Momentum and its conservation.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Momentum and its conservation

Forces

Lesson 19 of 20

openrevise.com

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?

openrevise.com

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

openrevise.com

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.

openrevise.com

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

A Collision

Forces

Lesson 19 of 20

Momentum before = momentum after.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

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.

openrevise.com