Pressure in a fluid - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Pressure in a fluid

Forces

Lesson 7 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 7 of 20

Answer each one, then check.

1

What is area?

2

What is the unit of area?

3

What is a fluid?

4

What is the unit of force?

5

What is the unit of pressure?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 7 of 20

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)

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Same Force, Different Pressure

Forces

Lesson 7 of 20

A smaller area gives a greater pressure for the same force.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Calculating Pressure

Forces

Lesson 7 of 20

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Finding the Force

Forces

Lesson 7 of 20

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Explaining Everyday Examples

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Pressure in Fluids

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Terms

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 7 of 20

YOUR TASK

Shoe Pressure

10 minutes

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.

WHAT 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.

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Can I...?

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

p = F ÷ 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)

Divide force by area.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Pressure in a fluid

Forces

Lesson 7 of 20

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Pressure in a fluid (cont.)

Forces

Lesson 7 of 20

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…

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Calculate

Forces

Lesson 7 of 20

“

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · mark scheme

Forces

Lesson 7 of 20

2 marks available. Award a mark for each point made.

Correct substitution

1 mark

15 000 Pa

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 2 marks · Explain

Forces

Lesson 7 of 20

“

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Forces

Lesson 7 of 20

2 marks available. Award a mark for each point made.

Small area

1 mark

Greater pressure for the same force

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 3 marks · Explain

Forces

Lesson 7 of 20

“

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · mark scheme

Forces

Lesson 7 of 20

3 marks available. Award a mark for each point made.

Weight is the same

1 mark

Larger area

1 mark

Smaller pressure

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 3 marks · Calculate

Forces

Lesson 7 of 20

“

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · mark scheme

Forces

Lesson 7 of 20

3 marks available. Award a mark for each point made.

Rearranges to F = pA

1 mark

Correct substitution

1 mark

1000 N

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Calculate

Forces

Lesson 7 of 20

“

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.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · mark scheme

Forces

Lesson 7 of 20

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

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