Experiment Planning

CIE A-Level Physics· Unit 15: Practical skills (AS)· 15 min read

1. Variables and Apparatus Selection★★☆☆☆⏱ 4 min

📘 Definition

Core Experimental Variables

All valid experiments have three key variable types: independent (changed by the experimenter), dependent (measured for effect), and control (kept constant to avoid confounding results)

Example:

For testing how extension changes with load, load is independent, extension is dependent

The first step in any plan is explicitly stating your variables — this is almost always worth 2-3 marks, so never skip this step. When selecting apparatus, you need to justify your choice based on measurement resolution, matching the apparatus to the expected range of measurements.

📐 Worked Example

For an experiment investigating how spring extension depends on added load, identify variables and select appropriate apparatus.

1
Identify all variables clearly:
2
- Independent variable: Load (force) added to the spring
- Dependent variable: Extension of the spring
- Control variables: Spring used, environmental temperature
3
Select appropriate apparatus with suitable resolution:
4
Force is measured with 0.1 N resolution slotted masses, appropriate for the 0-10 N range. Extension is measured with a 1 mm resolution metre rule, suitable for extensions of 1-10 cm. Additional apparatus: clamp stand, boss, set square to reduce parallax.

Exam tip:

Always mention the resolution of your apparatus if asked to justify your choice, this is a common hidden marking point.

2. Method and Control of Variables★★★☆☆⏱ 5 min

A good method is written as a clear step-by-step procedure that another person could follow exactly. You must explicitly state how you control variables that could affect results, and how you collect repeat data to reduce random error.

📘 Definition

Repeat Measurements

Multiple measurements of the same dependent variable for a fixed independent variable, used to reduce random error and calculate uncertainty in results.

📐 Worked Example

Write the method for the spring and load experiment, including control of variables.

1
1. Clamp the spring vertically to the stand, use a set square to align the rule vertically, measure and record the original unloaded length of the spring.
2
1. Add the first slotted mass, wait 30 seconds for the spring to reach equilibrium, then measure the new extended length.
3
1. Calculate extension = new length - original length, record the value. Remove the mass, repeat the measurement twice more for the same load to get three repeats.
4
1. Repeat steps 2-3 for at least 6 different load values across the 0-10 N range. Keep the same spring and measurement technique for all readings to control all other variables.

Exam tip:

Planning to take at least 6 readings for the independent variable and repeats for the dependent variable are almost always guaranteed marks, so never forget these.

3. Planning Data Analysis★★★☆☆⏱ 4 min

Most planning questions require you to explain how you will plot a graph and draw a conclusion. You must state what to plot on each axis, how you will test the given relationship, and how to calculate any unknown constant from your graph.

📐 Worked Example

How do you analyse the spring experiment results to find the spring constant $k$ and confirm Hooke's law $F = k x$ ?

1
1. Calculate the mean extension for each load value from your three repeat measurements.
2
1. Plot a graph with:
3
$F \text{ (load)} \text{ on the y-axis}, \quad x \text{ (mean extension)} \text{ on the x-axis}$
4
1. From Hooke's law $F = k x$ , the gradient of the straight line of best fit is equal to $k$ :
5
$k = \frac{\Delta F}{\Delta x} = \text{gradient}$
6
1. Confirm Hooke's law if all points lie close to a straight line that passes through the origin.

4. Error, Uncertainty and Safety★★★☆☆⏱ 2 min

You will almost always be asked to identify sources of uncertainty and relevant safety precautions. These are easy marks that you should never leave blank.

📐 Worked Example

Identify one major source of uncertainty and one relevant safety precaution for the spring experiment.

1
Major source of uncertainty: Parallax error when reading the end of the spring on the rule. This is reduced by using a set square aligned to the end of the spring to take the reading.
2
Safety precaution: Overloading the spring can cause it to snap and recoil violently. Wear safety goggles to protect your eyes from flying debris if the spring breaks.

5. Common Pitfalls

Wrong move:

Failing to explicitly state each variable type, only mentioning what you measure.

Why:

Mark schemes always award separate marks for correctly identifying independent, dependent and control variables.

Correct move:

Start your plan by clearly stating each variable by type.

Wrong move:

Planning fewer than 6 different readings for the independent variable.

Why:

A small range of data cannot produce an accurate graph or conclusion, and loses marks for insufficient data.

Correct move:

Always plan for at least 6 different values across the full available range of the independent variable.

Wrong move:

Forgetting to state what to plot on each graph axis.

Why:

This is a common 2-mark requirement that many students miss, even when they understand the relationship.

Correct move:

Always explicitly state which variable goes on each axis, and link the gradient or intercept to any required constant.

Wrong move:

Stating vague "human error" as a source of uncertainty.

Why:

Vague sources of error do not earn marks; you need to identify the specific cause.

Correct move:

Name the specific source, e.g. "parallax error when reading the rule" instead of just "human error".

Wrong move:

Including generic irrelevant safety precautions.

Why:

Mark schemes only award marks for safety precautions relevant to the specific experiment.

Correct move:

Identify the actual hazard (e.g. snapping spring, falling mass, hot wire) and give a matching precaution.

6. Quick Reference Cheatsheet

Planning Section	Required Marked Points
Variables	State independent, dependent, 1-2 control variables
Apparatus	List all, state resolution to justify choice
Method	≥6 readings, repeat measurements, control variables
Analysis	State axes, link gradient/intercept to constant, confirm relationship
Uncertainty	Name 1 specific source, explain how to reduce it
Safety	1 relevant hazard + matching precaution

7. Frequently Asked

How many marks is a planning question worth?

Planning is worth 15-20 marks out of 40 total for AS Paper 3, making up almost half the paper.

Do I need to draw a diagram for my plan?

Yes, a clear labelled diagram of your apparatus is almost always required and will earn you marks.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 3
Plan spring constant experiment
2023 · 3
Plan wire resistivity experiment
2024 · 3
Plan pendulum g experiment

Going deeper

syllabusCIE 9702 AS Practical SyllabusOfficial full planning requirements

What's Next

Experiment planning is the foundation of all practical work in CIE A-Level Physics, and the skills you learn here carry over to A2 practical work and analysis questions in written papers. Mastering planning gives you a huge advantage on Paper 3, where it makes up almost half the total marks. The skills of critical thinking about variables, error, and experimental design also help you answer unexpected questions across all physics papers. After mastering core planning, you can move on to deeper study of uncertainty, graph analysis and experiment evaluation.