### Aims…

**3.1** use the following units: degree (o), hertz (Hz), metre (m), metre/second (m/s), second (s).

**General Ideas…**

A **physical quantity **is something that can be measured. For any measurement, the unit being used must be stated to give an understanding of the scale of the measurement.

For example, distance can be measured in **kilometres** or in miles. They are similar, but not the same and it is important to identify which was used for the measurement, to know how far the distance actually is.

**Système Internationale d’Unités**

The units that scientists use all over the world are standardised in the Système Internationale d’Unités – **SI units**. It is important to remember these **six fundamental **(or ‘base’) units of measurement:

**metre**(m) – unit of length**kilograms**(kg) – unit of mass**second**(s) – unit of time**ampere**(A) – unit of electrical current**kelvin**(K) – unit of temperature**mole**(mol) – unit of the amount of substance

There are many quantities scientists **measure** that come from the base units. These derived units are very useful to quote as **measurements**, but they are not **fundamental** as they come from **fundamental** units.

For example, **frequency** is the number of times something happens per unit of time.

This is a useful quantity, but it is a **division** into the **time unit**.

The standard unit for **frequency** is considering the number **‘per second**‘, which is called **‘hertz**, Hz’, but this comes from the **fundamental** unit **‘second**‘.

Name | Unit | Abbreviation |

Frequency | hertz | Hz |

Force | newton | N |

Energy | joule | J |

Power | watt | W |

Pressure | pascal | Pa |

Electric charge | coulomb | C |

Work Done or Work | Nm or J | W |

Gravity or acceleration | Nkg^{-1}ms^{-2} | g or a |

Momentum | kgms^{-1} | p |

**Kilogram (kg): **the SI unit of mass, equivalent to the international standard kept at Sèvres near Paris (approximately 2.205 lb).

**Metre (m):** the fundamental unit of length in the metric system, equal to 100 centimetres or approximately 39.37 inches.

**Second (s): **the SI base unit of time which is used to determine progression of the universe. Defined from a complex atomic transition or as a division of an Earth day.

**Degree (****°****): **this is a way to measure an angle. With 360° in a circle (same as 2p radians) we can use this to measure angles from the normal from a surface to explain wave behaviours

**Speed: **the rate at which someone or something moves or operates or is able to move or operate in m/s or ms^{-1}

**Frequency: **This is a useful quantity, but it is a **division** into the **time ****unit**. The standard unit for **frequency** is considering the number **‘per second**‘, which is called **‘hertz**, Hz’, but this comes from the **fundamental** unit **‘second**‘.

**Period: **T or time for 1 whole cycle of a wave measured in s or hours etc..

### Resources….

Use this PowerPoint for a quick review… 3 Waves Part A Units