Overview
What you’ll learn
- Define pressure as force per unit area and calculate it with p = F/A.
- Explain everyday designs that increase or reduce pressure by changing area.
- Describe how liquid pressure grows with depth and with the density of the liquid.
- Use Δp = ρgΔh to calculate pressure changes under a liquid (Supplement).
Syllabus checklist (0625)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 1.8Define pressure as force per unit area; recall and use the equation p = F/A.
- 1.8Describe how pressure varies with force and area in the context of everyday examples.
- 1.8Describe, qualitatively, how the pressure beneath the surface of a liquid changes with depth and density of the liquid.
Supplement — Extended candidates only
- 1.8Recall and use the equation for the change in pressure beneath the surface of a liquid Δp = ρgΔh.
In 30 seconds
- 1Pressure = force ÷ area: p = F/A, measured in pascals (Pa), where 1 Pa = 1 N/m².
- 2A larger force gives more pressure; a larger area spreads the force and gives less — that is the whole design trick behind snowshoes and sharp knives.
- 3Pressure beneath a liquid surface grows with depth and with the density of the liquid.
- 4Δp = ρgΔh: every extra metre down in water adds about 9800 Pa (Supplement).
Notes · 1 of 3
Pressure, force and area
The same force concentrated on a small area bites; spread out, it barely dents.
Pressure is defined as force per unit area:
Everyday design is all about trading force against area. Increase the area to reduce pressure: snowshoes and skis stop you sinking, tractors have wide tyres, a camel’s broad feet pad across sand, and school bags have wide straps. Decrease the area to increase pressure: a knife’s sharp edge concentrates your hand’s force onto a tiny line of contact, drawing pins pierce boards through their points, and stiletto heels dent wooden floors far more than flat shoes carrying the same person.
Notes · 2 of 3
Pressure in everyday design
Engineers spend their lives moving p = F/A up or down on purpose.
| Design | Area change | Pressure effect | Why it helps |
|---|---|---|---|
| Wide tractor tyres | Larger | Lower | Soft soil is not squeezed into ruts |
| School bag with wide straps | Larger | Lower | Shoulders feel less force per area — no digging in |
| Football studs | Smaller | Higher | Studs bite into firm ground for grip |
| Needle point | Tiny | Very high | The skin is pierced with a gentle push |
| Building foundations | Much larger than the wall | Much lower | The building does not sink into the ground |
Notes · 3 of 3
Pressure in liquids
Dive deeper or dive in a denser liquid and the pressure climbs.
The pressure beneath the surface of a liquid comes from the weight of liquid above. So it grows the deeper you go, and it grows faster in a denser liquid. At any one depth the pressure pushes equally in all directions — which is why water squirts sideways from a hole in a can, not just downwards.
This is why a dam is built thickest at its base: the water there pushes hardest. It is also why your ears ache a few metres under the sea, and why submarines have a tested depth limit.
Supplement: the pressure increase with depth is Δp = ρgΔh, where ρ is the liquid density in kg/m³, g = 9.8 N/kg, and Δh is the depth change in metres.
Revise
Key terms
The words examiners expect you to use precisely.
- Pressure
- Force per unit area acting perpendicular to a surface, p = F/A, in pascals.
- Pascal (Pa)
- The unit of pressure: 1 Pa = 1 newton per square metre.
- Liquid pressure
- The pressure beneath the surface of a liquid, caused by the weight of liquid above, growing with depth and density.
- Density
- Mass per unit volume, ρ = m/V (from Topic 1); the ρ used in Δp = ρgΔh.Supplement
- Depth
- The vertical distance below a liquid’s surface, Δh in Δp = ρgΔh.Supplement
- Atmospheric pressure
- The pressure exerted by the weight of the air above us, about 100 kPa at sea level.
Show 3 more key termsShow fewer key terms
- Dam wall
- A barrier built thicker at the bottom because liquid pressure increases with depth.
- Contact area
- The area over which a force is spread; enlarging it lowers the pressure for the same force.
- Snowshoe
- A wide frame worn under a boot to spread the wearer’s weight and lower the pressure on soft snow.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Quoting the unit of pressure as the newton.
Pressure divides force by area, so its unit is N/m².
Instead: 1 Pa = 1 N/m². A pascal is quite small — kPa and MPa are common in real data.
Mistake: Using the area in cm² with the force in N and calling the answer pascals.
The pascal needs the area in m².
Instead: Convert areas first: 1 cm² = 0.0001 m², so 50 cm² = 0.0050 m².
Show 8 more mistakesShow fewer mistakes
Mistake: Saying heavier objects always exert more pressure.
Pressure depends on force AND the area it is spread over.
Instead: A light person in stiletto heels can exert more pressure than a heavy elephant with broad feet.
Mistake: Thinking liquid pressure pushes only downwards.
At a given depth the pressure acts equally in all directions.
Instead: Water in a can squirts sideways through a hole at the same pressure that pushes down on the base.
Mistake: Building the dam wall uniformly thick.
The pressure at the bottom of a reservoir is far greater than near the surface.
Instead: Dam walls are thickest at the base because liquid pressure increases with depth.
Mistake: Using depth in cm or the density in g/cm³ in Δp = ρgΔh.
The equation needs SI units to give pascals.
Instead: Convert: g/cm³ × 1000 → kg/m³, and cm ÷ 100 → m.
Mistake: Reporting Δp = ρgΔh as the total pressure at depth.
The equation gives the increase due to the liquid only.
Instead: Total pressure = atmospheric pressure at the surface + Δp.
Mistake: Believing pressure at a depth depends on the shape or width of the container.
Only depth and liquid density matter, not the vessel’s shape.
Instead: A narrow tube and a wide tank, filled to the same height with the same liquid, have equal pressures at the bottom.
Mistake: Dividing the force by the total surface area of an object instead of the contact area.
p = F/A uses the area over which the force actually acts.
Instead: Use the area of the face in contact with the surface.
Mistake: Converting cm² to m² by dividing by 100.
The conversion factor squares along with the unit: there are 100 cm in a metre, so 100 × 100 = 10 000 cm² in a square metre.
Instead: 1 m² = 10 000 cm², so divide an area in cm² by 10 000 to get m².
Check yourself
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 5 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Topic 5: Pressure (p = F/A, everyday examples, pressure in liquids, Δp = ρgΔh).
Mark scheme
PROTopic 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
What equations do I need for IGCSE Physics pressure?
p = F/A for pressure from a force on an area, and Δp = ρgΔh for the pressure increase with depth in a liquid (Supplement). Pressure is measured in pascals, 1 Pa = 1 N/m².
Why do sharp things cut and blunt things not, at the same force?
A sharp edge touches over a tiny area, so p = F/A gives an enormous pressure. The blunt edge spreads the same force over a large area and the pressure drops below what is needed to cut.
Does liquid pressure depend on the shape of the container?
No. At any depth the pressure depends only on the depth below the surface and the density of the liquid. A thin tube and a wide tank filled to the same height have the same pressure at the bottom.
What does Δp = ρgΔh actually give me?
The increase in pressure due to the liquid between two depths separated by Δh. To get the total pressure at depth you add the atmospheric pressure acting on the surface on top of this value.
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