# Science: Measurement in Science and Technology: SI Units and Derived Units (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Why Cannot the Parts of Human Body be Used for Accurate Measurement?

The results of the measurements vary from person to person because size of the unit is different for different person. For example, the units like a cubit or a foot would depend on the person making measurement. This created problems in trade between different countries and obviously in the day to day transactions.

## SI Units and Derived Units

• The International System of Units (SI, abbreviated from the French System international (d unites) is the modern form of the metric system and is the most widely used system of measurement.
• The SI units for all other physical quantities such as area, density, velocity can be derived in terms of the base SI units and are called derived units.

Since,

## Some Examples of Derived SI Units of the Commonly Used Physical Quantities

 Derived Quantity Dimensions Name of Unit Symbol of the Unit area Length × length square meter m2 volume Length × length × length cubic meter m3 speed, velocity meter per second m s – 1 acceleration meter per second squared m s – 2 wavenumber reciprocal meter m – 1 density kilogram per cubic meter kg m – 3 Work kilogram square meter per square second

## SI Prefixes

 Multiple Prefix Symbol Sub multiple Prefix Symbol 1024 yotta Y 10 – 1 deci d 1021 zetta Z 10 – 2 centi c 1018 exa E 10 – 3 milli m 1015 peta P 10 – 6 micro m 1012 tera T 10 – 9 nano n 109 giga G 10 – 12 pico p 106 mega M 10 – 15 femto f 103 kilo k 10 – 18 atto a 102 hecto h 10 – 21 zepto z 101 deca da 10 – 24 yocto y

The rule is that the prefix is chosen in such a way that the resulting value of the physical quantity has a value between 0.1 and 1000. Let us illustrate it with examples , . Radius of Sun = 6.96 × 108 m = 696 × 106 m = 696 Mm (696 mega meter) , Alternatively = 6.96 × 108 m = 0.696 × 109 m = 0.696 Gm (0.696 giga meter) .

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