Explain why a skydiver reaches a terminal velocity before opening their parachute.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A skydiver jumps from an aircraft and falls towards the ground. At first they accelerate, but after some time they fall at a constant speed called terminal velocity.
Model answer (4 marks)
1. At the start the skydiver’s weight (mg) is greater than the upward air resistance, giving a net downward force.
2. The downward force causes the skydiver to accelerate and their speed increases.
3. As speed increases, the air resistance rises, reducing the net downward force and therefore the acceleration.
4. When the air resistance equals the weight, the net force is zero and the skydiver falls at a constant speed – the terminal velocity.
2. The downward force causes the skydiver to accelerate and their speed increases.
3. As speed increases, the air resistance rises, reducing the net downward force and therefore the acceleration.
4. When the air resistance equals the weight, the net force is zero and the skydiver falls at a constant speed – the terminal velocity.
Examiner tips
- Use the word ‘net force’ to link weight and air resistance. Show the sequence: weight > resistance → acceleration → resistance ↑ → acceleration ↓ → resistance = weight → constant speed.
- Mention that terminal velocity is the point where forces balance, not a new force appears.
Common mistakes
- Forgetting to state that the net force becomes zero at terminal velocity. Confusing acceleration with speed – students may say the skydiver stops accelerating but still keep weight > resistance. Using vague terms like ‘slow down’ instead of ‘decrease acceleration’.
Mark scheme (4 marks)
- Weight (downwards) is greater than air resistance (upwards) at the start, so there is a resultant force downwards
- As the skydiver accelerates / speed increases, air resistance increases
- The resultant force decreases as air resistance increases, so acceleration decreases
- Terminal velocity is reached when air resistance equals weight, so the resultant force is zero and the skydiver moves at constant speed