What is the difference between elastic and plastic deformation?

Elastic deformation is reversible – when the force is removed, the object returns to its original shape. Plastic deformation is permanent – the object does not return to its original shape after the force is removed. For example, stretching a rubber band (elastic) vs. bending a paperclip too far (plastic).

How do I calculate the spring constant from a force-extension graph?

The spring constant k is the gradient of the straight-line portion of the force-extension graph. Choose two points on the line, calculate change in force divided by change in extension (k = ΔF/Δe). Make sure to use units of newtons and metres to get k in N/m.

What is the limit of proportionality?

The limit of proportionality is the point on a force-extension graph beyond which Hooke's Law no longer applies. Up to this point, extension is directly proportional to force. Beyond it, the graph curves, and the material may undergo plastic deformation.

How do I calculate elastic potential energy stored in a spring?

Use the formula EPE = ½ k e², where k is the spring constant in N/m and e is the extension in metres. For example, if k = 100 N/m and e = 0.2 m, then EPE = ½ × 100 × (0.2)² = 2 J.

Why does a spring not obey Hooke's Law if stretched too far?

When a spring is stretched beyond its elastic limit, the bonds between atoms are permanently damaged. The spring no longer returns to its original length, and the force-extension relationship becomes non-linear. This is because the material has undergone plastic deformation.

What is the difference between extension and compression?

Extension is an increase in length when a tensile force is applied (pulling). Compression is a decrease in length when a compressive force is applied (pushing). Hooke's Law applies to both, but extension is usually taken as positive and compression as negative in calculations.

Topic 15 – Forces and matter

EDEXCEL

GCSE

This topic covers the fundamental properties of waves, including the distinction between transverse and longitudinal waves and the transfer of energy without matter. It also explores wave characteristics such as frequency, wavelength, amplitude, and velocity, alongside the effects of reflection, refraction, transmission, and absorption at material interfaces.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Topic 15 – Forces and matter explores the relationship between forces and the deformation of objects. You'll learn how materials respond to stretching, compressing, and bending, and how to calculate key properties like spring constant and elastic potential energy. This topic is essential for understanding real-world applications such as suspension bridges, car springs, and even the elasticity of your skin.

The core of this topic is Hooke's Law, which states that the extension of a spring is directly proportional to the force applied, up to the limit of proportionality. You'll investigate how different materials behave under load, distinguishing between elastic and plastic deformation. Mastering these concepts is crucial for later topics in mechanics and materials science, and they frequently appear in both multiple-choice and calculation exam questions.

By the end of this topic, you should be able to describe how forces affect the shape of objects, calculate spring constants from experimental data, and explain the energy transfers involved when stretching or compressing a spring. This knowledge not only helps you ace your exams but also gives you insight into the physical world around you.

Key Concepts

Core ideas you must understand for this topic

→Hooke's Law: Force (F) = spring constant (k) × extension (e), where extension is proportional to force up to the limit of proportionality.
→Elastic vs. plastic deformation: Elastic deformation is reversible (object returns to original shape), while plastic deformation is permanent.
→Spring constant (k): A measure of stiffness; higher k means stiffer spring. Units: N/m.
→Elastic potential energy (EPE): Energy stored in a deformed elastic object, calculated as EPE = ½ k e².
→Force-extension graphs: The gradient gives the spring constant; the area under the graph equals work done (or EPE).

What You Need to Demonstrate

Key skills and knowledge for this topic

Waves transfer energy and information without transferring matter
Distinction between longitudinal and transverse waves
Use of wave speed equation v = f × λ
Use of wave speed equation v = x / t
Refraction at a boundary involves a change in speed and direction
Ultrasound and infrasound definitions and applications
Relationship between frequency, wavelength, and velocity when changing media

Marking Points

Key points examiners look for in your answers

Waves transfer energy and information without transferring matter
Distinction between longitudinal and transverse waves
Use of wave speed equation v = f × λ
Use of wave speed equation v = x / t
Refraction at a boundary involves a change in speed and direction
Ultrasound and infrasound definitions and applications
Relationship between frequency, wavelength, and velocity when changing media

Examiner Tips

Expert advice for maximising your marks

💡Always show working for calculations, especially when rearranging the wave speed equation
💡Use a ruler for drawing ray diagrams to ensure accuracy
💡Be precise with definitions of frequency and wavelength
💡Remember that the frequency of a wave remains constant when it changes medium
💡Always convert units to metres and newtons before using Hooke's Law. A common mistake is using cm without converting to m, leading to wrong spring constant values.
💡When drawing force-extension graphs, label axes correctly (force on y-axis, extension on x-axis) and include units. The line should start at the origin and be straight up to the limit of proportionality.
💡For calculation questions, show all working and include units in each step. If asked for elastic potential energy, use the formula EPE = ½ k e², not just force × extension.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the direction of particle oscillation with the direction of energy transfer
Incorrectly stating that waves transfer matter
Failing to convert units (e.g., kHz to Hz) before using the wave speed equation
Misinterpreting the relationship between frequency and wavelength in different media
Misconception: Hooke's Law applies for all forces. Correction: It only applies up to the limit of proportionality; beyond that, the relationship becomes non-linear.
Misconception: Extension and compression are the same. Correction: Extension is increase in length, compression is decrease; Hooke's Law works for both but with opposite signs.
Misconception: The spring constant changes with force. Correction: k is constant for a given spring (provided it's not permanently deformed).

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Topic 12 – Forces and motion: Understanding of force, mass, acceleration, and Newton's laws.
•Topic 13 – Energy: Basic energy concepts, including work done and energy transfer.
•Basic algebra: Rearranging equations and working with squares and square roots.

Likely Command Words

How questions on this topic are typically asked

Calculate

Describe

Explain

State

Compare

Ready to test yourself?

Practice questions tailored to this topic

Topic 15 – Forces and matter

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in EDEXCEL GCSE Physics

Topic 1 – Key concepts of physics

Topic 10 – Electricity and circuits

Topic 11 – Static electricity

Topic 12 – Magnetism and the motor effect

How to Revise Topic 15 – Forces and matter — Edexcel GCSE Physics

Examiner Tips for Topic 15 – Forces and matter

Common Mistakes in Topic 15 – Forces and matter

Key Marking Points

Topic 15 – Forces and matter

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between elastic and plastic deformation?

How do I calculate the spring constant from a force-extension graph?

What is the limit of proportionality?

How do I calculate elastic potential energy stored in a spring?

Why does a spring not obey Hooke's Law if stretched too far?

What is the difference between extension and compression?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in EDEXCEL GCSE Physics

Topic 1 – Key concepts of physics

Topic 10 – Electricity and circuits

Topic 11 – Static electricity

Topic 12 – Magnetism and the motor effect