Units are the third aspect that is important when measuring in Physics. It can be the mistake that will ruin the whole measurement process.

Let us recap.

- You need to identify the quantity that is to be measured eg length of the book.
- you need to use the correct instrument to measure the quantity. In this case the 30 cm ruler.
- Lastly you need to measure the length and write it with appropriate unit.

Imagine that you write the length of a 20 cm long book as 20 inches. Then it ruins the whole measurement process.

SI units

To avoid such mistakes scientist around the world have decided to use what is called the SI system of units. It is a standard that is used by all scientists such that when a particular quantity is being measures everyone uses the same units although the Americans and the British continue to use the old systems. This has resulted in costly mistakes and loss of human lives in the space industry.

Base units

The SI system is based on fundamental base units on which all other units can be derived.

The 7 fundamental base units are given in the table below.

Basic Quantity | Base unit | Symbol of base unit |

Length | Metre | m |

Time | Second | s |

Mass | Kilogram | kg |

Thermodynamic temperature | Kelvin | K |

Luminous intensity | Candela | Cd |

Electric current | Ampere | A |

Amount of matter | Mole | Mol |

These units are the 7 fundamental base units that are used in the SI system of units. Hence even though the quantities in the above table can be measured using other units, it is recommended that only these units are used.

The next part this series on derived units can be found here.

See you later my students.

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