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AQA GCSE Physics · Topic 4.5 Forces
Terminal Velocity
In this lesson you'll learn to: name the two forces acting on a falling object, explain why
air resistance increases with speed, use the resultant force to say whether something
is speeding up, slowing down or moving at a constant speed, and explain why a skydiver reaches a
terminal velocity — and a new, lower one when the parachute opens.
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Prefer to watch? Scroll to the bottom for the video on this topic
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Start here — have a play
Don't read anything yet. Click points A to E on the graph and size the two force arrows at each one. Notice what never changes (her weight) and what does (the air resistance). Then build the sentences, and put the 4-mark answer into the right order.
4.5.6.1.5 · AQA GCSE PHYSICS · FORCES
Terminal Velocity
1
Draw the forces on the parachutist at A
— choose the size of each arrow⬇ Weight — downwards
⬆ Air resistance — upwards
2
Compare the forces at A
— click a gap, then click a wordWords to use
3
Exam-style question
— put the answer in orderExplain question[4 marks]
A skydiver jumps from a helicopter. She falls and reaches terminal velocity. She then opens her parachute, and soon reaches a new, lower terminal velocity.
Explain, in terms of the forces acting on her, why opening the parachute makes her reach a new, lower terminal velocity.
F = m × a
"Explain" means use the equation — link resultant force to acceleration
Physics With Kate · physicswithkate.com
▲ Her weight is the same in all five boxes — only the air resistance arrow changes.
Seen what happens? Now learn how to say it.
Every mark in this topic comes from the same four-step chain: forces → resultant force → acceleration → what happens to her speed. The notes below walk through the whole skydive and give you the exact wording the examiner is looking for.
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Revision notes
Only two forces act on a falling object
Whatever else is going on, a skydiver — or a raindrop, or a ball dropped out of a window — has just two forces on it:
- Weight, acting downwards. It is caused by gravity, and W = m g. Her mass doesn't change and g doesn't change, so her weight is exactly the same at every moment of the fall.
- Air resistance (drag), acting upwards — always in the opposite direction to the motion. It is caused by her having to push air out of the way.
⭐ Key fact — air resistance depends on speed
The faster she falls, the more air she has to push out of the way each second, so the bigger the air resistance. When she is not moving, air resistance is zero. As she speeds up it increases; as she slows down it decreases.
It also depends on surface area — a bigger area means more air resistance at the same speed. That is the whole point of a parachute.
The resultant force decides everything
The two forces are in opposite directions, so the resultant force is just the difference between them, in the direction of the bigger one. Then Newton's Second Law, F = m a, turns that resultant force into an acceleration.
| The two forces | Resultant force | Acceleration | What she does |
|---|---|---|---|
| Air resistance is zero (only weight acts) | Large, downwards | Large, downwards | Speed increases |
| Air resistance < weight | Downwards, getting smaller | Downwards, getting smaller | Speed increases, but at a lower rate |
| Air resistance = weight | Zero (balanced forces) | Zero | Constant speed — terminal velocity |
| Air resistance > weight | Upwards | Upwards | Slows down |
⭐ Key definition — terminal velocity
(F = m a) When the air resistance acting upwards is equal to the weight acting downwards, the resultant force is zero, so the acceleration is zero and the object falls at a constant speed. This is called its terminal velocity.
The whole skydive, stage by stage
These are the five points marked A to E on the graph in the animation above. Learn the story in this order and you can answer any version of the question.
Free fall — before the parachute
The core of the topicA
She jumps
She isn't moving yet, so air resistance is zero. Only her weight acts.
Resultant force: large, downwards · acceleration = g ≈ 9.8 m/s², its maximum value.B
Speeding up
She is moving, so air resistance has grown — but weight is still bigger.
Resultant force: downwards but decreasing · she is still accelerating, just less and less. The graph curves over.C
Terminal velocity
She is now fast enough that air resistance = weight. The forces are balanced.
Resultant force: zero · acceleration zero · she falls at a constant speed. The graph is flat.After the parachute opens
Where most marks are lostD
Parachute open
The surface area suddenly increases, so at the same speed the air resistance is much bigger than her weight.
Resultant force: upwards · acceleration upwards · she slows down (she does not go back up). The graph drops steeply.E
New terminal velocity
As she slows, air resistance falls again, until air resistance = weight once more.
Resultant force: zero again · constant speed again — but a new, lower terminal velocity, slow enough to land safely.💡 Exam tip — her weight NEVER changes
Opening a parachute does not change her mass, so it does not change her weight. The only force that changes during the whole skydive is the air resistance. If you have written "her weight increases", you have lost the mark.
Reading the velocity–time graph
- Steep at the start — big resultant force, so big acceleration.
- Curving / getting less steep — air resistance is growing, so the resultant force and the acceleration are decreasing.
- Flat (horizontal) — constant velocity, so resultant force is zero. This is terminal velocity.
- Sudden steep drop — the parachute has opened; air resistance is now bigger than weight, so she decelerates.
- Flat again, but lower — the new, lower terminal velocity.
💡 Exam tip — three things students get wrong
1. "The forces swap over." They don't. Weight is always down, air resistance is always up — only their sizes change.
2. "At terminal velocity there are no forces on her." There are — two big ones. They are balanced, so the resultant is zero.
3. "When the parachute opens she stops." She doesn't. She decelerates until the forces balance again at a lower speed.
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How to write it in the exam
Marks come from the chain of reasoning, not from the conclusion. Every answer in this topic is the same four steps: compare the forces → resultant force → acceleration → what happens to her speed. Use this frame every single time:
The PWK sentence frame
Her air resistance is less than her
weight,
so the resultant force is downwards,
so her acceleration is downwards (F = ma),
so she speeds up.
so the resultant force is downwards,
so her acceleration is downwards (F = ma),
so she speeds up.
the forces — always name both bigger / smaller / equal, and the direction what happens to her speed
Explain why the skydiver reaches a terminal velocity [3 marks]
As she falls she speeds up, so her air resistance increases. Eventually the air resistance is equal to her weight, so the resultant force is zero. Her acceleration is therefore zero, so she falls at a constant velocity — her terminal velocity.
Explain why opening the parachute gives a new, lower terminal velocity [4 marks]
Opening the parachute increases her surface area, so the air resistance becomes greater than her weight. The resultant force is now upwards, so her acceleration is upwards and she slows down. As she slows, the air resistance decreases, until it is equal to her weight again. The resultant force is zero once more, so she falls at a constant, lower terminal velocity.
Why does she not stop moving when the parachute opens? [2 marks]
Air resistance is only bigger than weight while she is fast. As she slows down the air resistance decreases, so the forces balance again before she reaches zero speed.
Explain the shape of the graph between A and C [3 marks]
At A the graph is steepest because air resistance is zero, so the resultant force and the acceleration are at their maximum. Between A and C the air resistance increases, so the resultant force and the acceleration decrease — the graph gets less steep. At C the forces are balanced, so the acceleration is zero and the graph is horizontal.
A heavier skydiver jumps from the same helicopter
A bigger weight needs a bigger air resistance to balance it, and air resistance only gets bigger at higher speeds — so the heavier skydiver reaches a higher terminal velocity.
💡 Exam tip — the four-part check
Before you move on, check your answer says: 1) which force is bigger, 2) the direction (or zero) of the resultant force, 3) the acceleration, 4) what happens to her speed. One step missing = one mark missing.
📘 Now do it in your workbook
Draw the force arrows for yourself at every stage of the skydive, then write the four-step explanation — with the answers at the back so you can see exactly where the marks are given.
AQA GCSE Physics Workbook · FORCES · page 12Free sample = the page for this lesson. The full workbook covers the whole of Topic 5 Forces with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
▲ In GHL you can also use the lesson's built-in video field instead of this embed.
Now put it into practice
Watching is the easy bit — you only find out whether it's stuck when you have to draw the arrows and write the four steps yourself. Page 12 of the workbook is free, and the full book covers every spec point in Topic 5 Forces with worked answers.
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