An "explain" question in A-Level Physics 9702 wants a reason, not a description. Name the physics that links the cause to the effect, then say what follows from it. Write one separate point for each mark on offer, use the correct terms, and never simply restate the question in other words.
Read the command word first
Command words tell you what kind of answer earns marks. These are Cambridge's own meanings, taken from the 9702 syllabus:
| Command word | What Cambridge says it means |
|---|---|
| State | Express in clear terms |
| Describe | State the points of a topic, giving characteristics and main features |
| Explain | Set out purposes or reasons, make the relationships between things evident, and say why and/or how, supported by relevant evidence |
| Suggest | Apply knowledge and understanding to situations where there are a range of valid responses |
| Show (that) | Provide structured evidence that leads to a given result |
If the word is "explain", the question has already told you it wants a mechanism. A neat description of what happens will not earn the reasoning marks. For the full list, see Cambridge command words explained.
The cause and effect chain
A strong explanation connects a cause to an effect through a named piece of physics. A reliable pattern is:
- Name the principle, law or relationship that applies.
- Say what changes (the cause).
- Say what that change produces (the effect).
Example in the style of an exam question: explain why a metal wire extends more when the load on it increases. A strong answer says that, up to the limit of proportionality, extension is proportional to the applied force (Hooke's law, F = kx), so a larger load produces a larger extension. That names the law, the cause and the effect in one chain.
One point per mark
Mark schemes list separate creditable points. A 3 mark question needs three different statements. A flowing paragraph that says the same thing three ways can be worth one mark.
A quick check after writing: count the distinct physics ideas. If a 3 mark answer contains fewer than three, you probably have a mark missing. Reading how Cambridge mark schemes work shows how those points are laid out.
Use precise vocabulary
Everyday phrases can cost marks even when the idea underneath is right.
| Vague phrasing | More precise phrasing |
|---|---|
| The current gets used up | The current is the same at every point in a series circuit; it is energy that is transferred |
| It speeds up | The resultant force produces an acceleration |
| The energy is lost | Energy is transferred to the surroundings as thermal energy |
| It gets bigger | The quantity increases in proportion to the other quantity |
Worked example 1: circular motion
Question in the style of an exam: explain why a body moving in a circle at constant speed is accelerating.
Weak answer: "It is accelerating because it is moving in a circle." That restates the question and shows no physics.
Stronger answer: "Velocity is a vector, so it has direction as well as size. The direction of the velocity is continuously changing, so the velocity is changing. A changing velocity means there is an acceleration, and it is directed towards the centre of the circle." That gives separate points: velocity includes direction, direction changes, so velocity changes, so there is acceleration towards the centre. There is no calculation, because the marks are for reasoning about vectors.
Worked example 2: terminal velocity
Question in the style of an exam: explain why a ball falling through the air eventually reaches a constant velocity. The 9702 syllabus expects you to describe and explain qualitatively the motion of objects falling with air resistance, so this is fair game.
- The weight of the ball is constant and acts downwards.
- The drag force acts upwards and increases as the speed increases.
- The resultant force, which is weight minus drag, therefore decreases, so the acceleration decreases.
- When drag equals weight the resultant force is zero, so the acceleration is zero and the velocity is constant.
Each line is a separate point. Notice the sequence: cause (speed rises), effect (drag rises), consequence (resultant force falls), end state (zero resultant force, so constant velocity).
Using the number of marks as a guide
The marks beside a question are a budget. A 2 mark explain question wants two linked points, and a 4 mark question wants about four. If you have written one point and the question is worth three, ask yourself what else must be true: what causes the change, what quantity changes as a result, and what that means for the thing the question asks about. If you have written six points for a two mark answer, you are spending time you may need later in the paper.
Common traps
- Restating the question. If the answer could be written without knowing any physics, it scores nothing.
- Describing when asked to explain. A list of what happens misses the why.
- Contradicting yourself. A correct statement next to an incorrect one can cancel a mark.
- Padding. Repeating a point in different words fills the page but not the mark scheme.
- Missing the link word. Words like "so", "therefore" and "because" show the examiner you are reasoning, not listing.
A short checklist
- Did I name a principle, law or equation?
- Did I link a cause to an effect?
- Do I have at least one distinct point per mark?
- Did I use physics terms rather than everyday words?
If you want to practise projectile reasoning specifically, projectile motion explained has worked examples where the explanation matters as much as the number.
Practise explain questions on real papers
Open an A Level Physics past paper, write your explanations by hand and have them marked against the official mark scheme, so you can see which reasoning points scored. Reading and downloading papers is free, and a free account adds AI marking and Pal, which gives hints rather than answers. The 2023 and 2024 papers are marked first, with earlier years being added.
Frequently asked questions
- What is the difference between describe and explain in 9702?
- Cambridge defines describe as stating the points of a topic and giving its main features and characteristics. Explain means setting out reasons and making the relationships between things clear, supported by evidence. If you only describe when the question says explain, you miss the reasoning marks.
- How many points should I write for a 3 mark explain question?
- Aim for at least three distinct physics points, one for each mark, linked clearly together. Mark schemes award marks for separate creditable points, so three different statements beat one long paragraph that repeats the same idea.
- Do I need equations in explain questions?
- Not always, but naming the relevant relationship often supplies a point. Saying that acceleration is proportional to resultant force shows the mechanism even when no calculation is asked for.
- What does show that mean in 9702?
- Cambridge defines show that as providing structured evidence that leads to a given result. You must show the working that reaches the printed value, not just state it.