AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Pressure in a fluid
Forces · Lesson 7 of 20
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is area?
The amount of surface
2. What is the unit of area?
Square metres (m²)
3. What is a fluid?
A liquid or a gas
4. What is the unit of force?
Newton
5. What is the unit of pressure?
Pascal (Pa)
Learning Objectives
1. Define pressure and recall that a fluid can be a liquid or a gas.
2. Recall and apply p = F ÷ A.
3. Explain that pressure in a fluid causes a force at right angles to any surface.
4. Explain examples where force is spread over a large or small area.
Pressure
pressure = force normal to a surface ÷ area of that surface p = 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
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A smaller area gives a greater pressure for the same force. |
A nail with a tiny tip area giving high pressure, and a person in boots versus snow shoes.
Calculating Pressure
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A person of weight 600 N stands on an area of 0.04 m². Calculate the pressure on the ground. |
1. Write the equation
p = F ÷ A
2. Substitute
p = 600 ÷ 0.04
3. Answer
p = 15 000 Pa
Answer: 15 000 Pa
Finding the Force
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A pressure of 2000 Pa acts on a surface of area 0.50 m². Calculate the force. |
1. Rearrange
F = pA
2. Substitute
F = 2000 × 0.50
3. Answer
F = 1000 N
Answer: 1000 N
Explaining Everyday Examples
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Explain why a sharp knife cuts better than a blunt one. |
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
Answer: The sharp knife has a smaller area, so for the same force it exerts a higher pressure.
Pressure in Fluids
Liquids and gases.
▸ Direction. Pressure in a fluid causes a force normal (at right angles) to any surface.
▸ Gases. Produced by collisions of particles with surfaces.
▸ Liquids. Particles collide with the container walls in all directions.
▸ Uses. Tyres, hydraulics and sealed containers use fluid pressure.
Key Terms
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Pressure Force per unit area. |
Pascal The unit of pressure; 1 N/m². |
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Fluid A liquid or a gas. |
Normal At right angles to a surface. |
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Area The size of a surface. |
Force A push or pull. |
Your Task: Shoe Pressure
10 minutes
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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.
Note: Ask about tractors with wide tyres.
Can I...?
☐ Define pressure.
☐ Use p = F ÷ A.
☐ Rearrange for force or area.
☐ Convert cm² to m².
☐ Explain sharp and blunt objects.
☐ Explain wide tyres.
☐ State the direction of fluid pressure.
☐ Give the unit.
Summary
✓ p = F ÷ A.
✓ Smaller area, larger pressure.
✓ Fluid pressure acts at right angles to surfaces.
✓ 1 Pa = 1 N/m².
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EXAM FOCUS A person of weight 600 N stands on 0.04 m² of ground. Calculate the pressure. (2 marks) Divide force by area. |
Exam Practice: Pressure in a fluid
Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes
▸ Question 1 · 2 marks · Calculate. 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
▸ Question 2 · 2 marks · Explain. A nail has a sharp point. Explain why it is easier to push into wood than a blunt object.
▸ Question 3 · 3 marks · Explain. 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.
▸ Question 4 · 3 marks · Calculate. A pressure of 2000 Pa acts on a surface of area 0.50 m². Calculate the force on the surface.
▸ Question 5 · 4 marks · Calculate. 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…
Question 1 · 2 marks · Calculate
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“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” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 15 000 Pa. 1 mark
▸ Model answer. 600 ÷ 0.04 = 15 000 Pa
Question 2 · 2 marks · Explain
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“A nail has a sharp point. Explain why it is easier to push into wood than a blunt object.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ Small area. 1 mark
▸ Greater pressure for the same force. 1 mark
▸ Model answer. The point has a small area, so the same force gives a greater pressure.
Question 3 · 3 marks · Explain
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“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.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Weight is the same. 1 mark
▸ Larger area. 1 mark
▸ Smaller pressure. 1 mark
▸ Model answer. The wide tyres spread the weight over a larger area, so the pressure on the ground is smaller.
Question 4 · 3 marks · Calculate
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“A pressure of 2000 Pa acts on a surface of area 0.50 m². Calculate the force on the surface.” |
HOW TO ANSWER IT Command word: Calculate. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Rearranges to F = pA. 1 mark
▸ Correct substitution. 1 mark
▸ 1000 N. 1 mark
▸ Model answer. F = pA = 2000 × 0.50 = 1000 N
Question 5 · 4 marks · Calculate
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“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.” |
HOW TO ANSWER IT Command word: Calculate. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ 1 250 000 Pa. 1 mark
▸ 12 500 Pa. 1 mark
▸ Ratio 100. 1 mark
▸ States 100 times greater. 1 mark
▸ Model answer. 500 ÷ 0.0004 = 1 250 000 Pa; 500 ÷ 0.04 = 12 500 Pa; the pressure is 100 times greater.