A boat is floating motionless on a still lake. Explain why the boat remains stationary, referring to the forces acting on it.

Eduqas GCSE Physics — 3.1 Forces and their interactions · Explain · 4 marks · View as Markdown

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

A wooden boat sits on the surface of a calm lake without moving in any direction.

Model answer (4 marks)

1. Two forces act on the boat: its weight (gravity) acting downwards and the upthrust (buoyancy) acting upwards.
2. The weight of the boat pulls it towards the lake bottom.
3. The upthrust from the water pushes the boat upwards.
4. Because the two forces are equal in magnitude and opposite in direction, they balance each other, giving a net force of zero, so the boat remains stationary.

Examiner tips

  • Use the exact terms 'weight' and 'upthrust' or 'buoyancy'.
  • Show the forces are opposite and equal to justify the zero resultant.

Common mistakes

  • Confusing the direction of the upthrust with the direction of gravity.
  • Failing to mention that the forces are equal and opposite, or not stating that the resultant force is zero.

Mark scheme (4 marks)

  1. There are at least two forces acting on the boat (weight/gravity downwards AND upthrust/buoyancy force upwards)
  2. The weight of the boat acts downwards
  3. The upthrust from the water acts upwards on the boat
  4. The forces are balanced / equal in size, so the resultant force is zero, meaning there is no change in motion

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