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.