A skydiver jumps from an aircraft and falls towards the ground. Explain why the skydiver first accelerates, then reaches a terminal velocity.

Edexcel A-Level Physics (9PH0) — 9.1 Kinetic theory and ideal gases · Explain · 5 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 and falls vertically downward.

Model answer (5 marks)

Initially the only significant force on the skydiver is his weight, so the net force is downward and the skydiver accelerates.
As the speed increases, air resistance grows.
The upward air resistance reduces the net downward force, so the acceleration decreases.
When the air resistance equals the weight, the forces balance and the net force is zero.
At this point the skydiver moves at a constant terminal velocity.

Examiner tips

Show the sequence of forces and how they change with speed
Use the term ‘net force’ and ‘terminal velocity’
Explain the balance of weight and air resistance
Mention Newton’s second law implicitly

Common mistakes

Confusing air resistance with weight, or saying they are equal from the start
Forgetting to state that acceleration decreases before reaching terminal velocity
Using vague terms like ‘slow down’ instead of ‘decrease acceleration’

Mark scheme (5 marks)

Initially, the only significant force acting on the skydiver is weight (downward) / air resistance is zero or very small at the start
There is a resultant force downwards, so the skydiver accelerates (Newton's second law / F = ma implied)
As speed increases, air resistance increases
The resultant force decreases as air resistance increases, so acceleration decreases
Eventually air resistance equals weight, so the forces are balanced / resultant force is zero, and the skydiver moves at constant (terminal) velocity

Key terms in this question

terminal velocity