The shape of the lesson
By the end of the lesson, learners can
- Sink lower, feel more: Change the depth and the density of the liquid.
- Depth, density, direction: The defining equation and its rearrangement for depth:
- Linear with depth: twice as deep gives twice the pressure.
- Denser liquid: higher pressure at the same depth.
- Shape and area: make no difference, only vertical depth.
One picture every idea returns to
The change in pressure with depth in a liquid is Δp = ρgΔh, where ρ is the density of the liquid, g is the gravitational field strength and Δh is the depth below the surface. Pressure acts equally in all directions.
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, Deeper means harder squeezed.. Dive to the bottom of a pool and your ears feel the weight of all the water above you. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Sink lower, feel more | Change the depth and the density of the liquid. Project tab 2 of the lab (Pressure in liquids) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Depth, density, direction | The defining equation and its rearrangement for depth: Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 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, 14 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 Depth, density, direction, 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.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
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 |
|---|---|
| Liquid pressure depends on depth and density, never on the shape or the amount of liquid. | A narrow tube and a wide tank filled to the same depth give exactly the same pressure at the bottom. Use the vertical depth below the surface for Δh, and the density of the liquid for ρ, not the density of anything floating in it. |
Pressure in liquids (Δp = ρgΔh): worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Write the equation for the change in pressure with depth in a liquid, and state what each symbol represents.
Calculate the pressure due to the water at a depth of 5.0 m in a lake. Take the density of water as 1000 kg/m³ and g = 10 N/kg.
A tank holds oil of density 850 kg/m³. Calculate the pressure due to the oil at a depth of 3.0 m, using g = 9.8 N/kg.
The wall of a dam is built much thicker at the bottom than at the top. Explain why, in terms of pressure.
Two differently shaped containers, one narrow and one wide, are filled with the same liquid to the same depth. Compare the pressure at the base of each and explain your answer.
The pressure due to the water at a certain depth in a reservoir is 39200 Pa. Taking the density of water as 1000 kg/m³ and g = 9.8 N/kg, calculate the depth.
Answer key and marking notes
- Δp = ρgΔh. ✓
- ρ = density (kg/m³), g = gravitational field strength (N/kg), Δh = depth below the surface (m). ✓
- Δp = ρgΔh = 1000 × 10 × 5.0
50000 Pa
- Δp = 850 × 9.8 × 3.0
24990 Pa (about 25 kPa)
- Pressure increases with depth, since Δp = ρgΔh. ✓
- So the water pushes hardest near the bottom of the dam. ✓
- The wall is made thicker there to withstand the greater pressure and force. ✓
- The pressure at the base is the same in both. ✓
- Pressure depends only on depth and density (Δp = ρgΔh), not on the shape or width of the container. ✓
- Δh = Δp ÷ (ρg) = 39200 ÷ (1000 × 9.8)
Δh = 39200 ÷ 9800
Δh = 4.0 m
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.
What is the pressure due to water at a depth of 2.5 m? Take the density of water as 1000 kg/m³ and g = 9.8 N/kg.
- A2500 Pa
- B9800 Pa
- C24 500 Pa
- D245 000 Pa
Points X, Y and Z are at different depths in a tank of water, with X near the surface and Z near the bottom. How do the water pressures compare?
- Athe pressure is the same at all three points
- Bthe pressure is greatest at X
- Cthe pressure is greatest at Z
- Dthe pressure depends only on the width of the tank
Quiz key
p = ρgh = 1000 × 9.8 × 2.5 = 24 500 Pa. Option A omits g; B omits the depth; D has an extra factor of ten.
From p = ρgh, pressure increases with depth, so it is greatest at the deepest point Z. Option B reverses this. Pressure does not depend on the tank's width or shape, which rules out D.
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.