Hello! In the last lesson, you identified the result measured in an investigation as its dependent variable. If that result is “how tall a seedling grows,” you still need to decide how to measure its height and which unit to write beside the number.
This lesson is about making those choices. You will match common quantities to suitable instruments and SI units, while learning why the unit printed on an instrument is sometimes different from the SI unit requested in a science question. Allow about 35–40 minutes for the lesson and its short resources.
A measurement needs a quantity, an instrument, and a unit
A quantity is something measurable, such as length, mass, time, or temperature. An instrument obtains the reading; a unit tells you what the number means. “The seedling grew 12” is incomplete, but “The seedling is 12 cm tall” identifies a length.
Scientists use the International System of Units, or SI, so that measurements can be communicated consistently. Watch the opening of this video for the basic SI units. It names some quantities you will study much later; for now, concentrate on length, mass, time, temperature, and electric current.
SI Base Units and Derived Units - Physics and Chemistry
“SI Base Units and Derived Units – Physics and Chemistry,” from The Organic Chemistry Tutor, introduces the standard units behind scientific measurements.
Watch the SI introduction. Listen for metre, kilogram, second, kelvin, and ampere, and notice that the name of a quantity is different from the name of its unit.
Now read the two short subsections of Pearson’s Physics Study Notes: Measurement of Physical Quantities. They put instruments beside the quantities they measure. You do not need the later material on reading specialised scales.
Physics Measurements Study Guide: Tools, Errors & Units | Notes
Pearson’s notes provide a compact reference for common quantities, units, and measuring apparatus.
Under “Measurement of Physical Quantities,” read “Basic Quantities and SI Units” and “Measurement Apparatus.” Begin at quantities and apparatus, ending with the volume instruments. Focus on the everyday alternatives listed beside the SI units: these explain why an instrument might display centimetres, grams, or degrees Celsius.
The image below shows five of these instruments. Each has a different job, even though several display a numerical reading.
The crucial distinction is this: choose the instrument for the quantity, then state the appropriate unit. A digital balance might display grams, for example, even though the SI base unit of mass is the kilogram. Do not replace a unit actually shown on an instrument with another unit unless you also convert the number.
Length and time: choosing an instrument that fits
Return to the seedling investigation. Its height is a length, so a ruler or metre rule is suitable. A flexible measuring tape is more useful for measuring a person’s height or the distance around a larger object. The SI base unit of length is the metre, symbol m. For a small seedling, it is convenient to record centimetres (cm); if a question specifically asks for the SI base unit, give metres.
For the time taken for a ball to bounce, a stopwatch is more suitable than a wall clock because you need the duration of a short event. The SI base unit of time is the second, symbol s. A clock is useful for longer periods or times of day; a stopwatch or timer is useful when you must start and stop a measurement for an event.
The BBC Bitesize reading develops these choices with examples of short and long distances and durations.
Physical quantities and measurement techniques - IGCSE Physics - BBC Bitesize
BBC Bitesize explains which length and time instruments suit different scales of measurement.
In “Using standard units,” go to the subsection “Units of length and time.” Read length and time. Start with the explanation of standard units and stop after the paragraph about clocks and stopwatches; the following “Average values” subsection belongs to the next lesson. Notice why a ruler, measuring tape, measuring wheel, and stopwatch are not interchangeable.
Instrument choice depends on size and purpose, not just on the quantity’s name. Both a ruler and a measuring wheel measure length, but a ruler is suited to a seedling while a measuring wheel is suited to a much longer distance.
Mass and temperature: watch for familiar-looking units
A balance—such as a beam balance or electronic balance—measures mass. Mass concerns how much matter an object contains. Its SI base unit is the kilogram (kg), although a balance used for a small object may display grams (g). The relationship is:
A thermometer measures temperature. The SI base unit is the kelvin (K). Many laboratory and everyday thermometers instead show degrees Celsius. Both describe temperature, but their numbers are not interchangeable: you cannot write “25 K” simply because a thermometer reads . When a question asks for an instrument, “thermometer” is enough; when it asks for the SI base unit, answer “kelvin.”
One distinction prevents a common error: mass is not weight. Weight is the force of gravity on an object. If the question asks for weight, a spring balance or force meter is appropriate and the SI unit is the newton (N). If it asks for mass, choose a balance and kilograms. Read the quantity named in the question before choosing an instrument.
Volume and electric current
Volume describes the space something occupies. For a liquid, choose a measuring cylinder rather than a balance: the cylinder measures volume, while the balance measures mass. Choose a cylinder with a scale suited to the amount of liquid being measured.
The SI unit of volume is the cubic metre, written . A laboratory measuring cylinder is usually marked in millilitres (mL) or cubic centimetres, written . Those smaller units are much more practical for a small sample. In particular,
Read the relevant part of BBC Bitesize to see how a measuring cylinder can also help find the volume of an irregular solid, such as a stone.
Physical quantities and measurement techniques - IGCSE Physics - BBC Bitesize
This second part of the BBC Bitesize article connects units of volume to measuring liquids and irregular solids.
In “Units of area and volume,” find the paragraph beginning “Volume measures the space inside a 3-dimensional (3D) object.” From its sentence about standard units, read measuring volume, stopping after the explanation of water displaced by a solid. Focus on why the displaced water can be measured in a cylinder. You can leave the following gold-bar calculation and unit-conversion sections for another time.
The ammeter in the image measures electric current. The SI base unit of current is the ampere (A). You do not need to know how to connect an ammeter yet; that belongs with circuits later in the course. For now, keep the pairing clear: current—ammeter—ampere.
Here is a reference for the quantities covered so far:
| Quantity | Suitable instrument | SI unit |
|---|---|---|
| Length | Ruler, metre rule, or measuring tape | metre (m) |
| Mass | Beam or electronic balance | kilogram (kg) |
| Time interval | Stopwatch or timer | second (s) |
| Temperature | Thermometer | kelvin (K) |
| Volume of a liquid | Measuring cylinder | cubic metre () |
| Electric current | Ammeter | ampere (A) |
| Weight | Spring balance or force meter | newton (N) |
The volume row deserves particular care: is the SI unit, but a measuring cylinder for a small sample will normally be marked in mL or . Similarly, cm, g, and degrees Celsius are useful in everyday laboratory work even when a question asks you to name the SI base unit.
Putting the choice together
When you meet a measurement question, first identify exactly what is being measured. “The height from which a ball is dropped” and “the time before it hits the ground” concern different quantities, even though they describe the same investigation. Next, choose an instrument suited to that quantity and the size of the measurement. Finally, check whether the question wants the unit used for a practical reading or the SI unit.
In last lesson’s ball investigation, for example, a ruler or tape measure could measure drop height in centimetres or metres. A stopwatch could measure the time taken in seconds. Neither instrument measures how high the ball bounces automatically; you must decide which height you intend to record and place the measuring instrument accordingly. Being precise about the dependent variable makes the instrument choice much easier.
Key takeaways
A complete measurement states a number and a unit. Match the instrument to the quantity: a balance for mass, thermometer for temperature, measuring cylinder for liquid volume, stopwatch for a short time interval, and an appropriately sized ruler or tape for length. Know the principal SI pairings, but also recognise that practical instruments often display smaller, convenient units.
Next, you will use repeated measurements to calculate an arithmetic mean. The unit remains important there: an average of several heights is still a height and must still be reported with a length unit.