A coherent system of units consists of a set of base units (typically time, length, mass, electric current and temperature) and a set of derived units. The derived units are formed from the product of base units raised to specific powers with a constant factor of one. Some derived units have special names.
In SI units, velocity is a derived unit that is measured in metres per second but does not have a special name. On the other hand the SI unit of energy has the special name "joule". If we consider Albert Einstein's formula E=mc2, then, using SI units, the energy E is in joules, the mass m is in kilograms and the speed of light c is in metres per second, it can be seen that one joule is equivalent to one kg·m2/s2.
Coherence can be explained by comparing the curie and the becquerel. The curie, a non-coherent unit, is equal to 3.7 x 1010 decays per second while the becquerel, a coherent unit, is equivalent to one decay per second.
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