Changes in momentum - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Changes in momentum

Forces

Lesson 20 of 20

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Warm-up

Forces

Lesson 20 of 20

Answer each one, then check.

1

What is momentum?

2

What is acceleration?

3

What is F = ma?

4

Give a safety device in a car.

5

What is a collision?

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up - Answers

Forces

Lesson 20 of 20

1

What is momentum?

Mass times velocity

2

What is acceleration?

Change in velocity divided by time, a = Δv ÷ t

3

What is F = ma?

Resultant force equals mass times acceleration

4

Give a safety device in a car.

Seat belt, airbag or crumple zone

5

What is a collision?

Two objects hitting each other

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Learning Objectives

Forces

Lesson 20 of 20

1

Explain that force equals the rate of change of momentum.

2

Apply F = mΔv ÷ Δt.

3

Explain safety features in terms of increasing the time of a collision.

4

Explain how changes in force, mass, velocity and time are related.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Forces

Lesson 20 of 20

RATE OF CHANGE OF MOMENTUM

force = change in momentum ÷ time taken F = mΔv/Δt

Combining F = ma and a = (v − u)/t gives F = mΔv/Δt, where mΔv is the change in momentum. This equation is given on the equation sheet.

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Why Crumple Zones Help

Forces

Lesson 20 of 20

A longer time means a smaller force for the same momentum change.

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Force in a Crash

Forces

Lesson 20 of 20

A car of mass 1000 kg travelling at 20 m/s stops in 0.50 s. Calculate the average force.

1

Change in momentum

1000 × 20 = 20 000 kg m/s

2

Substitute

F = 20 000 ÷ 0.50

3

Answer

F = 40 000 N

ANSWER

40 000 N

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Increasing the Time

Forces

Lesson 20 of 20

The same car is stopped in 0.10 s by hitting a wall. Calculate the average force and compare with the answer above.

1

Substitute

F = 20 000 ÷ 0.10 = 200 000 N

2

Compare

Five times greater

ANSWER

200 000 N: a much shorter time gives a much greater force.

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Safety Features

Forces

Lesson 20 of 20

Explain how an air bag reduces injury in a collision.

1

Momentum

The change in the person's momentum is the same

2

Time

The air bag increases the time for the person to stop

3

Force

F = mΔv ÷ Δt: longer time gives a smaller force

ANSWER

It increases the collision time so the average force on the person is smaller.

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Other Safety Features

Forces

Lesson 20 of 20

Seat belts

Stretch slightly, increasing the stopping time.

Crumple zones

Deform on impact, increasing the stopping time.

Cycle helmets

The foam compresses, increasing the stopping time.

Crash mats and playground surfaces

Cushion the landing, increasing the time.

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Key Points

Forces

Lesson 20 of 20

For exam answers.

Same momentum change

The change in momentum is the same, whatever the time.

Force depends on time

Smaller rate of change of momentum means smaller force.

Say it clearly

'Increases the time of the collision, so decreases the force.'

Units

Force in N, momentum in kg m/s, time in s.

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Key Terms

Forces

Lesson 20 of 20

Rate of change of momentum

Change in momentum divided by time.

Air bag

A cushion that inflates in a collision.

Crumple zone

Part of a vehicle designed to deform in a crash.

Seat belt

A strap that holds a passenger and stretches slightly.

Impact time

The time a collision lasts.

Average force

The mean force over the collision time.

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Forces

Lesson 20 of 20

YOUR TASK

Ball Catch

10 minutes

A 0.40 kg ball moving at 15 m/s is stopped by a catcher in 0.020 s. A second catcher uses a glove that stops it in 0.10 s. Calculate the force in each case.

1

Find the momentum change.

2

Divide by time.

WHAT A GOOD ANSWER SHOWS

Change in momentum = 6.0 kg m/s. First: 300 N. Second: 60 N.

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Can I...?

Forces

Lesson 20 of 20

State F = mΔv ÷ Δt.

Calculate a change in momentum.

Calculate a force.

Explain an air bag.

Explain a seat belt.

Explain a crumple zone.

Explain a cycle helmet.

Explain a crash mat.

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Summary & Exam Focus

F = mΔv ÷ Δt.

Longer collision time means a smaller force.

Air bags, seat belts, crumple zones, helmets, crash mats.

Change in momentum is the same.

EXAM FOCUS

Explain how a crumple zone reduces the force on the passengers. (3 marks)

Increases the time of the collision.

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Exam Practice: Changes in momentum

Forces

Lesson 20 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 17 minutes

Question 1 · 3 marks · Calculate

A car of mass 1000 kg travelling at 20 m/s stops in 0.50 s. Calculate the average force on the car. Use the equation: force = mass × change…

Question 2 · 3 marks · Explain

Explain how an air bag reduces the force on a driver in a collision.

Question 3 · 3 marks · Explain

A cyclist wears a helmet containing soft foam. Explain how the helmet reduces the risk of head injury in a fall.

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Exam Practice: Changes in momentum (cont.)

Forces

Lesson 20 of 20

Question 4 · 4 marks · Calculate

A ball of mass 0.40 kg moving at 15 m/s is caught and stopped in 0.020 s. Calculate the average force on the ball. Then state the force if…

Question 5 · 3 marks · Explain

A gymnast lands on a thick crash mat rather than a hard floor. Use the idea of rate of change of momentum to explain why the mat reduces…

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Question 1 · 3 marks · Calculate

Forces

Lesson 20 of 20

“

A car of mass 1000 kg travelling at 20 m/s stops in 0.50 s. Calculate the average force on the car. Use the equation: force = mass × change in velocity ÷ time

HOW TO ANSWER IT

Command word: Calculate. Worth 3 marks, so plan before writing.

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Question 1 · mark scheme

Forces

Lesson 20 of 20

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

Change in momentum 20 000 kg m/s

1 mark

Correct substitution

1 mark

40 000 N

1 mark

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Question 2 · 3 marks · Explain

Forces

Lesson 20 of 20

“

Explain how an air bag reduces the force on a driver in a collision.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 2 · mark scheme

Forces

Lesson 20 of 20

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

Increases the time to stop

1 mark

Same change in momentum

1 mark

Force = change in momentum ÷ time, so smaller force

1 mark

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Question 3 · 3 marks · Explain

Forces

Lesson 20 of 20

“

A cyclist wears a helmet containing soft foam. Explain how the helmet reduces the risk of head injury in a fall.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 3 · mark scheme

Forces

Lesson 20 of 20

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

Foam compresses

1 mark

Increases the stopping time

1 mark

Reduces the force

1 mark

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Question 4 · 4 marks · Calculate

Forces

Lesson 20 of 20

“

A ball of mass 0.40 kg moving at 15 m/s is caught and stopped in 0.020 s. Calculate the average force on the ball. Then state the force if a soft glove makes the stopping time 0.10 s.

HOW TO ANSWER IT

Command word: Calculate. Worth 4 marks, so plan before writing.

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Question 4 · mark scheme

Forces

Lesson 20 of 20

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

6.0 kg m/s

1 mark

300 N

1 mark

60 N

1 mark

Comparison: longer time, smaller force

1 mark

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Question 5 · 3 marks · Explain

Forces

Lesson 20 of 20

“

A gymnast lands on a thick crash mat rather than a hard floor. Use the idea of rate of change of momentum to explain why the mat reduces the risk of injury.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 5 · mark scheme

Forces

Lesson 20 of 20

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

Mat increases the stopping time

1 mark

Same change in momentum

1 mark

Smaller rate of change of momentum, smaller force

1 mark

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