What is a physical quantity?

What is a physical quantity?

In physics, a physical quantity is a property of an object or phenomenon that can be measured using an instrument. For example: the length of a table, the mass of a bag, the duration of a journey, the temperature of water.

Please note: not everything is a physical quantity! The beauty of a landscape or the taste of a dish cannot be measured with an instrument: these are subjective experiences, not physical quantities.

Quantity and value: two different things

A quantity is a general concept (length, mass, etc.). A value is the concrete result of a measurement: a number associated with a unit. For example, for the quantity ‘length’, the measured value might be 1.5 m.

A value without a unit means nothing! Writing ‘the table measures 2’ is incomplete: 2 what? 2 metres? 2 centimetres?

Examples of common quantities

Quantity	Common unit	Symbol	Instrument
Length	metre	m	ruler, tape measure
Mass	kilogram	kg	scales
Duration	second	s	stopwatch
Temperature	degree Celsius	deg C	thermometer

A common pitfall to avoid

People often confuse mass and weight in everyday language (“I weigh 50 kg”). In physics, mass (in kg) measures the amount of matter, whereas weight is a force (in newtons) that depends on gravity. At secondary school, the main thing to remember is that you must always specify the unit of a measurement; never give a number on its own.