A student measures the acceleration of a toy car rolling down a ramp. She times how long the car takes to travel a fixed distance and repeats the measurement six times. Her results, in seconds, are: 1.23, 1.25, 1.24, 1.26, 1.58, 1.24. Explain how the student should process these results to obtain a reliable value for the time, and explain why her final value may still not be accurate.

AQA A-Level Physics (7408) — 3.1.2 Limitations of Physical Measurements · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Identify 1.58 s as an anomalous result and exclude it.

Calculate the mean of the remaining six values: (1.23+1.25+1.24+1.26+1.24)/5 = 1.244 s.

State that this mean is a more reliable estimate of the true time.

Explain that the mean may still be inaccurate because a systematic error (e.g. a mis‑set stopwatch or a slope error) would shift all readings by the same amount and cannot be reduced by averaging.

Examiner tips

  • Show the calculation of the mean explicitly; include units. Mention that the outlier is removed before averaging. Explain that systematic errors affect all readings equally and cannot be eliminated by more trials.

Common mistakes

  • Including the 1.58 s outlier in the mean. Failing to state that systematic errors cannot be removed by averaging. Using the wrong number of trials (e.g. averaging all seven values).

Mark scheme (5 marks)

  1. Identify 1.58 s as an anomalous result (anomaly / outlier)
  2. The anomalous result should be excluded / not included in the mean
  3. Calculate a mean (average) of the remaining results
  4. The result may still be inaccurate because of a systematic error
  5. Systematic error affects all readings in the same way / cannot be reduced by averaging

Key terms in this question

accurate

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