The shape of the lesson
By the end of the lesson, learners can
- Drag catches up with weight: Release the skydiver and watch the two arrows.
- Three stages of a fall.
- Weight is constant; drag increases with speed.
- Terminal velocity: drag equals weight, so the resultant force is zero.
- Zero resultant means zero acceleration, so the velocity stays constant (even though the object moves).
One picture every idea returns to
An object falling through a fluid has a constant weight downward and a drag (resistance) upward that grows with speed. When drag grows to equal the weight, the resultant force is zero, so there is no acceleration and the object falls at a constant terminal velocity.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, When the falling stops speeding up.. A skydiver does not get faster and faster forever. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Drag catches up with weight | Release the skydiver and watch the two arrows. Project tab 5 of the lab (Terminal velocity) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Three stages of a fall | Three stages of a fall. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Three stages of a fall, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| At terminal velocity the object is still moving, and moving fast, yet the resultant force is zero. | Drag equals weight, so they cancel. A common mistake is to think a moving object must have a forward or downward resultant force. It does not: constant velocity and balanced forces go together, the same idea as a car cruising at a steady speed. |
Terminal velocity: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define terminal velocity.
An object falls through air. State the two main forces acting on it, and describe how each changes as the object speeds up.
Explain, in terms of forces, why a falling object eventually reaches a terminal velocity.
At terminal velocity, state the resultant force on the object and whether it is moving. Explain how both statements can be true at once.
A skydiver is falling at terminal velocity and then opens a parachute. Describe what happens to the forces and the motion immediately afterward.
Describe the shape of the velocity-time graph for an object dropped from rest that reaches terminal velocity.
Answer key and marking notes
- The constant (maximum) velocity reached by an object falling through a fluid. ✓
- It occurs when the drag has grown to equal the weight, so the resultant force is zero. ✓
- Weight acts downward and stays constant. ✓
- Air resistance (drag) acts upward. ✓
- The drag increases as the object's speed increases. ✓
- As the object speeds up, the drag increases. ✓
- Eventually the drag grows to equal the weight, so the resultant force becomes zero. ✓
- With no resultant force there is no acceleration, so the velocity stays constant: the terminal velocity. ✓
- The resultant force is zero, and the object is still moving (at constant speed). ✓
- A zero resultant force means no acceleration, not no motion; balanced forces keep the velocity constant. ✓
- The parachute greatly increases the drag, so drag is now larger than the weight. ✓
- There is a resultant force upward, so the skydiver decelerates (slows down). ✓
- As they slow, drag decreases until it again equals weight, giving a new, lower terminal velocity. ✓
- It rises steeply at first (large acceleration). ✓
- The curve bends over as the acceleration decreases, then levels off to a horizontal line at the terminal velocity. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A skydiver falls and eventually reaches terminal velocity. Which statement describes the forces at terminal velocity?
- AWeight is greater than air resistance, so she accelerates.
- BAir resistance equals weight, so the resultant force is zero.
- CAir resistance is greater than weight, so she speeds up.
- DThere is no air resistance at terminal velocity.
The velocity-time graph for a falling object rises and then levels off to a constant value. What does the levelling-off represent?
- Athe object has stopped
- Bthe object has reached terminal velocity
- Cthe acceleration is increasing
- Dair resistance has fallen to zero
Quiz key
At terminal velocity the upward air resistance has grown until it equals the downward weight, giving zero resultant force and so a constant velocity. Option A describes the start of the fall, not terminal velocity. The forces balance, they do not vanish.
The flat top means a constant velocity: the terminal velocity, where air resistance balances weight. Option A is wrong because the object is still moving, just not accelerating. The gradient (acceleration) falls to zero rather than increasing.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.