A small stone is dropped from rest into a deep lake. Describe how the velocity of the stone changes from the moment it enters the water until it reaches terminal velocity. Explain the changes in terms of the forces acting on the stone.

Cambridge International IGCSE Physics (0625) — 1.2 Motion · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The stone starts from rest and immediately accelerates downwards.
1. When it first enters the water the weight (mg) is larger than the liquid resistance, so the net force is downward and the stone speeds up.
2. As the stone’s speed increases, the liquid resistance (drag) increases.
3. The increasing drag reduces the net downward force, so the acceleration decreases.
4. When the drag equals the weight the net force becomes zero; the stone then moves at a constant speed – the terminal velocity.

Examiner tips

  • Use the terms ‘weight’, ‘liquid resistance’ and ‘net force’. Show the sequence: acceleration → drag increases → net force decreases → terminal velocity. Remember to link force changes to velocity changes.
  • common_mistakes
  • :
  • Confusing drag with weight or ignoring that drag increases with speed. Failing to state that terminal velocity occurs when forces balance.

Mark scheme (4 marks)

  1. The stone initially accelerates (velocity increases) after entering the water
  2. Weight (downward force / gravitational force) is greater than liquid resistance at first, giving a resultant downward force
  3. As speed increases, liquid resistance increases, so the resultant force (and acceleration) decreases
  4. Terminal velocity is reached when liquid resistance equals weight, so resultant force is zero and velocity is constant

Key terms in this question

terminal velocity

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