Lesson overview
Pressure: Why does a dam need to be thicker at the bottom?
Weigh a brick and stand it on each of its three faces on fresh blocks of soft foam: predict which face sinks deepest, work out the pressure under one of them, and decide how to make the brick lying flat press as hard as it did on its side. Then lower a pressure sensor into tall tubes of water, brine and oil, 0.10 m at a time, and turn it to face the side, to find how the pressure in a liquid depends on depth and density.
Suggested sequence
- Lesson 1 · Force and area. Pressure as force per unit area, p = F/A; how pressure changes with the force and with the area, in everyday examples such as snowshoes and drawing pins. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Pressure in liquids. How the pressure beneath the surface of a liquid changes with depth and with density; Supplement: the change in pressure ∆p = ρg∆h. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Force and area
- Define pressure as force per unit area; recall and use the equation p = F / A (1.8.1)
- Describe how pressure varies with force and area in the context of everyday examples (1.8.2)
Part 2: Pressure in liquids
- Describe, qualitatively, how the pressure beneath the surface of a liquid changes with depth and density of the liquid (1.8.3)
- Supplement: recall and use the equation for the change in pressure beneath the surface of a liquid ∆p = ρg∆h (1.8.4)
Misconceptions to listen for
- “Force × area.” — Multiplying by the area would make a larger area give a larger pressure. Pressure is force divided by area: a larger area gives a smaller pressure.
- “Force per unit volume.” — A pressure acts on a surface, so the force is shared over an area in m², not a volume in m³.
- “Mass per unit area.” — A pressure comes from a force in newtons, not a mass in kilograms. Its unit is N/m², not kg/m².
- “p = 50 × 2.0 = 100 Pa.” — Pressure is force divided by area, not multiplied.
- “To change cm² into m², divide by 100.” — There are 100 cm in a metre, but 100 × 100 = 10 000 cm² in a square metre.
- “1.5 times as large.” — You divided the area change by the force change. Pressure = force ÷ area, so the change is 2 ÷ 3.
- “Six times as large.” — You multiplied the two changes. A larger area makes the pressure smaller, so divide by 3.
- “The same as before.” — The force doubles and the area triples. These do not cancel: work out 2 ÷ 3.
- “p = 30 ÷ 120 = 0.25 Pa.” — That uses the area in cm². Change it into m² first.
- “F = 125 000 × 0.10 = 12 500 N.” — That is the force on one foot. The elephant’s weight is shared by all four.
Scope
Pressure as force per unit area, p = F/A (1.8.1); how pressure varies with force and area in everyday examples (1.8.2); qualitatively, how the pressure beneath the surface of a liquid changes with depth and with the density of the liquid (1.8.3); Supplement: the change in pressure beneath the surface of a liquid, ∆p = ρg∆h (1.8.4). Atmospheric pressure beyond one sentence, hydraulics, upthrust and floating are not covered; gas pressure is in Gas pressure.