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In order to make the measurement of a physical quantity we have, first of all, to evolve a standard for that measurement so that different measurements of same physical quantity can be expressed relative to each other. That standard is called a unit of that physical quantity.

- System of Units:-
(a) C.G.S (Centimeter-Grand-Second) system.(b) F.P.S. (Foot-Pound-Second) system.(c) M.K.S. (Meter-Kilogram--Second) system.(d) M.K.S.A. (Meter-Kilogram-Second-Ampere) unit.

- Dimensional Formula:-
Dimensional formula of a physical quantity is the formula which tells us how and which of the fundamental units have been used for the measurement of that quantity.

- How to write dimensions of physical quantities:-
(a) Write the formula for that quantity, with the quantity on L.H.S. of the equation.(b) Convert all the quantities on R.H.S. into the fundamental quantities mass, length and time.(c) Substitute M, L and T for mass, length and time respectively.(d) Collect terms of M,L and T and find their resultant powers (a,b,c) which give the dimensions of the quantity in mass, length and time respectively.

- Characteristics of Dimensions:-
(a) Dimensions of a physical quantity are independent of the system of units.(b) Quantities having similar dimensions can be added to or subtracted from each other.(c) Dimensions of a physical quantity can be obtained from its units and vice-versa.(d) Two different physical quantities may have same dimensions.(e) Multiplication/division of dimensions of two physical quantities (may be same or different) results in production of dimensions of a third quantity.

Principle of homogeneity:-It states that “ the dimensional formulae of every term on the two sides of a correct relation must be same.”

Types of error:-(a) Constant errors:- An error is said to be constant error if it affects, every time, a measurement in a similar manner.(b) Systematic errors:- Errors which come into existence by virtue of a definite rule, are called systematic errors.(c) Random error or accidental error:- Error which takes place in a random manner and cannot be associated with a systematic cause are called random or accidental errors.(d) Absolute error:-

- Relative Error:-

- Percentage Error:-