A student investigates how the extension of a spring varies with the force applied to it. She repeats each measurement three times and calculates a mean extension for each force value. Explain why repeating measurements and calculating a mean improves the quality of the data collected, and describe one additional step the student should take to check the validity of her results.

Edexcel A-Level Physics (9PH0) — 1.1 Practical and investigative skills · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student hangs different weights from a spring and measures the extension each time. She repeats each measurement three times.

Model answer (5 marks)

1. Repeating each measurement reduces the influence of random errors and anomalous results.
2. Calculating the mean of the three readings gives a more reliable and precise value for the extension.
3. The student should identify and exclude any anomalous results before calculating the mean.
4. She should check that the data follow a straight‑line relationship (extension ∝ force) to confirm Hooke’s law and assess validity.
5. Additionally, she should take a reading with no weight (zero force) to establish a reference point for zero extension, or ensure the force is increased in equal steps for a fair comparison.

Examiner tips

  • Use the command word ‘explain’ to give reasons, not just statements. Show the logical chain: repeat → less random error → better mean. Mention the importance of checking for outliers and a zero‑force reference. Include Hooke’s law as a validity check.

Common mistakes

  • Failing to mention the role of random error. Not recognising the need to exclude outliers. Ignoring the requirement to check for a linear relationship or a zero‑force reference.

Mark scheme (5 marks)

  1. Repeating measurements reduces the effect of random errors / anomalous results
  2. Calculating the mean gives a more reliable / precise value for the extension
  3. The student should identify and exclude any anomalous results before calculating the mean
  4. The student should check her results show a pattern consistent with Hooke's law (extension proportional to force) to assess validity
  5. The student should take a reading with no weight applied (zero force) to establish a reference / zero extension point, OR ensure the independent variable (force) is changed in equal steps to allow a fair comparison

Key terms in this question

mean · validity · extension

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