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Physics · Forces
Pressure in a fluid
Use \(p = F \div A\), explain how pressure in fluids acts at right angles to surfaces, and explain everyday examples of pressure.
Warm-up
Answer each one, then check.
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1
What is area?
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The amount of surface
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2
What is the unit of area?
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Square metres (m²)
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3
What is a fluid?
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A liquid or a gas
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4
What is the unit of force?
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Newton
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5
What is the unit of pressure?
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Pascal (Pa)
Learning Objectives
PRESSURE
pressure \(=\) force normal to a surface \(\div\) area of that surface \(p = \dfrac{F}{A}\)
Pressure is in pascals (Pa), force in newtons (N) and area in square metres (m²). 1 Pa = 1 N/m².
Same Force, Different Pressure
A smaller area gives a greater pressure for the same force.
Calculating Pressure
A person of weight 600 N stands on an area of 0.04 m². Calculate the pressure on the ground.
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- 1 Write the equation \(p = F \div A\)
- 2 Substitute \(p = 600 \div 0.04\)
- 3 Answer \(p = 15\,000\) Pa
Answer15 000 Pa
Finding the Force
A pressure of 2000 Pa acts on a surface of area 0.50 m². Calculate the force.
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- 1 Rearrange \(F = pA\)
- 2 Substitute \(F = 2000 \times 0.50\)
- 3 Answer \(F = 1000\) N
Answer1000 N
Explaining Everyday Examples
Explain why a sharp knife cuts better than a blunt one.
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- 1 Same force The same force is applied to both
- 2 Sharp edge Has a very small area
- 3 Pressure A small area gives a large pressure, so it cuts
AnswerThe sharp knife has a smaller area, so for the same force it exerts a higher pressure.
Pressure in Fluids
Liquids and gases.
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Direction
Pressure in a fluid causes a force normal (at right angles) to any surface.
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Gases
Produced by collisions of particles with surfaces.
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Liquids
Particles collide with the container walls in all directions.
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Uses
Tyres, hydraulics and sealed containers use fluid pressure.
Shoe Pressure
A 500 N person wears stiletto heels of total area 0.0004 m² and then flat shoes of area 0.04 m². Calculate the pressure each exerts and explain which damages a floor.
1. Calculate each pressure.
2. Compare.
A good answer shows: Heels: 500 ÷ 0.0004 = 1 250 000 Pa. Flat: 500 ÷ 0.04 = 12 500 Pa. The heels exert 100 times more pressure, so they damage the floor.
Can I...?
- 1Define pressure.
- 2Use \(p = F \div A\).
- 3Rearrange for force or area.
- 4Convert cm² to m².
- 5Explain sharp and blunt objects.
- 6Explain wide tyres.
- 7State the direction of fluid pressure.
- 8Give the unit.
Summary & Exam Focus
- \(p = F \div A\).
- Smaller area, larger pressure.
- Fluid pressure acts at right angles to surfaces.
- 1 Pa = 1 N/m².
Exam focus
A person of weight 600 N stands on 0.04 m² of ground. Calculate the pressure. (2 marks) (2 marks)
Divide force by area.
Key terms
The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.
- Pressure
- Force per unit area.
- Pascal
- The unit of pressure; 1 N/m².
- Fluid
- A liquid or a gas.
- Normal
- At right angles to a surface.
- Area
- The size of a surface.
- Force
- A push or pull.
Practice questions
Have a go at each one before you open its answer.
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Question 1 Calculate 2 marks
A person weighs 600 N and stands on an area of 0.04 m². Calculate the pressure on the ground. Use the equation: pressure = force ÷ area
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Model answer
\(600 \div 0.04 = 15\,000\) Pa
Mark scheme
- Correct substitution — 1 mark
- 15 000 Pa — 1 mark
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Question 2 Explain 2 marks
A nail has a sharp point. Explain why it is easier to push into wood than a blunt object.
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Model answer
The point has a small area, so the same force gives a greater pressure.
Mark scheme
- Small area — 1 mark
- Greater pressure for the same force — 1 mark
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Question 3 Explain 3 marks
A tractor with wide tyres does not sink into soft ground as much as a car with narrow tyres of the same weight. Explain why.
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Model answer
The wide tyres spread the weight over a larger area, so the pressure on the ground is smaller.
Mark scheme
- Weight is the same — 1 mark
- Larger area — 1 mark
- Smaller pressure — 1 mark
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Question 4 Calculate 3 marks
A pressure of 2000 Pa acts on a surface of area 0.50 m². Calculate the force on the surface.
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Model answer
\(F = pA = 2000 \times 0.50 = 1000\) N
Mark scheme
- Rearranges to F = pA — 1 mark
- Correct substitution — 1 mark
- 1000 N — 1 mark
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Question 5 Calculate 4 marks
A 500 N person wears shoes with a total area of 0.0004 m² on a floor. Calculate the pressure. Then state how many times greater it is than the pressure from flat shoes of area 0.04 m².
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Model answer
\(500 \div 0.0004 = 1\,250\,000\) Pa; \(500 \div 0.04 = 12\,500\) Pa; the pressure is 100 times greater.
Mark scheme
- 1 250 000 Pa — 1 mark
- 12 500 Pa — 1 mark
- Ratio 100 — 1 mark
- States 100 times greater — 1 mark
Quick check
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Pressure is...
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D: force per unit area
p = F ÷ A.
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The unit of pressure is...
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A: pascal
1 Pa = 1 N/m².
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A smaller area with the same force gives...
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B: a greater pressure
p = F ÷ A.
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100 N on 2 m² gives...
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C: 50 Pa
100 ÷ 2.
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A fluid is...
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D: a liquid or a gas
Both can flow.
Downloads
Free to keep, print and annotate.
- Pressure in a fluid.pptx Built from the lesson script on 30 September 2026. View
- Pressure in a fluid - Completed Notes.docx The full notes for the lesson, to revise from. Built from the lesson script on 30 September 2026. View
- Pressure in a fluid - Exam Questions.docx Built from the lesson script on 30 September 2026. View
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