Physical quantities are of two types:
(i) Fundamental quantities Fundamental units are those units, which can neither be derived from one another nor can they be further resolved into any other units. The quantities mass, length and time are called fundamental physical quantities and their units are known as
fundamental units because:
(i) Mass, length and time cannot be obtained from one another.
(ii) All other physical quantities in mechanics can be obtained from them.
(iii) Derived quantities: The units of all such physical quantities, which can be expressed in terms of the fundamental units of mass, length and time are called derived units. For example force, work, power, energy, pressure, momentum, acceleration, density, etc. are all derived units and they can be obtained by writing their defining equations in terms of fundamental physical quantities. To explain, let us consider the defining equation of speed, we know that:
An international system of units (SI. system of units)
The international system of units (abbreviated as SI, from the French system international (d’ unites) is the modern form of the metric system and is the most widely used system of measurement. The SI is based on the following seven fundamental units and two supplementary units as listed in the table.
