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| Introduction - Systems of Units |
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| A unit of a quantity is the standard quantity used to measure a physical quantity. |
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| The process of measuring a physical quantity has two steps: |
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 Selection of unit |
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 To find the number of times that unit is contained in that physical quantity. |
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| In order to measure the length of a table, the unit selected is that of length. If metre is the unit used, a metre rod is used to measure the length and if the table length is thrice the length of the metre rod, then the numerical value of length of the table is 3. |
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| i.e., Table length = 3 x 1m = 3m |
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| In general, measure of a physical quantity = numerical value of physical quantity x size of its unit |
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| i.e., x = nu |
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| where x = quantity to be measured for selected unit |
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| n = numerical value of physical quantity |
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| u = size of the unit |
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| If size of the chosen unit is small, then numerical value is large and vice versa. But measure of physical quantity is always the same. |
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 nu = constant |
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| If n1 and n2 are numerical values of physical quantities for units u1 and u2, then, |
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| n1 u1 = n2 u2. |
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