A rowing boat moves along a straight river. It starts from rest and accelerates uniformly for 8 seconds, reaching a velocity of 4 m/s. It then travels at a constant velocity of 4 m/s for a further 12 seconds before decelerating uniformly to rest in 4 seconds. Describe the motion of the rowing boat during each of the three stages of its journey, referring to the velocity-time graph that would represent this motion.

Pearson Edexcel International GCSE Physics (4PH1) — 1.2 Movement and position · Describe · 4 marks · View as Markdown

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

A rowing boat moves along a straight river. It starts from rest and accelerates uniformly for 8 seconds, reaching a velocity of 4 m/s. It then travels at a constant velocity of 4 m/s for a further 12 seconds before decelerating uniformly to rest in 4 seconds.

Model answer (4 marks)

1. Acceleration stage: the boat starts from rest (0 m s⁻¹) and its speed increases uniformly to 4 m s⁻¹ over 8 s. On a velocity‑time graph this is a straight line rising from the origin to the point (8 s, 4 m s⁻¹) – a positive gradient.
2. Constant‑speed stage: the boat then moves at 4 m s⁻¹ for 12 s. The graph is a horizontal straight line at 4 m s⁻¹ from t = 8 s to t = 20 s.
3. Deceleration stage: finally the boat slows uniformly to rest in 4 s. The graph is a straight line falling from (20 s, 4 m s⁻¹) to (24 s, 0 m s⁻¹) – a negative gradient.
The area under the whole graph (a trapezoid + rectangle) gives the total distance travelled.

Examiner tips

  • Use the correct graph shapes – straight lines for uniform motion, horizontal for constant speed.
  • Mention the gradient sign for acceleration/deceleration.
  • State the time intervals and velocities explicitly.
  • Explain that the area under the graph equals distance.

Common mistakes

  • Drawing a curved line for uniform acceleration instead of a straight line.
  • Forgetting to include the horizontal line for the constant‑speed stage.
  • Not noting that the area under the graph represents distance.

Mark scheme (4 marks)

  1. During the first stage, the boat accelerates uniformly from rest, so the velocity-time graph shows a straight line with a positive gradient.
  2. During the second stage, the boat travels at constant velocity, so the velocity-time graph shows a horizontal straight line.
  3. During the third stage, the boat decelerates uniformly to rest, so the velocity-time graph shows a straight line with a negative gradient.
  4. The area under the velocity-time graph represents the total distance travelled by the boat.

Key terms in this question

velocity · velocity-time graph · constant velocity

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