Momentum and its conservation - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Momentum and its conservation

Forces

Lesson 19 of 20

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up

Forces

Lesson 19 of 20

Answer each one, then check.

1

What is velocity?

2

What is mass?

3

What is a closed system?

4

What does conserved mean?

5

What is a collision?

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up - Answers

Forces

Lesson 19 of 20

1

What is velocity?

Speed in a given direction

2

What is mass?

The amount of matter in an object

3

What is a closed system?

A system with no external forces or energy transfer

4

What does conserved mean?

Stays the same in total

5

What is a collision?

When two objects hit each other

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Learning Objectives

Forces

Lesson 19 of 20

1

Define momentum and recall the equation.

2

State the principle of conservation of momentum.

3

Calculate momentum before and after collisions and explosions.

4

Explain examples such as collisions using momentum.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Forces

Lesson 19 of 20

MOMENTUM

momentum = mass × velocity p = mv

In a closed system the total momentum before an event is equal to the total momentum after the event. Momentum is a vector, measured in kg m/s.

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A Collision

Forces

Lesson 19 of 20

Momentum before = momentum after.

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Calculating Momentum

Forces

Lesson 19 of 20

A car of mass 1500 kg moves at 20 m/s. Calculate its momentum.

1

Write the equation

p = mv

2

Substitute

p = 1500 × 20

3

Answer

p = 30 000 kg m/s

ANSWER

30 000 kg m/s

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Collision: Objects Stick Together

Forces

Lesson 19 of 20

A 2.0 kg trolley moving at 3.0 m/s hits a stationary 1.0 kg trolley. They stick together. Calculate their velocity after the collision.

1

Momentum before

2.0 × 3.0 + 1.0 × 0 = 6.0 kg m/s

2

Momentum after

(2.0 + 1.0) × v = 3.0v

3

Conservation

3.0v = 6.0

4

Answer

v = 2.0 m/s

ANSWER

2.0 m/s in the original direction.

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Explosion

Forces

Lesson 19 of 20

A 4.0 kg gun fires a 0.020 kg bullet at 200 m/s. Calculate the recoil velocity of the gun.

1

Momentum before

0 (both at rest)

2

Bullet

0.020 × 200 = 4.0 kg m/s forwards

3

Gun

4.0 × v = −4.0

4

Answer

v = −1.0 m/s

ANSWER

1.0 m/s backwards.

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Key Ideas

Forces

Lesson 19 of 20

Use direction carefully.

Vector

Choose a positive direction; motion the other way is negative.

Closed system

No external forces (such as friction) act.

Kinetic energy

Not always conserved in collisions; momentum is.

Units

kg m/s.

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Key Terms

Forces

Lesson 19 of 20

Momentum

Mass multiplied by velocity.

Conservation of momentum

Total momentum before an event equals total momentum after in a closed system.

Closed system

A system with no external forces.

Collision

Two objects hitting each other.

Recoil

The backward movement of a gun when fired.

Explosion

When objects push apart from rest.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Forces

Lesson 19 of 20

YOUR TASK

Stick Together

12 minutes

A 1.5 kg trolley moving at 2.0 m/s collides with a stationary 0.50 kg trolley and they stick together. Find the velocity after the collision.

1

Find the total momentum before.

2

Divide by the total mass.

WHAT A GOOD ANSWER SHOWS

Momentum before = 3.0; after (2.0 kg) v = 1.5 m/s.

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Can I...?

Forces

Lesson 19 of 20

Define momentum.

Use p = mv.

State conservation of momentum.

Solve a sticking collision.

Solve a recoil problem.

Use positive and negative directions.

Explain momentum in collisions.

Give units.

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Summary & Exam Focus

p = mv.

Closed system: momentum before = momentum after.

Momentum is a vector.

Explosion from rest: total momentum zero.

EXAM FOCUS

A car of mass 1500 kg moves at 20 m/s. Calculate its momentum. (2 marks)

Multiply mass by velocity.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Exam Practice: Momentum and its conservation

Forces

Lesson 19 of 20

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

Question 1 · 2 marks · Calculate

A car of mass 1500 kg travels at 20 m/s. Calculate the momentum of the car. Use the equation: momentum = mass × velocity

Question 2 · 4 marks · Calculate

The diagram shows a trolley A colliding with a stationary trolley B. After the collision they stick together. Calculate the velocity of the…

Question 3 · 3 marks · Explain

A moving ball hits a stationary ball and they move apart. Use the idea of conservation of momentum to explain what happens to the total…

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Exam Practice: Momentum and its conservation (cont.)

Forces

Lesson 19 of 20

Question 4 · 4 marks · Calculate

A gun of mass 4.0 kg fires a bullet of mass 0.020 kg at a speed of 200 m/s. Calculate the recoil speed of the gun.

Question 5 · 2 marks · Explain

Explain why momentum is a vector quantity.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Question 1 · 2 marks · Calculate

Forces

Lesson 19 of 20

“

A car of mass 1500 kg travels at 20 m/s. Calculate the momentum of the car. Use the equation: momentum = mass × velocity

HOW TO ANSWER IT

Command word: Calculate. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Forces

Lesson 19 of 20

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

Correct substitution

1 mark

30 000 kg m/s

1 mark

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Question 2 · 4 marks · Calculate

Forces

Lesson 19 of 20

The diagram shows a trolley A colliding with a stationary trolley B. After the collision they stick together. Calculate the velocity of the trolleys after the collision. (4 marks)

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Question 2 · mark scheme

Forces

Lesson 19 of 20

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

Momentum before 1.0 kg m/s

1 mark

Total mass 2.0 kg

1 mark

Equates momentum

1 mark

0.50 m/s

1 mark

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Question 3 · 3 marks · Explain

Forces

Lesson 19 of 20

“

A moving ball hits a stationary ball and they move apart. Use the idea of conservation of momentum to explain what happens to the total momentum.

HOW TO ANSWER IT

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

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Question 3 · mark scheme

Forces

Lesson 19 of 20

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

Total momentum before equals after

1 mark

Closed system

1 mark

Momentum transferred between the balls

1 mark

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Question 4 · 4 marks · Calculate

Forces

Lesson 19 of 20

“

A gun of mass 4.0 kg fires a bullet of mass 0.020 kg at a speed of 200 m/s. Calculate the recoil speed of the gun.

HOW TO ANSWER IT

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

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Question 4 · mark scheme

Forces

Lesson 19 of 20

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

Momentum of bullet 4.0 kg m/s

1 mark

Total momentum before is zero

1 mark

Equates momentum

1 mark

1.0 m/s backwards

1 mark

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Question 5 · 2 marks · Explain

Forces

Lesson 19 of 20

“

Explain why momentum is a vector quantity.

HOW TO ANSWER IT

Command word: Explain. Worth 2 marks, so plan before writing.

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Question 5 · mark scheme

Forces

Lesson 19 of 20

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

Mass times velocity

1 mark

Has direction

1 mark

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