What are the basic units of measurement used in science for motion, forces, and energy?

In the IB MYP Science curriculum, the basic units of measurement for motion, forces, and energy include meters (m) for distance, seconds (s) for time, kilograms (kg) for mass, newtons (N) for force, and joules (J) for energy. These units are part of the International System of Units (SI), which is widely used in scientific studies to ensure consistency and accuracy.

How do you measure speed in scientific experiments?

Speed is measured by calculating the distance traveled over a period of time. In scientific experiments, this is often done using the formula: speed = distance/time. The units typically used are meters per second (m/s). Accurate measurements of distance and time are crucial for determining speed, and tools like stopwatches and measuring tapes are commonly used.

Why is it important to use standard units of measurement in science?

Using standard units of measurement, such as those in the SI system, is crucial in science because it allows for clear communication and comparison of results across different studies and disciplines. It ensures that measurements are consistent and reliable, which is essential for replicating experiments and validating scientific findings.

What tools are commonly used to measure force in scientific experiments?

In scientific experiments, force is commonly measured using a device called a force meter or a spring scale. These tools measure force in newtons (N) and are essential for experiments involving motion, forces, and energy. They work by measuring the amount of force required to stretch or compress a spring, which is then translated into a force measurement.

How is energy measured in the context of motion and forces?

Energy, particularly in the context of motion and forces, is measured in joules (J). In scientific experiments, energy can be calculated using various formulas depending on the type of energy being measured. For example, kinetic energy is calculated using the formula: KE = 0.5 * mass * velocity^2. Understanding how to measure and calculate energy is crucial for analyzing the effects of forces and motion in scientific studies.

Why is "Using accuracy and precision interchangeably." a common mistake in Measurements in Science?

Why it happens: They sound similar in everyday speech. How to avoid it: Accuracy = closeness to the true value. Precision = closeness of repeats to each other. A balance that always reads 5.0 g for everything is precise but inaccurate.

Why is "Believing that repeating a measurement removes systematic error." a common mistake in Measurements in Science?

Why it happens: Repeating clearly helps with random error, so students extend the idea. How to avoid it: Repeats average out random scatter but don't fix a fixed offset (like a +0.2 g zero error). Calibrate the instrument to remove systematic error.

Why is "Writing a measurement without units." a common mistake in Measurements in Science?

Why it happens: Numbers feel like a complete answer. How to avoid it: Every measurement = number + unit. "21" is not an answer; "21 mm" is.

Motion, forces and energyIB MYP Sciences (Years 1-5)

Measurements in Science

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Short Study Notes — Measurements in Science

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Measurements in Science — IB MYP Sciences: SI units, instruments, uncertainty

Reliable measurement is the bedrock of science. This MYP Sciences study note covers the seven SI base units, common prefixes (milli, centi, kilo, mega), how to choose the right instrument, the difference between precision and accuracy, and how to express uncertainty correctly — the skills MYP criterion C (Processing & Evaluating) expects you to demonstrate in every investigation.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Recall the SI base units for length, mass, time, temperature and electric current.
MYP Sciences A — Convert between prefixed units (e.g. 4 500 mm = 4.5 m).
MYP Sciences B — Choose an appropriate measuring instrument for a given quantity and justify the choice.
MYP Sciences C — Distinguish between accuracy and precision; identify systematic and random errors.
MYP Sciences C — Express a measurement with an appropriate uncertainty (e.g. 23.4 ± 0.1 cm).

SI base units and prefixes

Science uses a single international system of units so anyone, anywhere, can repeat your experiment.

Every measurement has a number and a unit. Without the unit, a result is meaningless.

The SI system (Système International d'Unités) defines seven base units. The five you will meet most often in MYP Sciences are:

Quantity	SI base unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Electric current	ampere	A

All other units (m/s, N, J, W, m³, m/s², g/cm³) are built from these. For example, speed combines length and time → m/s.

Prefixes scale units up or down so the number is easy to read:

Prefix	Symbol	Multiplier
nano	n	× 10⁻⁹
micro	µ	× 10⁻⁶
milli	m	× 10⁻³
centi	c	× 10⁻²
kilo	k	× 10³
mega	M	× 10⁶
giga	G	× 10⁹

So 5 km = 5 × 10³ m = 5 000 m. And 25 cm = 25 × 10⁻² m = 0.25 m. Always check that the unit on your final answer matches the unit examiners are asking for.

Five common SI base units: m, kg, s, K, A.
All derived units come from these (m/s, N = kg⋅m/s², J = N⋅m, etc.).
Prefixes are powers of ten — milli (10⁻³), kilo (10³), mega (10⁶).

Choosing the right instrument

Each instrument has a range it can read and a smallest division (its resolution).

Range is the smallest to largest value an instrument can read. Resolution is the smallest difference it can detect — the smallest scale division on a ruler, or the last digit on a digital display.

Quantity	Common instruments	Typical resolution
Length	ruler	1 mm
	vernier callipers	0.1 mm
	micrometer screw gauge	0.01 mm
Mass	top-pan balance	0.01 g
Time	stopwatch (digital)	0.01 s
Volume (liquid)	measuring cylinder	1 cm³
	burette	0.05 cm³
Temperature	thermometer (alcohol)	1 °C
Voltage	voltmeter (digital)	0.01 V
Current	ammeter (digital)	0.001 A (1 mA)

For MYP investigations, choose the instrument with a range that covers your expected values AND a resolution small enough that random fluctuations are larger than one scale division. Otherwise you cannot tell whether the variable has truly changed.

A ruler, a vernier calliper and a micrometer screw gauge shown side by side at different scales of resolution

Range = the smallest-to-largest values an instrument can read.
Resolution = the smallest difference it can detect.
Pick an instrument whose range covers the expected values AND whose resolution is smaller than the change you want to see.

Precision vs accuracy

Precision is repeatability; accuracy is closeness to the true value. They are independent.

Accuracy is how close a measurement is to the true value. A balance that reads exactly the correct mass is accurate.

Precision is how close a set of repeated measurements are to each other. A balance that always reads 5.00 g (whether or not that is correct) is precise.

You can have one without the other:

Four dartboards showing accurate and precise, accurate but not precise, precise but not accurate, and neither

The two main causes of inaccuracy/imprecision in MYP Sciences:

Random error — readings vary slightly each time. Reduced by repeating and averaging.
Systematic error — every reading is shifted by the same amount (e.g. a balance with a +0.2 g zero error). Cannot be reduced by repeating; must be corrected by recalibrating the instrument.

Accuracy = closeness to true value. Precision = closeness of repeats to each other.
Random error: small, scattered. Reduce by repeating and finding the mean.
Systematic error: same direction every time. Fix by calibration, not repetition.

Expressing uncertainty

Every measurement has an uncertainty — usually half the smallest scale division for an analogue instrument.

When you write a measurement, also write its uncertainty:

$\text{value} \pm \text{uncertainty (with units)}$

Analogue instruments (rulers, thermometers): the absolute uncertainty is normally ± half the smallest scale division. A ruler with mm divisions → ± 0.5 mm. A thermometer with 1 °C divisions → ± 0.5 °C.

Digital instruments: the absolute uncertainty is usually ± the smallest displayed digit. A balance reading 4.23 g → ± 0.01 g. A digital stopwatch reading 12.34 s → ± 0.01 s.

For repeated readings, calculate the mean and use half the range (½ × (largest − smallest)) as the absolute uncertainty. Two thermometer readings of 21.0 °C and 22.0 °C → mean 21.5 ± 0.5 °C.

Percentage uncertainty is useful when comparing two measurements of different sizes:

$\%\text{ uncertainty} = \frac{\text{absolute uncertainty}}{\text{measured value}} \times 100$

A measurement of 5.0 ± 0.1 cm has a 2% uncertainty; a measurement of 50.0 ± 0.1 cm has only 0.2%.

Analogue: uncertainty = ± half the smallest scale division.
Digital: uncertainty = ± the last displayed digit.
Repeats: uncertainty = ± ½ × (max − min).
Percentage uncertainty = absolute uncertainty ÷ value × 100.

How it’s examined

Criterion C (Processing & Evaluating) tests measurement skills in every investigation report. The eAssessment also has criterion A items where you identify a suitable instrument, convert between prefixed units, and evaluate the source and effect of random vs systematic error in a given scenario.

Sources: IB MYP Sciences guide (IBO, official subject guide); BIPM SI Brochure (definition of SI units). Last reviewed 2026-05-25.

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Step-by-step worked examples — Measurements in Science

Step-by-step solutions to past-paper-style questions on measurements in science, written exactly the way a tutor would explain them at the board.

1Converting between prefixed units
Getting started• units, prefixes
Question
Convert (a) 4 500 mm to metres, (b) 0.25 km to centimetres, (c) 320 mg to kilograms.
Step-by-step solution
1. Step 1
  (a) 1 mm = 10⁻³ m, so 4 500 mm × 10⁻³ = 4.5 m.
2. Step 2
  (b) 0.25 km × 1 000 m/km × 100 cm/m = 25 000 cm.
3. Step 3
  (c) 320 mg = 320 × 10⁻³ g = 0.320 g = 0.320 × 10⁻³ kg = 3.2 × 10⁻⁴ kg.
Answer
(a) 4.5 m; (b) 25 000 cm; (c) 3.2 × 10⁻⁴ kg.
2Choosing the right instrument
Getting started• instruments, resolution
Question
Which instrument would you use to measure the diameter of a 5 cent coin (about 21 mm), and why? Choose from a ruler (mm), a vernier calliper (0.1 mm) or a micrometer screw gauge (0.01 mm).
Step-by-step solution
1. Step 1
  A ruler reads to ± 0.5 mm — that's about 2% of 21 mm. The micrometer would only fit objects up to ~25 mm so it works but is overkill.
2. Step 2
  The best balance is a vernier calliper: range up to 150 mm comfortably covers 21 mm, and 0.1 mm resolution gives ~0.5% uncertainty.
Answer
Vernier calliper — its 0.1 mm resolution gives suitable precision for a 21 mm coin without the awkwardness of a tiny micrometer.
3Mean and uncertainty from repeats
Building confidence• uncertainty, repeats
Question
Three readings of the temperature of warm water are 41.0 °C, 41.4 °C and 41.2 °C. Find the mean and the absolute uncertainty.
Step-by-step solution
1. Step 1
  Mean = (41.0 + 41.4 + 41.2) ÷ 3 = 123.6 ÷ 3 = 41.2 °C.
2. Step 2
  Absolute uncertainty = ½ × (max − min) = ½ × (41.4 − 41.0) = 0.2 °C.
Answer
41.2 ± 0.2 °C.
4Percentage uncertainty
Stretch• uncertainty
Question
A length is measured as 5.0 cm ± 0.1 cm. Calculate the percentage uncertainty.
Step-by-step solution
1. Step 1
  % uncertainty = (absolute uncertainty ÷ value) × 100.
  $(0.1 / 5.0) × 100$
2. Step 2
  = 2%.
Answer
2%.

Key Formulae — Measurements in Science

The formulae you need to memorise for measurements in science on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Percentage uncertainty
$% uncertainty = (absolute uncertainty / value) × 100$
$absolute uncertainty$
± value, same units as the measurement
$value$
the measured quantity
When to use
When comparing uncertainties on measurements of different sizes, or when combining uncertainties through multiplication or division.
Uncertainty from repeats
$uncertainty = ½ × (max − min)$
$max$
largest reading in the set of repeats
$min$
smallest reading in the set
When to use
When you have 3+ repeated measurements and want a fair estimate of the spread.

Key Definitions and Keywords — Measurements in Science

Definitions to memorise and the exact keywords mark schemes credit for measurements in science answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

SI units
Examiner keyword
The international system of units used by scientists everywhere. The seven base units include the metre (m), kilogram (kg), second (s), kelvin (K) and ampere (A).
Range (of an instrument)
The smallest to largest value the instrument can read.
Resolution
Examiner keyword
The smallest difference an instrument can detect — usually the smallest scale division on an analogue instrument or the last digit on a digital one.
Accuracy
Examiner keyword
How close a measurement is to the true (accepted) value.
Precision
Examiner keyword
How close repeated measurements are to each other. A precise reading is not necessarily accurate.
Random error
Examiner keyword
Small unpredictable variations in repeated measurements. Reduced by taking more repeats and using the mean.
Systematic error
Examiner keyword
An error that shifts every reading by the same amount, e.g. a balance not zeroed. Cannot be reduced by repeating.
Uncertainty
Examiner keyword
The range within which the true value is expected to lie, given as ± a value with the same units as the measurement.

Common Mistakes and Misconceptions — Measurements in Science

The traps other students keep falling into on measurements in science questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Using accuracy and precision interchangeably.
Why it happens
They sound similar in everyday speech.
How to avoid it
Accuracy = closeness to the true value. Precision = closeness of repeats to each other. A balance that always reads 5.0 g for everything is precise but inaccurate.
✕Believing that repeating a measurement removes systematic error.
Why it happens
Repeating clearly helps with random error, so students extend the idea.
How to avoid it
Repeats average out random scatter but don't fix a fixed offset (like a +0.2 g zero error). Calibrate the instrument to remove systematic error.
✕Writing a measurement without units.
Why it happens
Numbers feel like a complete answer.
How to avoid it
Every measurement = number + unit. "21" is not an answer; "21 mm" is.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Measurements in Science — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Measurements in Science — IB MYP Sciences: SI units, instruments, uncertainty

Quantity

SI base unit

Symbol

Length

metre

Mass

kilogram

Time

second

Temperature

kelvin

Electric current

ampere

Prefix

Symbol

Multiplier

nano

× 10⁻⁹

micro

× 10⁻⁶

milli

× 10⁻³

centi

× 10⁻²

kilo

× 10³

mega

× 10⁶

giga

× 10⁹

Quantity

Common instruments

Typical resolution

Length

ruler

1 mm

vernier callipers

0.1 mm

micrometer screw gauge

0.01 mm

Mass

top-pan balance

0.01 g

Time

stopwatch (digital)

0.01 s

Volume (liquid)

measuring cylinder

1 cm³

burette

0.05 cm³

Temperature

thermometer (alcohol)

1 °C

Voltage

voltmeter (digital)

0.01 V

Current

ammeter (digital)

0.001 A (1 mA)

When you write a measurement, also write its uncertainty:

$\text{value} \pm \text{uncertainty (with units)}$

Digital instruments: the absolute uncertainty is usually ± the smallest displayed digit. A balance reading 4.23 g → ± 0.01 g. A digital stopwatch reading 12.34 s → ± 0.01 s.

Percentage uncertainty is useful when comparing two measurements of different sizes:

$\%\text{ uncertainty} = \frac{\text{absolute uncertainty}}{\text{measured value}} \times 100$

A measurement of 5.0 ± 0.1 cm has a 2% uncertainty; a measurement of 50.0 ± 0.1 cm has only 0.2%.

Measurements in Science

Short Study Notes in the form of Slides

Short Study Notes — Measurements in Science

Detailed Notes

What you’ll learn

How it’s examined

1Converting between prefixed units

2Choosing the right instrument

3Mean and uncertainty from repeats

4Percentage uncertainty

Percentage uncertainty

Uncertainty from repeats

SI units

Range (of an instrument)

Resolution

Accuracy

Precision

Random error

Systematic error

Uncertainty

✕Using accuracy and precision interchangeably.

✕Believing that repeating a measurement removes systematic error.

✕Writing a measurement without units.

Practice questions

Past paper style quiz

Assess your understanding

Measurements in Science — frequently asked questions

Measurements in Science

Short Study Notes in the form of Slides

Short Study Notes — Measurements in Science

Detailed Notes

What you’ll learn

How it’s examined

1Converting between prefixed units

2Choosing the right instrument

3Mean and uncertainty from repeats

4Percentage uncertainty

Percentage uncertainty

Uncertainty from repeats

SI units

Range (of an instrument)

Resolution

Accuracy

Precision

Random error

Systematic error

Uncertainty

✕Using accuracy and precision interchangeably.

✕Believing that repeating a measurement removes systematic error.

✕Writing a measurement without units.

Practice questions

Past paper style quiz

Assess your understanding

Measurements in Science — frequently asked questions