SI Base Units.

Scientists all over the world use the same system of units, called SI units. SI stands for Système International d'Unités.The system is built on seven base quantities, each with its own base unit: length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol) and luminous intensity (candela, cd).All other units are derived units, made by combining base units. For example, speed is measured in m s^-1 and density in kg m^-3.Named derived units can always be written in base units. The newton is kg m s^-2, because F = ma, and the joule is kg m² s^-2, because work done = force × distance.Writing units in base units lets you check that an equation is homogeneous: every term must have the same base units. An equation that is not homogeneous cannot be correct.