One
of the “joys” of moving from the US to just about any other country is the need
to get used to SI, or metric, units.
Food is sold by grams, kilograms, and liters; street signs are marked in
kilometers and kilometers per hour. An
attendant joy is that periodically, someone will mock the fact that the US
still uses the old English units to your face.
As an American physicist, I have lived and worked with both, and also as
a physicist, I can be annoyingly focused on the use of units past the attention
span of those cracking the original joke.
So let’s discuss the metric system, shall we?
The
metric system was introduced in 1799 in France an attempt to move beyond older
and regional systems of units to something more logical and universal. Today, the metric system has been replaced by
le Système international
d'unités, often abbreviated SI units. The idea was to have units that related to
each other in a logical way, that could be derived from a set of basic units,
and (a bit more modern priority) that could be derived from natural phenomena. The original basic metric units were the meter
(m), the second (s), and the kilogram (kg); the SI system currently has seven
fundamental units, adding the Kelvin, the candela, the mole, and the ampere to
the original three. All other units
within the system can be derived from simple combinations of this basic
group. For example, the unit for force,
the Newton, is defined as the force needed to accelerate 1 kg of material to a
speed of 1 m/s in 1 s.
