The shape of the lesson
By the end of the lesson, learners can
- Will it float?: Change the object and the fluid, then predict.
- Reading a density value: Water is the benchmark.
- Density: ρ = m ÷ V | Mass: m = ρV | Volume: V = m ÷ ρ.
- Float or sink: compare the object's density with the fluid's.
- Units check: g ÷ (g/cm³) leaves cm³, confirming V = m ÷ ρ.
One picture every idea returns to
Density is the mass per unit volume: ρ = m / V, measured in kg/m³ or g/cm³. An object floats in a fluid if its density is less than the density of the fluid, and sinks if its density is greater.
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, How tightly the matter is packed.. A kilogram of feathers and a kilogram of lead have the same mass, yet one fills a sack and the other a thimble. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Will it float? | Change the object and the fluid, then predict. Project tab 2 of the lab (Density and floating) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Reading a density value | Water is the benchmark. 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, 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 Reading a density value, 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 |
|---|---|
| Water is the benchmark, with a density of exactly 1.0 g/cm³. | But what is that in kilograms per cubic metre? Because a cubic metre is so large, it takes 1000 kg/m³ of water to fill it. Do not let the large number fool you: 1.0 g/cm³ and 1000 kg/m³ are the same density. To convert g/cm³ to kg/m³, multiply by 1000. |
Density and floating: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define density and state two units in which it can be measured.
A block has a mass of 540 g and a volume of 200 cm³. Calculate its density.
A liquid has a density of 3.0 g/cm³. Calculate the volume occupied by 750 g of it.
An object has a density of 0.8 g/cm³. State whether it floats or sinks in water (1.0 g/cm³), and explain why.
To find a volume, a student rearranges ρ = m ÷ V into V = ρ × m. Explain why this is wrong, give the correct rearrangement, and use a units check to support it.
A material has a density of 2.5 g/cm³. Calculate the mass of a 40 cm³ sample.
Answer key and marking notes
- Density is the mass per unit volume (ρ = m ÷ V). ✓
- Units: kg/m³ or g/cm³. ✓
- ρ = m ÷ V = 540 ÷ 200
ρ = 2.7 g/cm³
- V = m ÷ ρ = 750 ÷ 3.0
V = 250 cm³
V = m ÷ ρ, not ρ × m
- It floats. ✓
- Its density (0.8 g/cm³) is less than the density of water (1.0 g/cm³). ✓
- The correct rearrangement is V = m ÷ ρ. ✓
- Multiplying ρ × m gives the wrong quantity. ✓
- Units check: g ÷ (g/cm³) = cm³ (a volume), whereas g × (g/cm³) does not. ✓
- m = ρ V = 2.5 × 40
m = 100 g
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 solid blocks are placed in water of density 1.0 g/cm³. Their densities are listed. Which block floats?
| block | density / g/cm³ |
|---|---|
| A | 0.7 |
| B | 1.0 |
| C | 2.7 |
| D | 8.9 |
- Ablock A
- Bblock B
- Cblock C
- Dblock D
Quiz key
Only block A (0.7 g/cm³) is less dense than water, so it floats. Block B has the same density and stays submerged at any depth; C and D are denser and sink. Floating depends on density, not on mass.
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.