A rowing boat moves along a straight river at a constant velocity. The rowers then stop rowing. Explain why the boat decelerates and eventually comes to rest.

Edexcel GCSE Physics (1PH0) — 2.2 Forces and motion · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When the rowers stop, the driving force from the oars is removed, so the only horizontal force left is the resistive force of the water (friction/drag) acting opposite to the motion. This resultant force is therefore opposite to the boat’s velocity, giving a negative acceleration and causing the boat to decelerate. As the boat slows, the drag force decreases, but it continues to remove kinetic energy until the velocity reaches zero and the boat comes to rest.

Examiner tips

  • State that the driving force disappears when rowing stops
  • Identify the resistive force and its direction
  • Explain that the opposite force causes a negative acceleration
  • Mention that kinetic energy is dissipated by friction until velocity is zero

Common mistakes

  • Forgetting to mention that the driving force is removed
  • Confusing drag with a forward force
  • Saying the boat stops because the drag becomes zero instead of because it removes kinetic energy

Mark scheme (4 marks)

  1. When the rowers stop rowing, the driving force (from the oars/water) is removed / becomes zero
  2. There is still a resultant force acting on the boat (friction / water resistance / drag) in the opposite direction to motion
  3. The resultant force is in the opposite direction to the boat's motion, so the boat decelerates
  4. The boat eventually stops because the resultant force has reduced the velocity to zero / the boat comes to rest when all kinetic energy has been transferred away by friction

Key terms in this question

constant velocity

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