The shape of the lesson
By the end of the lesson, learners can
- Push against the resistance: Drive the object across different surfaces and through the air.
- Two resistive forces, compared.
- Friction opposes the relative motion of surfaces in contact.
- Drag opposes motion through a fluid and increases with speed.
- Energy lost to friction is transferred to heat (and some sound).
One picture every idea returns to
Friction is a force that opposes the relative motion of two solid surfaces in contact. Drag (air or water resistance) is the resistive force on an object moving through a fluid. Drag increases sharply as the object speeds up, while solid sliding friction stays roughly constant.
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, The force that always pushes back.. Friction is the quiet opponent of every moving thing. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Push against the resistance | Drive the object across different surfaces and through the air. Project tab 3 of the lab (Friction and drag) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Two resistive forces, compared | Two resistive forces, compared. Model the worked example Worked example on the board, one line at a time. | Frayer Model |
| 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, 13 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 Two resistive forces, compared, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| On a moving object, friction and drag act opposite to the direction of motion, never along it. | A common slip is to draw friction helping the motion. It always resists relative motion. Drag also increases with speed, so the resistive force on a fast moving car is much larger than on a slow one. |
Friction and drag: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
What is friction? State the direction in which it acts on a moving object.
Give one situation where friction is useful and one where it is a nuisance.
State how the drag (air resistance) on a moving object changes as its speed increases, and give one consequence of this for a vehicle.
A box is given a push along the floor and then slides to a stop. Explain, in terms of forces, why it slows down.
When friction acts between two surfaces that rub together, energy is transferred. State the main form the energy is transferred to, and one effect this can have.
A car travels at a steady high speed, then later at a steady low speed, both on a level road. Explain why a larger driving force is needed at the higher speed.
Answer key and marking notes
- Friction is a force that opposes the relative motion of two surfaces in contact. ✓
- It acts in the direction opposite to the motion. ✓
- Useful: gripping the road / walking / braking. ✓
- Nuisance: wearing down moving parts / wasting energy as heat. ✓
- The drag increases as the speed increases. ✓
- A larger driving force is needed to keep going at higher speeds (or fuel use is greater). ✓
- Once released, friction acts backward, opposite to the motion. ✓
- This gives a resultant force backward, so the box decelerates until it stops. ✓
- Mainly to heat (thermal energy), with some sound. ✓
- It warms the surfaces, which can cause wear or overheating of parts. ✓
- At any steady speed the resultant force is zero, so the driving force equals the total resistive force. ✓
- Drag increases with speed, so the resistive force is larger at the higher speed. ✓
- Therefore a larger driving force is needed to balance it. ✓
1 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 book is pushed to the right across a table and is slowing down. In which direction does the friction force from the table act on the book?
- Ato the right, in the direction of motion
- Bto the left, opposing the motion
- Cupward, away from the table
- Ddownward, into the table
Quiz key
Friction acts opposite to the motion, so to the left, which is why the book slows down. Option A would speed it up; C describes the normal contact force, not friction.
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.