Physics (Combined)

Forces and Motion

# Units

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Units

**Understanding Units**

- Units are standard quantities used to specify the measure of a particular physical quantity.
- The basic units we use are determined by the
**International System of Units (SI)**. - Every physical quantity, from displacement to force, is measured in terms of these base units.

**Units Used in Forces and Motion**

**Displacement**is measured in**metres (m)**.**Time**is measured in**seconds (s)**.**Speed and Velocity**are measured in**metres per second (m/s)**.**Acceleration**is measured in**metres per second squared (m/s²)**.**Mass**is measured in**kilograms (kg)**.**Force**is measured in**newtons (N)**.

**Unit Conversions**

- Always remember to convert units where necessary. For example, if time is given in minutes, convert to seconds for calculations involving speed or acceleration.
- Likewise, convert kilometres to metres when necessary.

**Compound Units**

- Some units are compound units – they are derived from more than one base unit. For example, the
**Newton**is a compound unit derived from the equation F=ma (Force equals mass times acceleration). Therefore 1 Newton equals 1 kg m/s². - Another common compound unit in Forces and Motion is
**metres per second squared (m/s²)**, used for acceleration.

**The Importance of Units**

- Always include units in your answer unless the question specifies otherwise, as marks often are awarded for correct use of units.
- When carrying out calculations, ensure the units you're using are consistent throughout in order to avoid errors.

**Unit Prefixes**

- Some units may have prefixes such as kilo (k), mega (M), giga (G), micro (μ), nano (n) and so on. These alter the base unit by a particular factor.
- For instance, 1 kilogram (kg) equals 1000 grams and 1 millimetre (mm) equals 0.001 metres.