cars and power plants.
At the end of this chapter, you should be able to:
• state the seven SI units
• understand common prefixes used in engineering
• use engineering notation and prefix form with engineering units
• define density and relative density
• perform simple calculations involving density
As discussed in chapter 3, the system of units used in engineering and science is the Système Internationale d’Unités (International system of units), usually abbreviated to SI units, and which is based on the metric system. It was introduced in 1960 and is now adopted by the majority of countries as the official system of measurement.
The basic units in the SI system are listed below with their symbols:
Quantity | Unit | Symbol |
Length | metre | m | (1m = 100cm = 1000mm) |
Mass | kilogram | kg | (1kg = 1000g) |
Time | second | s | |
Electric current | ampere | A | |
Thermodynamic temperature | kelvin | K | (K=°C+273) |
Luminous intensity | candela | cd | |
Amount of substance | mole | mol | |
SI units may be made larger or smaller by using prefixes which denote multiplication or division by a particular amount. The eight most common multiples, with their meaning, are listed below:
Length is the distance between two points. The standard unit of length is the metre, although the centimetre (cm), millimetre (mm) and kilometre (km), are often used.
Area is a measure of the size or extent of a plane surface and is measured by multiplying a length by a length. If the lengths are in metres then the unit of area is the square metre (m2).
Conversely, 1 cm2 = 10−4 m2 and 1 mm2 = 10−6 m2.
Volume is a measure of the space occupied by a solid and is measured by multiplying a length by a length by a length. If the lengths are in metres then the unit of volume is in cubic metres (m3).
Conversely, 1 cm3 = 10−6m3 and 1mm3 = 10−9m3. Another unit used to measure volume, particularly with liquids, is the litre (l), where 1 litre = 1000 cm3.
Mass is the amount of matter in a body and is measured in kilograms (kg).
and 1 tonne (t) = 1000 kg.
Problem 1. Express (a) a length of 36 mm in metres, (b) 32,400 mm2 in square metres, and (c) 8,540,000 mm3 in cubic metres
Hence,
Problem 2. Determine the area of a room 15m long by 8m wide in (a) m2, (b) cm2, (c) mm2
(Note, it is usual to express the power of 10 as a multiple of 3, i.e. × 103 or × 106 or × 10−9 and so on....
