Theory:

A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities.
The common systems of units are given below:
  • CGS System: In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.
  • SI System: This system contains seven fundamental units and two supplementary fundamental units.
  • MKS System: In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.
  • FPS System: In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.
Important!
In the system of units, the first three systems of units (CGS, SI, MKS systems) are the metric systems of units. FPS is not a metric system of units, it is a British system of units.
SI Units:
SI system is also known as the International system of units and it is abbreviated from the french word 'systeme International'.

Base quantities and unit: 
The seven physical quantities are known as 'Base Quantities' and also defined as 'Standard Unit' to measure each one. They are known as 'Base Units' or 'Fundamental Units', which are given below: 
  • Length: The SI unit of  the length is metre and it is denoted as 'm'.
  • Mass: The SI unit of  the mass is kilogram and it is denoted as 'kg'.
  • Time: The SI unit of  the time is second and it is denoted as 's'.
  • Temperature: The SI unit of the tempearture is kelvin and it is denoted as 'K'.
  • Electric current: The SI unit of the electric current is ampere and it is denoted as 'A'.
  • Amount of substance: The SI unit of the amount of asubstane is mole and it is denoted as 'mol'.
  • Luminous instensity: The SI unit of the luminous intensity is candela and it is denoted as 'cd'.
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