Fundamental and derived units

The units of fundamental quantities can't be expressed in terms of other units i.e. they do not depend upon other units. Hence, they are basic unit or fundamental units. For example kilogram (kg), second (s), meter (m) etc. The units used to measure the derived quantities involve the combination of two or more than two fundamental units. For example, unit of velocity depends upon the unit of distance and time i.e. the unit of velocity (m/s) can be expressed in terms of two fundamental units m and s. So, it is a derived unit. Thus, derived units are those units which can be expressed in terms of two or more than two fundamental units and are used for measuring derived quantities.

Systems Likewise, the unit of force, newton, involves three fundamental units kg, m and s, the unit of pressure, pascal, also involves kg, m and s. of units. Different people use different systems of units to measure different physical quantities. There are only a few system of units which are more scientific and are accepted as the standard system of units. Following are the most popular system of units for scientific purposes, derived on the basis of units used for measuring fundamental quantities.

FPS system
This system is also known as British system of units. In this system, length, mass and time are measured in foot, pound and second respectively.

CGS System
This is a French system of units. In this system, length, mass and time are measured in centimeter, gram and second respectively.

MKS System
This system is also known as metric system of units. In this system, length, mass and time are measured in meter, kilogram and second respectively.

SI System FPS, CGS and MKS systems involve only three quantities (length, mass and time) as fundamental quantities. These three quantities are insufficient to measure some physical quantities in thermodynamics, current electricity and optics. To solve this problem, in 1960 A.D., the 11th general conference of weight and measure was organized which introduced a logical and rationalized system of units called SI system (System International 'd' units). This system is an extended form of MKS system and involves the several fundamental quantities.

Unit

The process of measurement involves a numerical value associated with a standard reference terms. For example, measurement of mass involves kilogram associated with numerical value i.e. 1kg, 5kg, 10kg etc. So, kilogram is the unit of mass. Likewise meter is the unit of length, second is the unit of time, newton is the unit of force, watt is the unit of power etc. Thus, unit is a standard reference quantity of a substance in terms of which a physical quantity can be measured. The unit of a physical quantity is the most convenient amount of that quantity. Different physical quantities have different units for their measurement.

Fundamental Quantities and Derived Quantities

There are certain physical quantities which are independent of other physical quantities. Length, mass, time, temperature, electric current, luminous intensity and amount of substances are the physical quantities which are independent upon other quantities and are the examples of fundamental quantities. All other quantities are dependent upon two or more than two fundamental quantities. For example, velocity is physical quantity which depends upto two fundamental quantities length and time. Similarly, force depends upon mass and acceleration and the acceleration inturn depends upon length and time. Thus, velocity, force, acceleration, work, area, volume, pressure etc are derived quantities.

Physical quantities

Physical phenomenon of the nature can be explained either qualitatively or quantitatively. But only the qualitative explanation is not sufficient and today's advancement of science would not have possible only with qualitative study of natural phenomenon. A quantity is an observation of natural phenomenon that can be expressed quantitatively. "A physical quantity is that which can be measured directly or indirectly." Length, mass, time, speed, force, work, pressure, volume etc are some example of physical quantities. Physical quantities are the physical properties of matter.