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Planning an A-Level Physics Investigation

Plan A-Level Paper 4 investigations by defining variables, a workable method, measurement range, repeats, data processing and specific risk controls.

  • GCE A-Level H2 Physics 2027
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Learning objectives

  • Plan a practical investigation with controlled variables and a workable method

1. What a plan must establish

A practical plan is an argument that the proposed measurements can answer the stated physical question. It must connect the independent variable, dependent variable, control variables, measurement technique and processing method.

Paper 4 practical workflowFour connected stages show planning, manipulation and measurement, presentation and processing, then analysis, conclusion and evaluation.PPlanvariables, method,risk controlsMMOMeasurerange, repeats,techniquePDOPresenttable, graph,processingACEAnalyseconclusion, uncertainty,evaluationEvery stage must preserve the meaning of the original physical question.
Scroll diagram horizontally to read all labels.
A complete investigation links the question to a controlled method, defensible measurements, transparent processing and a conclusion whose limitations are evaluated.

2. Planning sequence

  1. State the relationship being tested. Name the independent and dependent variables with symbols and units.
  2. Choose a measurable range. Use enough well-spaced settings to reveal the trend without exceeding apparatus or safety limits.
  3. Control relevant variables. State how each important control is kept constant; a list without methods is incomplete.
  4. Describe how readings are obtained. Name the instrument, connection or alignment and the technique used to reduce measurement uncertainty.
  5. Include repeats where they add evidence. State how repeats are combined and how an anomalous result would be checked.
  6. Explain the processing. Name the graph or calculation and identify what its gradient, intercept or area represents, including units.
  7. Address risk. Link a real hazard to a control that reduces its likelihood or severity.

There is no universal requirement for a fixed number of readings or repeats. Choose and justify a range and repetition strategy suited to the apparatus, available time and expected scatter.

3. Variables and controls

RoleQuestion to answerExample for a pendulum investigation
Independent variableWhat is deliberately changed?pendulum length L
Dependent variableWhat is measured in response?period T
Control variableWhat else could change the result?release angle kept small and similar
Derived variableWhat is calculated before plotting?T²

“Keep conditions the same” is too vague. Write the control and the action: “Use the same bob so its shape and mass distribution remain unchanged.”

4. From theory to graph

For a simple pendulum at small amplitude, T = 2π square root of (L/g) .

Square the relationship: T² = (4π²/g)L.

A graph of T² against L should be linear, with gradient 4π²/g. The plan must state this before data collection so the table contains the quantities needed to test it.

5. Risk controls

A useful safety statement contains both parts:

  • Hazard: what can cause harm, such as a hot component, falling mass, stretched wire or high current.
  • Control: the specific action, such as limiting current and switching off between readings, securing a mass above a tray, or wearing eye protection while tensioning a wire.

Do not invent hazards unrelated to the proposed method. Follow the laboratory’s instructions and do not improvise apparatus outside supervised practical work.

5A. Common planning mistakes

  • Listing apparatus without explaining how it produces the required measurements.
  • Naming controls without stating how they are maintained.
  • Saying “repeat for accuracy”; repeats address random scatter, not systematic bias.
  • Choosing a graph but not identifying the physical meaning and unit of its gradient.
  • Giving a generic precaution such as “be careful” or “wear goggles” without a relevant hazard.

6. Worked Examples

Modelled example 1

Structuring a pendulum plan

Core

Problem

Outline the essential evidence and controls needed to determine g from a simple pendulum.
Study the worked solution

  1. Choose variables and range

    Method

    Vary pendulum length L over a practical, well-spaced range and measure from pivot to the bob’s centre.

    Reason

    L is the independent variable in T² = (4π²/g)L, and its definition must match the model.

    Working

    L: pivot to centre of bob

  2. Control the model conditions

    Method

    Use the same bob and keep the release angle small and similar.

    Reason

    This maintains mass distribution and the small-angle condition behind the stated relationship.

    Working

    θ₀ small and controlled

  3. Measure period reliably

    Method

    Time several complete oscillations, divide by their number, and repeat to calculate a mean T.

    Reason

    Timing multiple cycles reduces percentage timing uncertainty; repeats reveal random scatter.

    Working

    T = t_N/N; T bar = ∑ Tᵢ/n

  4. Linearise and plot

    Method

    Calculate T² and plot T² against L with a best-fit line.

    Reason

    The squared model predicts a linear graph whose gradient contains g.

    Working

    T² = (4π²/g)L

  5. Extract the result

    Method

    Find gradient m and calculate g = 4π²/m with units.

    Reason

    Comparing T² = mL with the model gives m = 4π²/g.

    Working

    g = 4π²/m

  6. Control the risk

    Method

    Keep the oscillation path clear and secure the support against toppling.

    Reason

    This addresses the moving-bob and unstable-support hazards in the proposed method.

    Working

    hazard → specific control

Next: Measurement and Observation