Forces and elasticity - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces and elasticity

Forces

Lesson 5 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 5 of 20

Answer each one, then check.

1

What is a spring?

2

What does proportional mean?

3

Convert 6 cm to m.

4

What is elastic potential energy?

5

What is the unit of force?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 5 of 20

1

What is a spring?

A coiled wire that can be stretched or compressed

2

What does proportional mean?

Doubling one doubles the other

3

Convert 6 cm to m.

0.06 m

4

What is elastic potential energy?

Energy stored in a stretched spring

5

What is the unit of force?

Newton

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 5 of 20

1

Explain why more than one force is needed to stretch, bend or compress an object.

2

Describe elastic and inelastic deformation.

3

Recall and apply F = ke and E_e = ½ke².

4

Interpret force-extension graphs and describe Required Practical 6.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 5 of 20

HOOKE'S LAW

force = spring constant × extension F = ke

The extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded. The same applies to compression.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Force and Extension

Forces

Lesson 5 of 20

Beyond the limit, the spring does not return to its original length.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Required Practical 6: Stretching a Spring

Forces

Lesson 5 of 20

Measure the length with each extra mass added.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Required Practical 6: Method

Forces

Lesson 5 of 20

Investigate force and extension.

1

Measure the original length

With no masses on the spring.

2

Add a mass

Record the force (weight = mg) and the new length.

3

Calculate the extension

New length minus original length.

4

Repeat

Add masses one at a time, keeping below the limit of proportionality at first.

5

Plot

Force (y-axis) against extension (x-axis).

6

Gradient

Gradient of the straight part = spring constant k.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Spring Constant

Forces

Lesson 5 of 20

A force of 3.0 N stretches a spring by 6.0 cm (within the limit of proportionality). Calculate the spring constant.

1

Convert

6.0 cm = 0.060 m

2

Rearrange

k = F ÷ e

3

Substitute

k = 3.0 ÷ 0.060

4

Answer

k = 50 N/m

ANSWER

50 N/m

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Elastic Potential Energy

Forces

Lesson 5 of 20

Calculate the energy stored in the spring when the extension is 0.040 m and k = 50 N/m.

1

Write the equation

E_e = ½ke²

2

Substitute

E_e = 0.5 × 50 × 0.040²

3

Answer

E_e = 0.040 J

ANSWER

0.040 J

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Elastic or Inelastic?

Forces

Lesson 5 of 20

ELASTIC DEFORMATION

• The object returns to its original shape when the force is removed.

• Happens within the limit of proportionality.

INELASTIC DEFORMATION

• The object does not return to its original shape.

• Happens beyond the limit of proportionality.

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Key Ideas

Forces

Lesson 5 of 20

Learn these.

Two forces

To stretch, bend or compress a stationary object, more than one force is needed (for example pull at both ends).

Work done

The work done on an elastic object equals the elastic potential energy stored, provided it is not inelastically deformed.

Linear

A straight line through the origin: proportional.

Non-linear

A curve: not proportional.

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Key Terms

Forces

Lesson 5 of 20

Extension

The increase in length of a spring.

Spring constant

The force needed per metre of extension (N/m).

Limit of proportionality

The point beyond which force and extension are no longer proportional.

Elastic deformation

A change of shape that is reversed when the force is removed.

Inelastic deformation

A permanent change of shape.

Linear

Following a straight line.

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Forces

Lesson 5 of 20

YOUR TASK

Spring Test

12 minutes

A spring stretches 4.0 cm when a 2.0 N force is applied. (a) Calculate k in N/m. (b) What force gives an extension of 10 cm if the limit is not exceeded? (c) What energy is stored at 10 cm?

1

Convert cm to m.

2

Use F = ke and Ee = ½ke².

WHAT A GOOD ANSWER SHOWS

(a) 50 N/m. (b) F = 50 × 0.10 = 5.0 N. (c) E = 0.5 × 50 × 0.10² = 0.25 J.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Can I...?

Forces

Lesson 5 of 20

State F = ke.

Convert cm to m.

Calculate a spring constant.

Calculate stored energy.

Describe elastic and inelastic deformation.

Read a force-extension graph.

Describe Required Practical 6.

Find the gradient.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

F = ke.

E_e = ½ke².

Beyond the limit of proportionality: non-linear, may not return.

Spring constant = gradient of the linear part.

EXAM FOCUS

A spring has a spring constant of 50 N/m. Calculate the force needed to stretch it by 0.10 m. (2 marks)

Use F = ke.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Forces and elasticity

Forces

Lesson 5 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 18 minutes

Question 1 · 2 marks · Calculate

A spring has a spring constant of 50 N/m. Calculate the force needed to stretch the spring by 0.10 m. Use the equation: force = spring…

Question 2 · 5 marks · Use the graph

A student investigates a spring. The graph shows how the force changes with extension. (a) State the extension at the limit of…

Question 3 · 2 marks · Describe

Describe the difference between elastic deformation and inelastic deformation of a spring.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Forces and elasticity (cont.)

Forces

Lesson 5 of 20

Question 4 · 6 marks · Describe

Describe how to investigate the relationship between the force applied to a spring and the extension of the spring. Your answer should…

Question 5 · 2 marks · Explain

Explain why two forces are needed to stretch a spring that is fixed to a wall.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Calculate

Forces

Lesson 5 of 20

“

A spring has a spring constant of 50 N/m. Calculate the force needed to stretch the spring by 0.10 m. Use the equation: force = spring constant × extension

HOW TO ANSWER IT

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

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Question 1 · mark scheme

Forces

Lesson 5 of 20

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

Correct substitution

1 mark

5.0 N

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 5 marks · Use the graph

Forces

Lesson 5 of 20

A student investigates a spring. The graph shows how the force changes with extension. (a) State the extension at the limit of proportionality. (b) Calculate the spring constant of the spring. (c) Calculate the elastic potential energy stored when the extension is 0.040 m. (5 marks)

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Question 2 · mark scheme

Forces

Lesson 5 of 20

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

6 cm (accept 5.5 to 6.5)

1 mark

Uses 3.0 N at 0.060 m

1 mark

50 N/m

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme (cont.)

Forces

Lesson 5 of 20

Correct substitution

1 mark

0.040 J

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 2 marks · Describe

Forces

Lesson 5 of 20

“

Describe the difference between elastic deformation and inelastic deformation of a spring.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · mark scheme

Forces

Lesson 5 of 20

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

Elastic: returns to original shape

1 mark

Inelastic: does not return

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 6 marks · Describe

Forces

Lesson 5 of 20

“

Describe how to investigate the relationship between the force applied to a spring and the extension of the spring. Your answer should include the apparatus, the measurements, and how you would use them.

HOW TO ANSWER IT

Command word: Describe. Worth 6 marks, so plan before writing.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · mark scheme

Forces

Lesson 5 of 20

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

Level 3 (5 to 6 marks): a complete method with stand, ruler and masses, original length measured, several forces (weights) applied with the new length each time, extension calculated, graph plotted and spring constant from the gradient, with a precaution (eye level, avoid overstretching)

5 to 6 marks

Level 2 (3 to 4 marks): a method covering force and extension with some detail missing

3 to 4 marks

Level 1 (1 to 2 marks): simple statements about adding masses and measuring length

1 to 2 marks

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Question 5 · 2 marks · Explain

Forces

Lesson 5 of 20

“

Explain why two forces are needed to stretch a spring that is fixed to a wall.

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 5 · mark scheme

Forces

Lesson 5 of 20

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

Forces act at both ends

1 mark

Equal and opposite forces

1 mark

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