The shape of the lesson
By the end of the lesson, learners can
- Stretch the time, soften the blow: Set the same change in momentum, then vary the time it takes.
- Why time is the lever.
- Resultant force: equals the rate of change of momentum.
- Longer time: smaller force for the same change in momentum.
- Rebounds: use signed velocities, so Δp is larger than for stopping alone.
One picture every idea returns to
The resultant force equals the rate of change of momentum: F = Δp ÷ Δt = (mv − mu) ÷ t. For a given change in momentum, a longer time produces a smaller force, and a shorter time a larger one.
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, Spread the change over more time.. A china plate shatters on concrete but survives on a carpet, from the very same fall. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Stretch the time, soften the blow | Set the same change in momentum, then vary the time it takes. Project tab 2 of the lab (Impulse and force) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why time is the lever | Why time is the lever. Model the worked example Worked example on the board, one line at a time. | Round Table |
| 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 Why time is the lever, 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.
Round Table
One sheet travels round the team. Each student adds one line of working or one idea and passes it on, so the solution is built four hands at a time and every contribution is visible.
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 |
|---|---|
| When a question asks why a safety feature reduces the force, the answer is the time. | The change in momentum is set by the masses and speeds and cannot be altered. It is the longer time of the collision that lowers the force, since F = change in momentum divided by time. |
Impulse and force: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what the resultant force on an object is equal to, and write the equation linking force, change in momentum and time.
The momentum of an object changes by 24 kg m/s in a time of 0.40 s. Calculate the average resultant force.
A 2.0 kg trolley speeds up from 3.0 m/s to 8.0 m/s in 2.0 s. Calculate the resultant force acting on it.
Explain, using the idea of change of momentum, how the crumple zone of a car reduces the force on the passengers in a crash.
A 0.20 kg ball hits the floor moving downward at 5.0 m/s and bounces straight up at 3.0 m/s. The contact with the floor lasts 0.10 s. Taking upward as positive, calculate the average force the floor exerts on the ball.
Two identical eggs are dropped from the same height. One lands on a hard tiled floor and breaks; the other lands on a soft cushion and survives. Explain the difference in terms of force and time.
Answer key and marking notes
- The resultant force equals the rate of change of momentum. ✓
- F = Δp ÷ t (= (mv − mu) ÷ t). ✓
- F = Δp ÷ t = 24 ÷ 0.40
F = 60 N
- Δp = m(v − u) = 2.0 × (8.0 − 3.0) = 10 kg m/s
F = Δp ÷ t = 10 ÷ 2.0
F = 5.0 N
- The change in momentum is fixed by the mass and speed of the car. ✓
- The crumple zone folds, increasing the time over which the car stops. ✓
- Since F = Δp ÷ t, a longer time gives a smaller force on the passengers. ✓
- Δp = m(v − u) = 0.20 × (3.0 − (−5.0))
Δp = 0.20 × 8.0 = 1.6 kg m/s
F = 1.6 ÷ 0.10
F = 16 N upward
downward velocity is −5.0, so the change is 8.0, not 2.0
- Both eggs have the same change in momentum on landing. ✓
- The cushion increases the stopping time, so F = Δp ÷ t gives a smaller force, low enough not to break the egg. ✓
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 force of 50 N acts on a ball for 0.30 s. What is the impulse, which equals the change in momentum?
- A0.0060 N s
- B15 N s
- C17 N s
- D167 N s
A car airbag increases the time taken to bring a passenger to rest in a crash. For the same change in momentum, what is the effect on the force on the passenger?
- Athe force increases
- Bthe force decreases
- Cthe force is unchanged
- Dthe force becomes zero
Quiz key
impulse = Ft = 50 × 0.30 = 15 N s, equal to a change in momentum of 15 kg m/s. Option A inverts the calculation; C adds the numbers; D divides.
With the same change in momentum spread over a longer time, F = Δp ÷ t is smaller, so the force on the passenger decreases. This is why airbags reduce injury. Option A reverses the relationship.
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.