Physical quantities, units and measurement
Start G2 Science Physics with SI units, prefixes, physical scales and vectors, then choose and read suitable measuring instruments.
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Before you begin
Every measurement in physics is a number with a unit, and a number on its own means nothing. The lessons start with physical quantities, their SI units and the prefixes that make very large and very small values easy to write, compare the sizes of things from an atom to the Earth, and tell scalars from vectors. They finish by choosing a measuring instrument with a suitable range and precision and using it carefully.
Learning goals
- Represent a physical quantity with a numerical magnitude and unit
- Recall the six prescribed SI base quantities and their units
- Use the prescribed SI prefixes from nano to tera
- Compare orders of magnitude from a typical atom to the Earth
- Distinguish scalar and vector quantities and give examples
- Select and justify measuring instruments by range and precision
Syllabus statements covered
- show an understanding that physical quantities typically consist of a numerical magnitude and a unit
- recall the following base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K), amount of substance (mol)
- use the following prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units: nano (n), micro (µ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T)
- show an understanding of the orders of magnitude of the sizes of common objects ranging from a typical atom to the Earth
- select and explain the use of appropriate measuring instruments to measure or determine physical quantities listed in “Summary of key quantities, symbols and units” taking into consideration the range and precision of the instrument
- state what is meant by scalar and vector quantities and give common examples of each
Lessons
Work through them in order.
- Physical quantities, units and scaleWrite quantities with SI units and prefixes, compare sizes from an atom to the Earth by powers of ten, and sort quantities into scalars and vectors.
- Measuring instruments: range and precisionChoose an instrument by its range and precision, read it correctly, and use repeated readings, many oscillations or a stack to get a reliable value.
Practise and check
Or choose
Topic reference
Quick reference
| Base quantity | SI unit |
|---|---|
| mass | kilogram, kg |
| length | metre, m |
| time | second, s |
| current | ampere, A |
| temperature | kelvin, K |
| amount of substance | mole, mol |
| Prefix | Symbol | Factor | Prefix | Symbol | Factor |
|---|---|---|---|---|---|
| nano | n | 10⁻⁹ | kilo | k | 10³ |
| micro | µ | 10⁻⁶ | mega | M | 10⁶ |
| milli | m | 10⁻³ | giga | G | 10⁹ |
| centi | c | 10⁻² | tera | T | 10¹² |
| deci | d | 10⁻¹ |
Core knowledge to remember
- A physical quantity consists of a numerical magnitude and a unit.
- A scalar quantity has magnitude only, such as mass, distance, speed and time. A vector quantity has magnitude and direction, such as force, velocity, acceleration and weight.
- Choose a measuring instrument by its range, so that it can measure the whole quantity, and its precision, the smallest change it can show.
Common mistakes
- Writing a value without its unit.
- Applying a prefix to the number but not the unit, or converting cm² or cm³ as if they were cm.
- Calling speed a vector, or velocity a scalar.
- Choosing an instrument that is more precise than needed but cannot cover the whole range, or the other way round.