Welcome! A skydiver jumps from a plane 4000 m up. Does she keep getting faster until she hits the ground? In this lesson you’ll find out what really happens, then commentate on a skydive using the physics of forces. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen, a ruler and a calculator.
- Write the title and date: Terminal velocity – Skydive Challenge.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. calculate weight and resultant force
2. explain why a falling object reaches terminal velocity
3. describe and explain the velocity–time graph of a skydiver.
Step 1: Starter (5 minutes)
Guess: a skydiver falls for about 60 seconds before opening her parachute. Does she keep speeding up for the whole minute? Write down your prediction and a reason.
Show answer
No! She speeds up at first, but after about 12 seconds she stops speeding up and falls at a steady speed of roughly 55 m/s (about 120 mph). This top speed is called her terminal velocity. You’ll find out why in this lesson.
📝 Copy into your book
weight = mass × gravitational field strength W = m g
W in N, m in kg, g = 9.8 N/kg on Earth
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What two forces act on a falling skydiver?
- What happens to air resistance as the skydiver speeds up?
- What is terminal velocity?
📖 Read: Why falling objects stop speeding up
Two forces act on a falling skydiver: her weight, acting downwards, and air resistance, acting upwards. Air resistance is a type of friction caused by the air pushing against her as she moves. The resultant force is the difference between the two forces. If the resultant force is downwards, she accelerates downwards.
The faster an object moves through the air, the bigger the air resistance becomes. So as the skydiver speeds up, the air resistance grows and the resultant force gets smaller. Her acceleration gets smaller too. Eventually the air resistance becomes equal to her weight. The forces are balanced, the resultant force is zero, and she falls at a steady speed. This steady speed is called her terminal velocity.
When she opens her parachute, its large surface area makes the air resistance much bigger than her weight. The resultant force is now upwards, so she slows down. As she slows, the air resistance falls, until it balances her weight again – at a new, much lower terminal velocity, slow enough to land safely.
Show answers
(a) Weight (downwards) and air resistance (upwards).
(b) It increases.
(c) The steady speed a falling object reaches when air resistance equals its weight, so the resultant force is zero.
📝 Copy and complete
As a skydiver speeds up, air resistance __________. When air resistance equals her __________, the resultant force is __________ and she falls at a steady speed called her __________ __________.
Check your gaps
increases · weight · zero · terminal velocity
Step 3: Key facts (8 minutes)
Your task: copy this table into your book. It describes the whole skydive, stage by stage.
| Stage | Forces | Resultant force | Motion |
|---|---|---|---|
| A. Just jumped | Weight much bigger than air resistance | Large, downwards | Speeds up quickly |
| B. Falling faster | Air resistance increasing | Getting smaller | Still speeding up, but more slowly |
| C. Terminal velocity | Air resistance = weight | Zero | Steady high speed |
| D. Parachute opens | Air resistance much bigger than weight | Upwards | Slows down |
| E. New terminal velocity | Air resistance = weight again | Zero | Steady low speed |
📝 Copy into your book
Resultant force = the difference between the forces.
Resultant force downwards → speeds up.
Resultant force zero → constant speed (NOT stopped!).
Resultant force upwards → slows down.
Watch out: a resultant force of zero does not mean the skydiver stops. It means her speed stays the same.
Step 4: Prove it with a calculation (8 minutes)
The skydiver has a mass of 75 kg. Use g = 9.8 N/kg. Show every step.
Q1 Calculate her weight.
Show answer
W = m g = 75 × 9.8 = 735 N
Q2 A few seconds into the jump, the air resistance is 300 N. Calculate the resultant force.
Show answer
735 − 300 = 435 N downwards
Q3 Calculate her acceleration at this moment.
Show answer
F = m a, so a = 435 ÷ 75 = 5.8 m/s²
Q4 What is the air resistance when she reaches terminal velocity? Explain.
Show answer
735 N – at terminal velocity the forces are balanced, so air resistance equals her weight.
✅ Score out of 4.
Step 5: Main task – Skydive Challenge (12 minutes)
You’re the TV commentator for a charity skydive!
- Sketch the velocity–time graph for the whole skydive. Put time on the x-axis and velocity on the y-axis. Label the five stages A to E from your table.
- Write your commentary: one or two sentences for each stage, explaining what the forces are doing and why the skydiver moves the way she does.
Stuck? A steep line on a velocity–time graph means a big acceleration. A flat line means constant speed. A line going down means slowing down.
Show a model answer
The graph: starts at zero and rises steeply (A), then curves over and gets less steep (B), then goes flat at a high velocity (C). When the parachute opens it drops steeply (D), then curves and goes flat again at a much lower velocity (E).
A: “She’s out of the plane! Her weight is much bigger than the air resistance, so there’s a big downward resultant force and she’s speeding up fast.”
B: “As she speeds up, the air resistance is growing, so the resultant force is shrinking. She’s still speeding up, but more slowly now.”
C: “Air resistance now equals her weight – the forces are balanced. She’s reached terminal velocity and is falling at a steady 55 m/s.”
D: “Parachute open! The huge area means air resistance is now much bigger than her weight. The resultant force is upwards, so she’s slowing down.”
E: “As she slows, air resistance drops until it balances her weight again. A new, lower terminal velocity – nice and safe for landing!”
✅ Check: did you mention weight, air resistance and resultant force at every stage?
Step 6: Exam practice (12 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) Explain why a skydiver falls at a constant speed at terminal velocity.
Show model answer
The air resistance is equal to the weight (1), so the resultant force is zero and there is no acceleration (1).
Q2 (3 marks) An 80 kg skydiver has a weight of 784 N. At one moment the air resistance on him is 584 N. Calculate his acceleration.
Show model answer
Resultant force = 784 − 584 = 200 N (1)
a = F ÷ m = 200 ÷ 80 (1)
a = 2.5 m/s² (1)
Q3 (6 marks) Describe and explain the motion of a skydiver from the moment she jumps until she lands.
Hint: your table from Step 3 and your commentary are a perfect plan. For each stage, say what happens to the forces AND to her speed.
Show model answer
- When she jumps, her weight is bigger than the air resistance, so the resultant force is downwards and she accelerates.
- As her speed increases, the air resistance increases, so the resultant force and her acceleration decrease.
- Eventually air resistance equals her weight, the resultant force is zero and she falls at terminal velocity.
- When the parachute opens, its larger surface area increases the air resistance so it is bigger than her weight.
- The resultant force is upwards, so she decelerates, and as she slows the air resistance decreases.
- Air resistance becomes equal to her weight again, so she falls at a new, lower terminal velocity until she lands.
✅ Add up your marks out of 11 and write the score in your book.
Step 7: Exit ticket (7 minutes)
Think about it: a feather and a hammer are dropped at the same time. On Earth the feather falls much more slowly – but on the Moon, where there is no air, they land together. Explain why in two sentences. Then finish:
- At terminal velocity, the resultant force is… because…
- A parachute slows a skydiver down because…
- One thing I’m still not sure about is…
Check the feather and hammer
On Earth, the feather has a large surface area for its small weight, so air resistance quickly balances its weight and it reaches a very low terminal velocity. On the Moon there is no air, so there is no air resistance – both objects accelerate at the same rate and land together.
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Forces questions.
