System of Units
creation of the decimal Metric System at the time of the French Revolution and
the subsequent deposition of two platinum standards representing the meter and
the kilogram, on 22 June 1799, in the Archives de la République in Paris can be
seen as the first step in the development of the present International System
of Units.. read on...
quantity in the general sense is a property ascribed to phenomena,
bodies, or substances that can be quantified for, or assigned to, a particular
phenomenon, body, or substance. Examples are mass and electric charge.
quantity in the particular sense is a quantifiable or assignable
property ascribed to a particular phenomenon, body, or substance. Examples are
the mass of the moon and the electric charge of the proton.
quantity is a quantity that can be used in the mathematical equations of science
A unit is a particular physical quantity, defined
and adopted by convention, with which other particular quantities of the same
kind are compared to express their value.
All physical quantities can be
expressed in terms of seven base units.
Other quantities, called derived quantities, are defined
in terms of the seven base quantities via a system of quantity equations. The
SI derived units for these derived quantities are obtained from these equations
and the seven SI base units. Examples of such SI derived units are given in Table
2, where it should be noted that the symbol 1 for quantities of dimension 1 such
as mass fraction is generally omitted.
in terms of SI units|
velocity||meter per second||m/s|
|acceleration||meter per second squared||m/s2|
|mass density||kilogram per cubic meter||kg/m3|
|energy, work||joule (J) N-m||m2·kg·s-2|
|electric potential||volt (V)||m2·kg·s-3·A-1|