# 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 | _{m}2 |

volume | Length × length × length | cubic meter | _{m}3 |

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 |

10^{24} | yotta | Y | 10 ^{– 1} | deci | d |

_{10}21 | zetta | Z | 10 ^{– 2} | centi | c |

10^{18} | exa | E | 10 ^{– 3} | milli | m |

_{10}15 | peta | P | 10 ^{– 6} | micro | m |

10^{12} | tera | T | 10 ^{– 9} | nano | n |

10^{9} | giga | G | _{10} – 12 | pico | p |

10^{6} | mega | M | _{10} – 15 | femto | f |

10^{3} | kilo | k | _{10} – 18 | atto | a |

10^{2} | hecto | h | _{10} – 21 | zepto | z |

10^{1} | 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 × 10^{8} m = 696 × 10^{6} m = 696 Mm (696 mega meter) , Alternatively = 6.96 × 10^{8} m = 0.696 × 10^{9} m = 0.696 Gm (0.696 giga meter) .