The shape of the lesson
By the end of the lesson, learners can
- Same mass, different weight: Switch worlds and watch the two instruments.
- Telling them apart: Gravitational field strength g is the weight per unit mass, g = W / m, measured in N/kg.
- Mass is the matter (kg) and is the same everywhere.
- Weight is a force (N), and depends on g: W = mg.
- If a force is asked for, substitute mg, never the mass on its own.
One picture every idea returns to
Mass is the amount of matter in an object, measured in kilograms, and is the same everywhere. Weight is the gravitational force on the object, measured in newtons, and is found from W = mg, where g is the gravitational field strength in N/kg.
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 same matter, a different pull.. Take a 2 kg bag to the Moon and it is still 2 kg of matter. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Same mass, different weight | Switch worlds and watch the two instruments. Project tab 1 of the lab (Mass and weight) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Telling them apart | Gravitational field strength g is the weight per unit mass, g = W / m, measured in N/kg. | Cooperative Concept Mapping |
| 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 Telling them apart, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| The most common error here is using mass where a force is needed. | If a question asks for a force, such as the weight, the answer must be in newtons, found with W = mg. Saying an object weighs 5 kg mixes the two up: 5 kg is its mass, and its weight would be about 49 N on Earth. In any equation that needs weight, substitute mg, not the mass. |
Mass and weight: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State the difference between the mass and the weight of an object, including the unit of each.
Write the equation linking weight, mass and gravitational field strength, and state the unit of g.
A bag has a mass of 6.0 kg. Taking g = 9.8 N/kg, calculate its weight.
An astronaut travels from Earth to the Moon, where the gravitational field strength is smaller. State what happens to the astronaut's mass and to their weight, and explain why.
On Earth (g = 10 N/kg) an object has a weight of 500 N.
(a)Calculate its mass. (b)Calculate its weight on the Moon, where g = 1.6 N/kg.A student writes: "The box has a weight of 12 kg." Identify the error and write a corrected statement, taking g = 10 N/kg.
Answer key and marking notes
- Mass is the amount of matter in the object, measured in kilograms (kg). ✓
- Weight is the gravitational force on the object, measured in newtons (N). ✓
- Mass is the same everywhere; weight depends on the gravitational field strength g. ✓
- W = m g (weight = mass × gravitational field strength). ✓
- g is measured in newtons per kilogram (N/kg). ✓
- W = m g = 6.0 × 9.8
W = 59 N (58.8 N)
- The mass stays the same, because the amount of matter does not change. ✓
- The weight decreases. ✓
- Weight is W = mg, and g is smaller on the Moon, so the weight is less. ✓
(a) m = W ÷ g = 500 ÷ 10
m = 50 kg
(b) W = m g = 50 × 1.6
W = 80 N
- The kilogram is a unit of mass, not weight; weight is a force, measured in newtons. ✓
- Correct: the box has a mass of 12 kg and a weight of W = mg = 12 × 10 = 120 N. ✓
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.
An object has a mass of 6.0 kg. Taking g = 9.8 N/kg, what is its weight?
- A0.61 N
- B6.0 N
- C16 N
- D59 N
Quiz key
W = mg = 6.0 × 9.8 = 59 N. Option B treats weight as equal to mass; A divides instead of multiplying; C adds the numbers.
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.