INTRODUCTION TO THE S.I. SYSTEM
1. WHAT IS THE SI SYSTEM? – S.I. are the initial letters for System Internationale or in plain
English the Metric System. It is a system of measurement based on the following seven
base units:
Metre (m) - unit of LENGTH *SEE NOTE BELOW
Kilogram (kg) - unit of MASS
Second (s) - unit of TIME
Ampere (A) - unit of ELECTRIC CURRENT
Kelvin (K) - unit of THERMODYNAMIC TEMPERATURE
Candela (cd) - unit of LUMINOUS INTENSITY
Mole (mol) - unit of AMOUNT OF SUBSTANCE
From these seven base units numerous other units, known as derived units, are obtained.
For example:
combining metres with metres *NOTE - words shown in bold
gives the derived unit of AREA. upper case italics are known
i.e. metres x metres = m² as QUANTITIES.
combining metres with seconds
gives the derived unit of VELOCITY.
i.e. metres ÷ seconds = m/s (or m.s-1)
combining kilograms with cubic metres
gives the derived unit of DENSITY.
i.e. kilograms ÷ cubic metres = kg/m³ (or kg.m-3)
combining metres per second with seconds
gives the derived unit of ACCELERATION.
i.e. metres per second per second = m/s² (or m.s-2)
combining kilograms with metres per second²
gives the derived unit of FORCE.
i.e. kilograms x metres per second² = kg x m/s² = newtons
2. MULTIPLES AND SUB-MULTIPLES OF SI UNITS – there are many occasions when
quantities are too big or too small to measure conveniently in the base or derived units,
resulting in either:
Lots of noughts (or zeroes) 58 000 metres
Lots of decimal places 0.000 12 metres
In such situations multiples and sub-multiples are introduced. Therefore:
The distance from Stockport to Sheffield is approximately 58 000 metres
or more conveniently 58 kilometres (i.e. a multiple of a metre)
The thickness of this piece of paper is approximately 0.000 12 metres
or more conveniently 0.12 millimetres (i.e. a sub-multiple of a metre)
Both kilo and milli are examples of a prefix and indicate that a multiple and/or sub-multiple
is being used. It is recommended that they are used to keep numerical values of quantities
generally between 0.1 and 1000. Note how the decimal point has moved in the two examples
above. The table on page 2 shows most of the SI unit prefixes.
1
, MULTIPLICATION FACTOR PREFIX SYMBOL
1 000 000 000 000 = 1012 TERA T
1 000 000 000 = 109 GIGA G
Use these to
1 000 000 = 106 MEGA M move the
decimal point
1 000 = 103 KILO k this way.
100 = 102 HECTO h
10 = 101 DECA da
......................................................................................................................................................
0.1 = 10-1 DECI d
0.01 = 10-2 CENTI c
Use these to
0.001 = 10-3 MILLI m move the
decimal point
0.000 001 = 10-6 MICRO this way.
0.000 000 001 = 10-9 NANO n
0.000 000 000 001 = 10-12 PICO p
EXAMPLES - change each of the following to a numerical value between 0.1 and 1000 and
include the appropriate prefix.
1. 72 000 newtons 2. 0.004 newtons 3. 95 000 000 watts
= = =
= = =
4. 0.000 007 watts 5. 0.005 metres 6. 15 650 metres
= = =
= = =
7. 1 348 000 000 newtons 8. 0.000 001 583 958 farads
= =
= =
Or Or
= =
= =
3. CONVERSION OF METRIC UNITS
2
1. WHAT IS THE SI SYSTEM? – S.I. are the initial letters for System Internationale or in plain
English the Metric System. It is a system of measurement based on the following seven
base units:
Metre (m) - unit of LENGTH *SEE NOTE BELOW
Kilogram (kg) - unit of MASS
Second (s) - unit of TIME
Ampere (A) - unit of ELECTRIC CURRENT
Kelvin (K) - unit of THERMODYNAMIC TEMPERATURE
Candela (cd) - unit of LUMINOUS INTENSITY
Mole (mol) - unit of AMOUNT OF SUBSTANCE
From these seven base units numerous other units, known as derived units, are obtained.
For example:
combining metres with metres *NOTE - words shown in bold
gives the derived unit of AREA. upper case italics are known
i.e. metres x metres = m² as QUANTITIES.
combining metres with seconds
gives the derived unit of VELOCITY.
i.e. metres ÷ seconds = m/s (or m.s-1)
combining kilograms with cubic metres
gives the derived unit of DENSITY.
i.e. kilograms ÷ cubic metres = kg/m³ (or kg.m-3)
combining metres per second with seconds
gives the derived unit of ACCELERATION.
i.e. metres per second per second = m/s² (or m.s-2)
combining kilograms with metres per second²
gives the derived unit of FORCE.
i.e. kilograms x metres per second² = kg x m/s² = newtons
2. MULTIPLES AND SUB-MULTIPLES OF SI UNITS – there are many occasions when
quantities are too big or too small to measure conveniently in the base or derived units,
resulting in either:
Lots of noughts (or zeroes) 58 000 metres
Lots of decimal places 0.000 12 metres
In such situations multiples and sub-multiples are introduced. Therefore:
The distance from Stockport to Sheffield is approximately 58 000 metres
or more conveniently 58 kilometres (i.e. a multiple of a metre)
The thickness of this piece of paper is approximately 0.000 12 metres
or more conveniently 0.12 millimetres (i.e. a sub-multiple of a metre)
Both kilo and milli are examples of a prefix and indicate that a multiple and/or sub-multiple
is being used. It is recommended that they are used to keep numerical values of quantities
generally between 0.1 and 1000. Note how the decimal point has moved in the two examples
above. The table on page 2 shows most of the SI unit prefixes.
1
, MULTIPLICATION FACTOR PREFIX SYMBOL
1 000 000 000 000 = 1012 TERA T
1 000 000 000 = 109 GIGA G
Use these to
1 000 000 = 106 MEGA M move the
decimal point
1 000 = 103 KILO k this way.
100 = 102 HECTO h
10 = 101 DECA da
......................................................................................................................................................
0.1 = 10-1 DECI d
0.01 = 10-2 CENTI c
Use these to
0.001 = 10-3 MILLI m move the
decimal point
0.000 001 = 10-6 MICRO this way.
0.000 000 001 = 10-9 NANO n
0.000 000 000 001 = 10-12 PICO p
EXAMPLES - change each of the following to a numerical value between 0.1 and 1000 and
include the appropriate prefix.
1. 72 000 newtons 2. 0.004 newtons 3. 95 000 000 watts
= = =
= = =
4. 0.000 007 watts 5. 0.005 metres 6. 15 650 metres
= = =
= = =
7. 1 348 000 000 newtons 8. 0.000 001 583 958 farads
= =
= =
Or Or
= =
= =
3. CONVERSION OF METRIC UNITS
2
