A steel ball bearing is released from rest and falls through a tall cylinder of thick oil. Describe how the speed and acceleration of the ball bearing change from the moment it is released until it reaches terminal velocity.

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

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

Model answer (4 marks)

The ball bearing starts from rest and accelerates downwards.
1. Immediately after release the only force acting is its weight, so the acceleration is maximum and the speed increases.
2. As the speed rises, the viscous resistance of the oil increases.
3. The increasing resistance reduces the net downward force, so the acceleration decreases.
4. When the weight is exactly balanced by the liquid resistance the net force becomes zero; the acceleration is zero and the speed is constant – this is the terminal velocity.

Examiner tips

  • Use the word ‘accelerates’ for the initial phase and ‘decreases’ for the acceleration later.
  • Show the relationship between speed, resistance and net force.
  • Mention that terminal velocity occurs when weight equals resistance and acceleration is zero.
  • Keep the answer concise – 4 points, 1 sentence each.

Common mistakes

  • Confusing acceleration with speed – e.g. saying the speed decreases.
  • Forgetting to state that the acceleration becomes zero at terminal velocity.
  • Using vague terms like ‘slow down’ instead of ‘decrease’ for acceleration.

Mark scheme (4 marks)

  1. The ball bearing accelerates (from rest) / speed increases after release
  2. As speed increases, liquid resistance increases
  3. The resultant force decreases, so acceleration decreases
  4. Terminal velocity is reached when weight equals liquid resistance / when resultant force is zero, so acceleration is zero and speed is constant

Key terms in this question

terminal velocity · acceleration

Related

More Motion questions

▶ Try answering this question with AI marking (free) →