A skydiver jumps from an aeroplane and falls vertically downwards. At first, the skydiver accelerates. After some time, the skydiver reaches terminal velocity. Explain why the skydiver's acceleration decreases and eventually becomes zero as the skydiver falls.

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)

As the skydiver speeds up, the air resistance (drag) acting on her increases.
The resultant force on the skydiver is the weight minus the drag; as the drag increases, this resultant force decreases.
Because acceleration is proportional to the resultant force (F=ma), the acceleration therefore decreases.
When the skydiver reaches terminal velocity, the drag equals the weight, so the resultant force is zero and the acceleration becomes zero.

Examiner tips

  • Use the word "decreases" to show the change in acceleration. Mention that drag increases with speed. Show the balance of forces at terminal velocity. Keep the answer concise and use the exact terminology: drag, resultant force, terminal velocity.

Common mistakes

  • Confusing drag with weight. Failing to state that drag increases with speed. Using vague terms like "slow down" instead of "decrease in acceleration". Not recognising that the resultant force becomes zero at terminal velocity.

Mark scheme (4 marks)

  1. As the skydiver speeds up, air resistance (drag) increases
  2. The resultant force on the skydiver decreases as air resistance increases
  3. Because the resultant force decreases, the acceleration decreases
  4. At terminal velocity, air resistance equals weight (gravity), so the resultant force is zero and acceleration is zero

Key terms in this question

terminal velocity · acceleration

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