Explain how a student could estimate the speed of a car travelling along a road, without using any electronic equipment. Give a reasonable value for the speed of a car on a typical UK road.

AQA A-Level Physics (7408) — 3.1.3 Estimation of Physical Quantities · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Measure a known distance on the road, such as between two lamp posts or road markings. Time how long the car takes to travel that distance using a stopwatch or by counting seconds. Calculate the speed using the formula speed = distance ÷ time. Note that reaction time introduces uncertainty; improve reliability by repeating the measurement or using a longer distance to reduce percentage error. A typical UK road speed is about 30–70 mph, i.e. 13–31 m s⁻¹ (≈50 km h⁻¹).

Examiner tips

  • Use the speed formula explicitly; show units. Mention reaction time as an uncertainty. Give a realistic speed in the required range. Show the calculation steps if numbers are used.

Common mistakes

  • Failing to state the speed formula. Ignoring reaction time as a source of error. Providing a speed outside the 10–35 m s⁻¹ range.

Mark scheme (5 marks)

  1. Measure or estimate a known distance along the road (e.g. between two lamp posts or road markings)
  2. Time how long the car takes to travel that distance (e.g. using a stopwatch or counting seconds)
  3. Use speed = distance ÷ time to calculate the speed
  4. Acknowledge a source of uncertainty or suggest a way to improve reliability (e.g. reaction time affects timing; repeat measurements; use a longer distance to reduce percentage error)
  5. Give a reasonable value for car speed on a typical UK road in the range 10–35 m/s (or equivalently 30–70 mph / approximately 50 km/h for a standard road)

Key terms in this question

speed

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