The shape of the lesson
By the end of the lesson, learners can
- Tilt it until it tips: Tilt the object and watch the line of action of its weight.
- Stable, until the line escapes.
- Centre of gravity: the point where all the weight appears to act.
- Toppling: happens when the line of action of the weight falls outside the base.
- More stable: a wider base and a lower centre of gravity.
One picture every idea returns to
The centre of gravity is the point at which all of an object’s weight can be taken to act. An object topples when the line of action of its weight passes outside its base. A wider base and a lower centre of gravity make an object more stable.
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 point where the weight acts.. Every object behaves as if its entire weight pulls down from a single point, its centre of gravity. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Tilt it until it tips | Tilt the object and watch the line of action of its weight. Project tab 6 of the lab (Centre of gravity) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Stable, until the line escapes | Stable, until the line escapes. Model the worked example Worked example · required practical on the board, one line at a time. | 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, 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 Stable, until the line escapes, 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 |
|---|---|
| An object does not topple simply because it is tilted. | It stays stable as long as the line of action of its weight stays within the base. It tips over only once that line passes outside the base, beyond the edge it pivots on. Lowering the centre of gravity or widening the base makes this tipping point much harder to reach. |
Centre of gravity: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
What is meant by the centre of gravity of an object?
State the condition under which a tilted object will topple over.
State two changes that would make an object more stable.
Describe an experiment to find the centre of gravity of a flat, irregular piece of card.
Explain, in terms of centre of gravity and base, why a racing car is very difficult to tip over.
A tall, narrow box and a short, wide box are made of the same material. State which is more stable and explain why.
Answer key and marking notes
The point at which all of the object's weight can be taken to act. ✓✓
- It topples when the line of action of its weight falls outside its base. ✓
- While that line stays within the base, it settles back. ✓
- Make the base wider. ✓
- Lower the centre of gravity. ✓
- Hang the card freely from a pin through a hole and let it settle. ✓
- Hang a plumb line from the same pin and draw the vertical line on the card. ✓
- Repeat from another hole; the centre of gravity is where the lines cross. ✓
- It has a low centre of gravity and a wide base. ✓
- The car must tilt through a very large angle before the line of action of its weight reaches the edge of the base, so it rarely topples. ✓
- The short, wide box is more stable. ✓
- It has a lower centre of gravity and a wider base, so a much larger tilt is needed before the weight line leaves the base. ✓
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.
Four objects have the same mass and the same base width but different heights. The dot marks the centre of gravity of each. Which object is the most stable on a flat surface?
- Aobject P
- Bobject Q
- Cobject R
- Dobject S
Quiz key
Object Q has the lowest centre of gravity, so it is hardest to topple and is the most stable. Taller objects such as S have a high centre of gravity and topple more easily.
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.