A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Fundamental and Derived Units in Physics — mark scheme explained
The short answer
In physics, measurements are essential for understanding the natural world. The International System of Units (SI) provides a standardized framework for these measurements. This explainer will cover fundamental units, derived units, SI prefixes, and unit conversions.
The question
A car travels at a speed of 80 km/h. Convert this speed to metres per second (m/s).
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For conversion questions, award marks for identifying the correct conversion factor, performing the calculation correctly, and providing the final answer with the correct units. For derived unit questions, ensure students show their working and use the appropriate base units.
What the command words demand
- Convert
- Change a quantity from one unit to another using the appropriate conversion factor.
- Calculate
- Perform a mathematical operation to find a specific value, ensuring units are consistent.
- Identify
- Recognize and name the correct SI base or derived unit for a given physical quantity.
- Explain
- Provide a clear and concise explanation of how to convert between units or use SI prefixes.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 2-3 minutes per mark to ensure you have enough time to carefully read the question, perform calculations, and check your answers.
- Identify the conversion factor between kilometres and metres.1 mark1 kilometre = 1000 metres
- Identify the conversion factor between hours and seconds.1 mark1 hour = 3600 seconds
- Convert the speed from km/h to m/s using these factors.2 marks80 km/h × (1000 m/km) ÷ (3600 s/h) = 22.22 m/s
Final answer: 22.22 m/s
Work through every step correctly and you earn all 4 marks.
Another worked example
A force of 50 N is applied to an object, causing it to accelerate at 10 m/s 2 . Calculate the mass of the object.
- Recall Newton's second law of motion.0 marksF = ma
- Rearrange the equation to solve for mass (m).1 markm = F/a
- Substitute the given values into the equation.1 markm = 50 N ÷ 10 m/s 2
- Calculate the mass.1 markm = 5 kg
Final answer: 5 kg
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing the base unit for mass with grams instead of kilograms.
Why it happens: Students often use grams (g) in everyday life, leading to confusion with the SI base unit, which is kilograms (kg).
Fix: Always remember that the SI base unit for mass is the kilogram (kg), not grams (g).
Forgetting to convert units before performing calculations.
Why it happens: Students sometimes forget to ensure all units are consistent before performing calculations, leading to incorrect results.
Fix: Always check and convert units to a common standard (e.g., metres, kilograms, seconds) before performing any calculations.
Using the wrong conversion factor for unit conversions.
Why it happens: Students may use incorrect or approximate conversion factors, leading to significant errors in their calculations.
Fix: Memorize and use the correct conversion factors. For example, 1 km = 1000 m, 1 hour = 3600 seconds, 1 eV = 1.602 × 10 -19 J.
Misusing SI prefixes in calculations.
Why it happens: Students may incorrectly apply or forget to use the appropriate power of ten when using SI prefixes, leading to errors in their results.
Fix: Always double-check the correct power of ten for each SI prefix. For example, 1 μA = 10 -6 A, 1 kN = 10 3 N.
Failing to use standard form for very large or small numbers.
Why it happens: Students may write out very large or small numbers in full, which can be cumbersome and prone to errors.
Fix: Use standard form (scientific notation) to express very large or small numbers. For example, 300,000,000 m/s should be written as 3 × 10 8 m/s.
Confusing the derived unit for energy with power.
Why it happens: Students may confuse the units of energy (joules) and power (watts), leading to incorrect calculations and answers.
Fix: Remember that energy is measured in joules (J) and power is measured in watts (W). Energy = Power × Time. For example, 1 kWh = 3.6 × 10 6 J.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Unit Conversion | Convert a speed from kilometres per hour into metres per second. | 4 |
| Calculate Mass | Use Newton's second law to find an object's mass from force and acceleration. | 3 |
| Unit Conversion | Convert a current from microamperes to amperes using the correct conversion factor. | 2 |
| Unit Conversion | Convert an energy value from joules into kilowatt-hours using the correct conversion factor. | 2 |
| Calculate Energy | Convert time to seconds, then multiply power by time to find energy in joules. | 3 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.