Fundamental Quantities and Derived Quantities, Advantages and Disadvantages

( i ) Fundamental Quantities : A fundamental quantity is quantity which cannot be expressed in terms of any other physical quantity. e.g., mass, length and tune arc chosen as three fundamental quantities because these quantities are independent of each other. In other words, they cannot be expressed in terms of any other fundamental quantity. However with the passage of time ; mass. length and time were not only the Fundamental quantities. quantities like temperature. current, luminous intensity, amount or substance and angle were also included in the list of fundamental quantities.

( ii ) Derived Quantities : All the physical quantities which can be expressed in term's of fundamental quantities are called derived quantities. e.g.,
Area = Length x Breadth = Length x Length
Speed = Distance/Time = Length/Time
In other words, all the physical quantities which are derived from fundamental quantities are called derived quantities.

The chosen standard of the same kind (as that of physical quantity) taken as reference in order to measure a physical quantity is called unit of that quantity.
Measurement of a physical quantity :-
For measuring a physical quantity, first we select the reference (standard or unit) quantity of the same kind as that of given physical quantity. Then we find by comparing with this quantity the number of times chosen standard (unit) is contained in the given physical quantity. Thus. the measurement of a physical quantity involves .

(i) The selection of unit.

(ii) A number or numerical factor : The number which indicates that how many times a unit is repeating in a physical quantity is called numerical factor or numerical value of physical quantity.
For example, if we measure mass of an object as 10 kg, this indicates that I kg unit is repeating 10 times in the physical quantity. Similarly, if we measure length of an object as 5 m, this indicates that I m unit of length is repeating 5 times in the physical quantity. Hence, in general, we can write

Measurement of a physical quantity :- Numerical value of the physical quantity less (n) x Unit (u)
Characteristics of a Standard Unit :-
A unit selected for measuring a physical quantity should have the following properties :-

(i) It should be well-defined i.e., its concept should be clear.

(ii) It should be of suitable size i.e., neither too large nor too small.