The shape of the lesson
By the end of the lesson, learners can
- Balance the seesaw: Slide the weights along the beam and watch the moments.
- Balancing the moments.
- Principle of moments: clockwise moments = anticlockwise moments about the pivot.
- Equilibrium: no resultant force AND no resultant moment, both at once.
- Distance: always the perpendicular distance from the pivot to the line of action.
One picture every idea returns to
The moment of a force is the force multiplied by the perpendicular distance from the pivot: moment = F × d, in newton metres (N m). For an object in equilibrium, the principle of moments states that the total clockwise moment equals the total anticlockwise moment about any pivot.
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 turning effect of a force.. A small child can balance a heavier one on a seesaw, simply by sitting further out. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Balance the seesaw | Slide the weights along the beam and watch the moments. Project tab 5 of the lab (Moments) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Balancing the moments | Balancing the moments. Model the worked example Worked example on the board, one line at a time. | Connect-Extend-Challenge |
| 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 Balancing the moments, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Two traps in one topic. | First, a moment uses the perpendicular distance from the pivot to the line of action of the force, not the length of the beam or a slanted distance. Second, balancing moments alone is not full equilibrium. An object is in equilibrium only when there is both no resultant force and no resultant moment. An answer that mentions only one of these is incomplete. |
Moments: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define the moment of a force and state its unit.
A force of 20 N acts at a perpendicular distance of 0.30 m from a pivot. Calculate the moment.
State the principle of moments.
A uniform beam is pivoted at its centre. A 12 N weight hangs 0.50 m to the left of the pivot. A second weight hangs 0.30 m to the right and the beam balances. Calculate the second weight.
State the two conditions that must both be satisfied for an object to be in equilibrium.
When calculating a moment, a student measures the distance along a beam that is tilted, rather than the horizontal distance from the pivot. Explain why this gives the wrong moment.
Answer key and marking notes
- The moment of a force = force × perpendicular distance from the pivot. ✓
- Unit: newton metre (N m). ✓
- moment = F × d = 20 × 0.30
6.0 N m
For an object in equilibrium, the total clockwise moment about a pivot equals the total anticlockwise moment about the same pivot. ✓✓
Balance. clockwise = anticlockwise
W × 0.30 = 12 × 0.50 W = 6.0 ÷ 0.30W = 20 N
- There is no resultant force on it. ✓
- There is no resultant moment (clockwise moments = anticlockwise moments). ✓
- A moment uses the perpendicular distance from the pivot to the line of action of the force. ✓
- The slanted distance along the beam is longer than the perpendicular distance, so the calculated moment is too large. ✓
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 spanner turns a nut. A force of 25 N is applied at a perpendicular distance of 0.20 m from the nut. What is the moment of the force about the nut?
- A0.0080 N m
- B5.0 N m
- C12.5 N m
- D125 N m
A uniform beam is balanced on a central pivot. A 4.0 N weight sits 30 cm to the left of the pivot. At what distance to the right must a 6.0 N weight be placed to balance the beam?
- A12 cm
- B20 cm
- C45 cm
- D80 cm
Quiz key
moment = 25 × 0.20 = 5.0 N m. Option D divides instead of multiplying; A inverts the calculation.
4.0 × 30 = 6.0 × d, so d = 120 ÷ 6 = 20 cm. The heavier weight sits closer to the pivot. Option C scales the wrong way (multiplying by 1.5 instead of dividing).
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.