Forces
Lesson overview
This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.
What you will learn
Spring practical data supplied on this page
Use the spring and rubber band examples to practise gradients, proportionality and elastic energy calculations.
Hooke's law and elastic energy infographic
Clear explanation
Hooke's law says force is proportional to extension until the limit of proportionality is reached. On a force-extension graph, this is the straight-line region through the origin.
The spring constant tells you how stiff a spring is. A larger spring constant means more force is needed for the same extension.
Elastic deformation is reversible when the force is removed. Inelastic deformation leaves the object permanently changed.
Key graph
Worked examples
Spring constant
A force of 12 N produces an extension of 0.04 m.
force = spring constant x extension
spring constant = force ÷ extension = 12 ÷ 0.04
Quick checks
Choose an answer, then check your thinking.
1. What does the straight-line part of a force-extension graph show?
2. What does a larger spring constant mean?
Practice questions
Question 1
A spring has k = 200 N/m and extension 0.03 m. Calculate force.
Reveal answer and marking guidance
Answer: 6 N.
Marking: Credit F = k x e and 200 x 0.03 = 6 N.
Question 2
A 5 N force extends a spring by 0.02 m. Calculate spring constant.
Reveal answer and marking guidance
Answer: 250 N/m.
Marking: Credit k = F ÷ e and 5 ÷ 0.02 = 250 N/m.
Question 3
What is meant by elastic deformation?
Reveal answer and marking guidance
Answer: The object returns to its original shape when the force is removed.
Marking: Credit reversible deformation.
Question 4
How can you identify the limit of proportionality on a graph?
Reveal answer and marking guidance
Answer: It is where the graph stops being a straight line through the origin.
Marking: Credit departure from proportional straight-line behaviour.
Practice ladder
Answers and marking guidance
The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.
Common mistakes
- Using total length instead of extension.
- Forgetting to convert centimetres to metres.
- Assuming Hooke's law applies after the graph curves.
- Confusing elastic deformation with elastic potential energy.
Extension challenge
Sketch a force-extension graph with the limit of proportionality labelled, then explain the gradient in the straight-line region.
Reveal answer
Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.
Exam-board guidance
Short board notes only. Learn the core physics above first.
AQA GCSE Physics
AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for hooke's law, spring constant and elastic energy.
OCR GCSE Physics
OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.
Pearson Edexcel GCSE Physics
Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.
Eduqas GCSE Physics
Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.
WJEC Wales
WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.
CCEA GCSE Physics
CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.
Next lesson
Next, continue with Work Done, Power and Energy Transfer.