Braking distances and large decelerations - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Braking distances and large decelerations

Forces

Lesson 18 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 18 of 20

Answer each one, then check.

1

What is friction?

2

What is kinetic energy?

3

What is deceleration?

4

What is work done?

5

What does the brake do to the wheel?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 18 of 20

1

What is friction?

A force opposing motion

2

What is kinetic energy?

Energy stored in a moving object

3

What is deceleration?

Slowing down

4

What is work done?

Energy transferred by a force

5

What does the brake do to the wheel?

Uses friction to slow it

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 18 of 20

1

Explain how road conditions and the condition of the vehicle affect braking distance.

2

Explain how the brakes reduce the kinetic energy of a vehicle by work done against friction.

3

Explain why greater speed needs a greater braking force to stop in the same distance.

4

Explain the dangers of large decelerations, and estimate the forces involved (Higher tier).

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 18 of 20

BRAKING

When a force is applied to the brakes, the work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and increases the temperature of the brakes.

The greater the braking force, the greater the deceleration. Large decelerations may cause brakes to overheat or a loss of control.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Braking Distance Depends on Conditions

Forces

Lesson 18 of 20

Less friction means a longer braking distance.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

What Affects Braking Distance

Forces

Lesson 18 of 20

INCREASES BRAKING DISTANCE

• Higher speed.

• Wet or icy roads (less friction).

• Worn tyres (less grip).

• Worn brakes (less braking force).

• A heavier vehicle (more kinetic energy).

REDUCES BRAKING DISTANCE

• Lower speed.

• Dry road.

• Good tyres with deep treads.

• Well-maintained brakes.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Braking Force

Forces

Lesson 18 of 20

A car of mass 1200 kg travelling at 25 m/s stops in 50 m. Estimate the average braking force.

1

Kinetic energy

E_k = ½mv² = 0.5 × 1200 × 25² = 375 000 J

2

Work done by the brakes

W = Fs, so F = 375 000 ÷ 50

3

Answer

F = 7500 N

ANSWER

About 7500 N.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Deceleration

Forces

Lesson 18 of 20

For the same car, calculate the deceleration.

1

Use F = ma

a = F ÷ m = 7500 ÷ 1200

2

Answer

a = 6.25 m/s²

ANSWER

6.25 m/s² (deceleration).

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Dangers of Large Decelerations

Forces

Lesson 18 of 20

Explain the dangers of a very large deceleration.

1

Large force

A large braking force is needed

2

Overheating

Brakes can overheat and fade

3

Loss of control

Tyres may lose grip and the car may skid

4

Passengers

Occupants experience large forces that can cause injury

ANSWER

Brakes overheat, the car can skid, and people in the vehicle are harmed.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Energy

Forces

Lesson 18 of 20

What happens to the energy.

Kinetic to thermal

Work done against friction transfers the car's kinetic energy to the thermal energy of the brakes and tyres.

Speed

Doubling speed quadruples kinetic energy, so the braking distance is about four times as long.

Braking force

A greater speed needs a greater braking force to stop in the same distance.

Estimate (Higher)

Use Fs = ½mv² to estimate forces.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Terms

Forces

Lesson 18 of 20

Braking distance

The distance travelled while the brakes slow a vehicle.

Deceleration

A negative acceleration: slowing down.

Friction

A force between surfaces that opposes motion.

Kinetic energy

Energy of a moving object.

Brake fade

Loss of braking power when brakes overheat.

Skid

When tyres lose grip and slide.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 18 of 20

YOUR TASK

Why Is Wet Worse?

10 minutes

A car's braking distance at 30 mph is 14 m on a dry road and 28 m on a wet road. Explain the difference. Estimate the braking distance on a wet road at 60 mph, where the dry distance is 55 m.

1

Link to friction.

2

Use the ratio.

WHAT A GOOD ANSWER SHOWS

Wet roads have less friction so the deceleration is less and the distance is longer. At 60 mph, wet braking distance is about double the dry: about 110 m.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Can I...?

Forces

Lesson 18 of 20

Explain braking distance factors.

Explain brakes and kinetic energy.

Explain why speed matters.

Explain wet and icy roads.

Explain worn tyres and brakes.

Explain brake overheating.

Estimate a braking force.

Explain dangers of deceleration.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

Braking: work done by friction reduces kinetic energy.

Wet or icy roads and worn tyres reduce friction.

Higher speed needs a larger braking force.

Large decelerations can overheat brakes and cause skids.

EXAM FOCUS

Explain why the braking distance of a car is greater on a wet road than on a dry road. (2 marks)

Less friction between the tyres and the road.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Braking distances and large decelerations

Forces

Lesson 18 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 16 minutes

Question 1 · 2 marks · Explain

Explain why the braking distance of a car is greater on a wet road than on a dry road.

Question 2 · 3 marks · Use the graph

The graph shows the braking distance of a car at different speeds on a dry road and on a wet road. (a) Use the graph to find the braking…

Question 3 · 3 marks · Explain

When a car brakes, the brakes become hot. Explain why in terms of energy and work.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Braking distances and large decelerations (cont.)

Forces

Lesson 18 of 20

Question 4 · 4 marks · Calculate

A car of mass 1000 kg travelling at 20 m/s is brought to rest in a distance of 40 m. Estimate the average braking force.

Question 5 · 3 marks · Explain

Explain the dangers of a very large deceleration of a car.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Explain

Forces

Lesson 18 of 20

“

Explain why the braking distance of a car is greater on a wet road than on a dry road.

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 1 · mark scheme

Forces

Lesson 18 of 20

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

Less friction on a wet road

1 mark

Smaller deceleration, so longer distance

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 3 marks · Use the graph

Forces

Lesson 18 of 20

The graph shows the braking distance of a car at different speeds on a dry road and on a wet road. (a) Use the graph to find the braking distance at 50 mph on a wet road. (b) Explain the difference between the two lines. (3 marks)

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Forces

Lesson 18 of 20

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

About 76 m (accept 72 to 80)

1 mark

Less friction on the wet road

1 mark

So greater braking distance

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 3 marks · Explain

Forces

Lesson 18 of 20

“

When a car brakes, the brakes become hot. Explain why in terms of energy and work.

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 18 of 20

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

Work done by the friction force

1 mark

Kinetic energy store decreases

1 mark

Thermal energy store of brakes increases

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 4 marks · Calculate

Forces

Lesson 18 of 20

“

A car of mass 1000 kg travelling at 20 m/s is brought to rest in a distance of 40 m. Estimate the average braking force.

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 4 · mark scheme

Forces

Lesson 18 of 20

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

Kinetic energy 200 000 J

1 mark

Work done = Fs

1 mark

Correct substitution

1 mark

5000 N

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 3 marks · Explain

Forces

Lesson 18 of 20

“

Explain the dangers of a very large deceleration of a car.

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 5 · mark scheme

Forces

Lesson 18 of 20

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

Brakes overheat

1 mark

Loss of control or skidding

1 mark

Risk of injury

1 mark

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