In physics, all natural and physical phenomena are defined exactly and accurately. And these phenomena are represented in various relationships of physical quantities. The accuracy and exactness of these relationships is always dependent on the measurement one makes. Further the accuracy of the measurement will depend upon the accuracy of the technique and instruments used for taking the measurement. We use the term physical quantity which is defined as any quantity which can be measured using various techniques and can be expressed numerically. There are two kinds of physical quantities: fundamental, which are independently defined for e.g. length, mass and time and derived, which are derived from fundamental quantities for e.g. force, momentum, acceleration, electric current, etc.

Fundamental units are those which are used to denote the measure of fundamental physical quantities. Fundamental units are not those which can be derived from other units nor can they be further dissolved in simpler unit. We can also say that the units of fundamental physical quantities are known as fundamental unit. Example of fundamental unit is meter, second, kilogram, candela, kelvin, ampere and mole.

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Fundamental dimensions are used as a measure of physical quantities without any number. Like fundamental units, fundamental dimensions are independent from other and cannot be resolved further. In other words, dimension used to denote the fundamental quantities are called fundamental dimensions. Example of fundamental dimensions are length [L], mass [M], time [T], temperature [K], electric current [A], luminous intensity [cd] and quantity of matter [mol].

Basics of measurement
Physics is said to be the exact science because it provides accuracy in defining all physical or natural phenomena. All-natural phenomena that occur in nature are predicted in terms of the relationships among involved quantities. The accuracy and exactness of these relationships is always dependent on the measurement one makes. Further the accuracy of the measurement will depend upon the accuracy of the technique and instruments used for taking the measurement. So, if one person has knowledge about something and is able to measure and express it in number then it is said to be complete knowledge otherwise the knowledge is not satisfactory

There is an important term called physical quantity which is defined as any quantity which can be directly or indirectly measured using the measurement techniques based on laws of physics. Some of the physical quantities are mass, speed, force, electric current, etc. These are the physics basic building blocks because of these quantities; the various laws of physics can be expressed in mathematical expressions. There are two types of physical quantities: fundamental which are independently defined for e.g. length, mass and time and derived which are derived from fundamental quantities for e.g. force, momentum, acceleration, electric current, etc.