In learning physics we need to carry out investigations. we gather information through observations and taking measurements. We measure many types of physical quantities. A **physical quantity** is a physical characteristic that can be measured. The value of the measurement consists of a numerical magnitude and unit. All physical quantities can be classified inti two groups : **base quantities** and **derived quantities**. **Base quantity** are quantity that cannot be defined interms of other base quantities. There are five base quantities : **length**, **mass**, **time**, **current** and **temperature**. Table 1.1 below shows the five base quantities and their respective** SI units.**

Base Quantity | Quantity symbol | SI unit | Symbol |

length | l |
metre | m |

Mass | m |
kilogram | kg |

Electric current | I |
ampere | A |

Temperature | T | kelvin | K |

Time | t |
second | s |

A **derived quantity** is one which obtained by combining base quantities by multiplication, division or both these operations.

**Prefixes**

We use **prefixes** to simplify the expression of very big or very small numerical values of pysical quantities. Prefixes are usually used to express some physical quantities that are either **very big** or **very small**.

Prefixes |
Symbol |
Value |

nano | n | 10^{-9} |

micro | µ | 10^{-6} |

milli | m | 10^{-3} |

centi | c | 10^{-2} |

deci | d | 10^{-1} |

kilo | k | 10^{3} |

mega | M | 10^{6} |

giga | G | 10^{9} |

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