The International System of Units
“… a single practical system of units of measurement suitable for adoption by all countries …”
From Resolution 6 of the 9th CGPM, 1948
The International System of Units is called SI after the French title, Systéme Internationale d'Unités. This system of units has been established, and is modified from time to time, by an international body called Conference Generale des Poids et Mesures (CGPM).
It has an executive arm – Bureau International des Poids et Mesures (BIPM) – which was established in 1875 and is located in Paris. It has 54 member states which support its operations.
The US was an original member of the convention setting up the Bureau while the UK joined nine years later.
Units
SI units comprise base units and derived units.
SI base units are the well-known ones of kilogram (symbol, kg), second (s) and metre (m), for the physical quantities mass, time and length.
In the US, standard spelling is “meter”.
In addition, there are four other base units for thermodynamic temperature, electrical current, amount of substance and luminous intensity.
Derived units are constructed from the base units, e.g. velocity, in metres per second (m/s).
The whole structure of science and engineering is built on the seven base units. All other units can be reduced to a combination of these units.
Mass
The kilogram is the unit of mass in the International System of Units. It was originally conceived as the mass of a litre of water at maximum density (4 °C).
In the US, standard spelling is “liter”.
Following a decision of the French National Assembly in 1791, a physical standard in the form of a platinum prototype of the kilogram was produced and was held in Paris from 1799.
Nearly a hundred years later in 1889, the 1st Conference Generale des Poids et Mesures adopted a technically new standard, copies of which were distributed among the standards laboratories of the member countries:
The international prototype of the kilogram.
http://www.bipm.org/en/scientific/mass/prototype.html
In 1901, because of the continuing ambiguity between mass and “weight”, the 3rd Conference Generale took steps to eliminate the confusion.
What did you say? 1901?!
It declared:
The kilogram is the unit of mass
The word “weight” denotes a quantity of the same nature as a “force”:
the weight of a body is the product of its mass and the acceleration due to gravity
Mass / density is a characteristic or property of matter.
Density: mass per unit volume.
The physical reality of our world is that masses exert force on each other; this is most immediately observed in the gravitational force of planet Earth on us: We call this force “weight”.
Mass is physically sensed by us as a force necessary to move something or lift something:
That something includes ourselves.
In terms of the units that are in current everyday use at a personal level,
state your weight in pounds (or stones and pounds),
state your body mass in kilograms.
For more on:
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Time
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Length
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Velocity
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Acceleration / Deceleration
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SI Prefixes
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Units, Numbers, Symbols and Prefixes: Some SI Rules
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The EU, the UK and the US
Go to: eBook
More information on SI units from the Bureau International des Poids et Mesures and Wikipedia: