A skydiver jumps from an aircraft and falls towards the Earth. Initially, the skydiver accelerates downwards. After some time, the skydiver reaches terminal velocity. Explain why the skydiver first accelerates and then reaches terminal velocity, using Newton's First and Second Laws.

OCR GCSE Physics A: Gateway Science (J249) — P2.2 Newton's laws · Explain · 4 marks · View as Markdown

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

A skydiver jumps from an aircraft at high altitude. At the moment of jumping, air resistance is very small. As the skydiver falls faster, air resistance increases. Eventually, the skydiver stops accelerating and falls at a constant speed known as terminal velocity.

Model answer (4 marks)

At the start of the fall the skydiver’s weight (mg) is greater than the small air resistance, so the forces are unbalanced and the resultant force is downwards.
By Newton’s Second Law (F=ma) this unbalanced force causes the skydiver to accelerate downwards.
As the speed increases the air resistance grows until it equals the weight; the two forces then balance and the resultant force becomes zero.
When the resultant force is zero, Newton’s First Law states that the skydiver moves at a constant speed – the terminal velocity.

Examiner tips

  • Use the exact terms ‘weight’, ‘air resistance’, ‘resultant force’, ‘unbalanced’, ‘balanced’, ‘Newton’s Second Law’, ‘Newton’s First Law’.
  • Show the sequence: unbalanced → acceleration → balanced → constant speed.
  • Mention that terminal velocity is the speed at which weight equals air resistance.

Common mistakes

  • Confusing Newton’s First and Second Laws – e.g. saying the skydiver stops accelerating because of the First Law.
  • Omitting the step that air resistance increases with speed.

Mark scheme (4 marks)

  1. At the start of the fall, weight (gravitational force) is greater than air resistance, so there is a resultant force acting downwards.
  2. By Newton's Second Law, the resultant force causes the skydiver to accelerate downwards.
  3. As the skydiver's speed increases, air resistance increases until it equals the skydiver's weight, giving a resultant force of zero / balanced forces.
  4. By Newton's First Law, when the resultant force is zero, the skydiver continues at a constant velocity — this is terminal velocity.

Key terms in this question

terminal velocity

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