Derived SI units
Absolutely any measurement can be expressed using combinations of the seven base units (and angle if needed). These combinations are called derived units.
However - physical measurement of something like ‘happiness’ can’t be expressed in these units but some people try to measure it.
Examples of derived SI units
| Quantity | Unit | 1 such unit equals: |
|
Area |
square metre | 1 metre x 1 metre |
|
Volume |
cubic metre |
1 metre x 1 metre x 1 metre |
|
Speed |
metre per second |
1 metre ÷ 1 second |
|
Acceleration |
metre per second per second |
1 metre ÷ 1 second ÷ 1 second |
|
Force |
newton |
1 kilogram x 1 metre ÷ 1 second ÷ 1 second |
|
Energy |
joule |
1 kilogram x 1 metre x 1 metre ÷ 1 second ÷ 1 second |
|
Power |
watt |
1 kilogram x 1 metre x 1 metre ÷ 1 second ÷ 1 second ÷ 1 second |
Prefixes used for multiples of units
A shorthand system of prefixes was agreed as part of the SI system. All prefixes are related to each other by the power of 10 making it very easy to use.
| Prefix | Symbol | Decimal | Power of 10 |
|
yotta |
Y | 1000000000000000000000000 | 1024 |
|
zetta |
Z |
1000000000000000000000 | 1021 |
|
exa |
E |
1000000000000000000 | 1018 |
|
peta |
P |
1000000000000000 | 1015 |
|
tera |
T |
1000000000000 | 1012 |
|
giga |
G |
1000000000 | 109 |
|
mega |
M |
1000000 | 106 |
|
kilo |
k |
1000 | 103 |
|
hecto |
h |
100 | 102 |
|
deca |
da |
10 | 101 |
|
deci |
d |
0.1 | 10-1 |
|
centi |
c |
0.01 | 10-2 |
|
milli |
m |
0.001 | 10-3 |
|
micro |
µ |
0.000001 | 10-6 |
|
nano |
n |
0.000000001 | 10-9 |
|
pico |
p |
0.000000000001 | 10-12 |
|
femto |
f |
0.000000000000001 | 10-15 |
|
atto |
a |
0.000000000000000001 | 10-18 |
|
zepto |
z |
0.000000000000000000001 | 10-21 |
|
yocto |
y |
0.000000000000000000000001 | 10-24 |
Throughout the SI system, the same prefixes are used for the same multiples, no matter what the unit – except for the kilogram and time.
We don’t refer to a thousandth of a kilogram as a millikilogram! It’s simply milligram.
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