1.1 Physical quantities
- Syllabus
- 9702–2028–2029
- Topic
- 1.1
- Level
- AS
Learning objectives
A physical quantity combines a numerical magnitude with a unit
Every physical quantity is written as a numerical magnitude together with a unit. In 7.0 N, 7.0 is the magnitude and N is the unit of force.
The magnitude tells how many of the chosen unit make up the quantity. Changing unit changes the number but not the physical quantity: 2.5 km and 2500 m describe the same distance.
A number alone or a unit alone is not a complete physical quantity. The unit does not have to be an SI base unit, and the magnitude does not have to be written in scientific notation.
Estimate a physical quantity from familiar scales and relationships
A reasonable estimate is a reasoned approximate value, not a random guess. It combines a familiar scale or physical relationship with a suitable unit and sensible order of magnitude.
- Identify the quantity and unit.
- Choose familiar values or a relationship that links the quantity to known values.
- Round to simple numbers and calculate.
- Check whether the order of magnitude is physically plausible.
To estimate an adult’s weight, use a mass of about 70 kg and g ≈ 10 N kg⁻¹. From W = mg, W ≈ 70 × 10 = 700 N = 7 × 10² N. A value near 7 N is too small for an adult, while 7 × 10⁴ N is too large.
An estimate should communicate scale, not false precision. Extra decimal places do not make an estimate more reliable.