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| The seven base units
in the SI system. Arrows point from units to
those that depend on them. |
Units of Measurement
Measurement is a process that uses numbers to describe
things based on what we can observe about them. This is
done to be able to compare them to each other. We can
measure how big things are, how warm they are, how heavy
they are, and lots of other features as well. Units of
Measurement provide standards for our comparisons, so
that the numbers from our measurements refer to the same
thing. For example, the metre is a standard unit to
measure length. Before 1982, it was defined as the
distance between two markers on a special rod. Now
scientists define the metre as a fraction of the speed
of light.
Saying something has a length of 2 metres means that it
is exactly twice as long as that rod used to define the
metre, or that light takes twice the time defined for a
metre to travel that distance.
Today, most units of measure fall into one of three
systems. The older two, the British imperial system and
the closely related US customary system use the foot as
a measure of length, the pound as a measure for weight
and the second as a measure for time. There are other
units as well. In these systems the number of smaller
units that make the bigger units varies. For example,
there are 12 inches in a foot and 16 ounces in a pound.
A newer system is the metric system or SI system which
usually use 10 of the smaller unit to make the bigger
one. It uses the metre for length, the kilogram for
weight, and, as with the others, the second for time. |
|
Number and Unit of measure
The property of the thing being measured is given as a
number of units of measure. The number only has sense
when the unit of measurement is also given.
For example, The Eiffel Tower in Paris, France is 300
meters tall. That is, the distance from the top to the
bottom of the Eiffel Tower is 324 meters. The property
of the Eiffel Tower being measured is a distance. The
number measured is 324. This number does not make sense
without the unit of measure. The unit of measure is the
meter.
Measurement Standards
Standards are special objects that are used to make
measurements in terms of fixed units of measurement. A
metre stick is an example of a standard. When you
measure something with a metre stick, you can compare
that measurement to anything else that is also measured
with a metre stick. This makes measurement easier and
comparisons between measurements easier.
Size of Units of Measurement
There are units of measurement of different sizes. There
are small units of measurement to measure small things.
There are big units of measurement to measure big
things.
Science, medicine and engineering use smaller units of
measurement to measure small things with less error. It
is easier to measure large things using larger units of
measurement. Large measurements like the width of a
galaxy and small measurements like the mass of an atom
use special units of measurement. |
|
Systems of Units of
Measurement
There are many different standards and units used all
over the world.
Metric System
The metric system is a system of measurement used in
most of the world. It is also called the International
System of Units, or SI.
Units of measure in the metric system include: |
- The units of length or linear
size are based on the metre. They include the
kilometre (km) which is 1000 meters, the centimetre
(cm), and the millimetre (mm) which is 1/1000th of a
meter.
- The unit of volume is the litre.
It is used for measuring an amount of liquid. A
millilitre (abbreviated as ml) is the amount of
liquid that would fill up a cube that measures 1
centimetre on each side. One l litre of liquid would
fill up a cube that is 10 cm on each side.
- The unit of mass is the
kilogram. A kilogram (kg) weighs the same as a litre
of water (at normal temperature, and pressure). 1
gram (g) is the weight of 1 millilitre of water at 0
degrees Celsius. The metric tonne is 1000 kilograms
or a million grams.
|
British imperial Units
Imperial units were defined in the United Kingdom in
1825. These units sometimes based on similar units that
were in use before 1825. Imperial units were used in
countries that were part of the British Empire. While
many of these countries, including the United Kingdom,
have officially adopted SI, the older system of units
are still used.
US customary units
US customary units are the official units used in the
US. These are similar to the British imperial units and
also based on the units used in the United Kingdom from
before American Independence. But some of the units are
different to the British ones. For example, there are 20
imperial fluid ounces in an imperial pint, but 16 US
fluid ounces in a US pint. Additionally, the US fluid
ounce is slightly bigger than the imperial fluid ounce.
The result is that US pints and gallons are smaller than
imperial pints and gallons. In the United States, the
metric system has been legal for trade since 1866 but
other measurements such as the gallon, inch, and the
pound are still widely used.
Imperial and US units of measurement include: |
- Length - inch (in), foot (ft), yard
(yd), and mile.
- 1 foot
= 12 inches
- 1 yard
= 3 feet (plural of foot) = 36 inches
- 1 mile
= 1760 yards = 5280 feet
- Imperial volume - imperial fluid
ounce (fl oz), imperial pint (pt), and imperial gallon
(gal).
- 1 imperial
pint = 20 imperial fluid ounces
- 1 imperial
gallon = 8 imperial pints
- US volume - US fluid ounces (fl oz),
US cup (cp), US pint (pt), US quart (qt), and US gallon
(gal).
- 1 US cup
= 8 US fluid ounces
- 1 US pint
= 2 US cups = 16 US fluid ounces
- 1 US quart
= 2 US pints
- 1 US gallon
= 4 US quarts = 8 US pints
- Weight and mass are measured in
ounces (oz) and pounds (lb), and stone (st) in imperial
only.
- 1 pound
= 16 ounces
- 1 stone
= 14 pounds
|
|
The ounces for weight and volume are different. Even when
measuring water, the number of ounces of weight is not the
same as the number of fluid ounces. |
|
Converting Between Systems
Metric to US |
- 1 meter
= 1.09 yards = 39.37 inches
- 1 liter
= 33.3 fluid ounces = 1.76 pints = .26 US gallons
- 1
kilogram = 35.32 ounces = 2.2 pounds
|
|
US to metric |
- Length
- 1 inch
= 2.54 centimetres
- 1 foot
= 30.48 centimetres
- 1 yard
= .914 metres
- 1 mile
= 1.61 kilometres
- Volume
- 1 fluid
ounce = 29.6 millilitres
- 1 pint
= 473.1 millilitres
- 1 gallon
= 3.79 litres
- 1 cup
= 16 ounces
- Mass
- 1 ounce
= 28.35 grams
- 1 pound
= .45 kilograms
|
Other Units of Measure
Time
The unit of time is the second. The minute (60 seconds)
and hour (60 minutes or 3600 seconds) are larger units.
A day is usually said to be 24 hours, but is actually a
little bit longer than that. This difference is
corrected at the end of some years with what is called a
leap second. A week (7 days) and month are also standard
units.
Money
A unit of measurement that applies to money is called a
unit of account. This is normally a currency issued by a
country. For instance, the United States use dollars.
Each dollar is 100 cents. The United Kingdom uses
pounds. Each pound is 100 pennies or pence. Other
countries in Europe use the Euro which is 100 cent for
the Euro (they are not called "cents"). |
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