Applies to: AQA GCSE Physics and Combined Science (Required Practical 6). Edexcel, OCR and the IGCSE boards all cover force and extension of a spring too, so this one is useful for everyone.

Springs are everywhere once you start looking: in pens, mattresses, trampolines and car suspension. They all follow the same simple rule, at least until you pull them too far. You can find that rule for yourself with a rubber band, a sandwich bag and a handful of coins.

What it tests

How the extension of a spring (or elastic band) depends on the force pulling on it, and the point where it stops behaving predictably.

What you need

A long rubber band (or a plastic slinky), a small sandwich bag tied to a paper clip, a wooden spoon or broom handle resting across two chair backs, a 30 cm ruler taped upright to a chair leg or wall, kitchen scales, coins or uncooked rice, and some sticky tape.

How to do it

Hang the rubber band from the middle of the wooden spoon and hook the empty bag onto the bottom with the paper clip. Read the position of the bottom of the band at eye level. This is your starting reading. Weigh out 50 g of coins or rice, put it in the bag, wait five seconds and read the new position. Keep adding 50 g at a time up to 400 g. Work out the extension each time (new reading minus starting reading) and the force (mass in kg multiplied by 9.8, so 50 g gives 0.49 N). Then plot force on the y-axis against extension on the x-axis.

Safety note

Keep your face away from the band, because a stretched band can snap and flick back. Stop adding weight if the band looks very thin or starts to split, make sure the spoon can't slide off the chairs, and keep your feet clear of the bag in case it falls.

What you should find

For a spring, the graph starts as a straight line through the origin, so extension is directly proportional to force. That's Hooke's law. Past a certain point the line curves, because the spring has gone beyond its limit of proportionality. A rubber band usually gives a curve from the start, so it doesn't obey Hooke's law very well, and noticing that is good science in itself. The PDF includes sample results from a real spring if you'd like to see the straight line.

Why it matters for the exam

The big one is F = k × e, and the classic mistake is forgetting to change the extension from centimetres into metres before using it. You'll also need to find the spring constant from the gradient of the graph, identify the limit of proportionality, and Higher students should be ready to calculate the energy stored using E = ½ × k × e². The PDF has all of these, plus a 6-mark method question with a mark scheme.

On motivation: adding weight a little at a time and watching the band stretch gives instant feedback with every step. For students who lose focus with long written tasks, that steady rhythm of "add, wait, read, record" can be really settling.

Free download: the Practicals at Home PDF

I've put everything for this practical into one free PDF: the home method, a results table, sample results if you don't have the kit, the school lab method the exam questions are based on, exam-style questions and a full mark scheme so you can check your own answers.

Download the Stretching a spring (Hooke's law) PDF

Watch and listen: Key Ideas

To help the theory stick, I've made a short Key Ideas video and a podcast on elastic potential energy and Hooke's law, including the equations you need to learn. Both are short enough to fit in a bus journey or a dog walk.

• Watch the Key Ideas video: Elastic potential energy and Hooke's law

• Listen to the Key Ideas podcast: Elastic potential energy and Hooke's law

These are a little taster of what's inside the Science Cafe revision portal, where every lesson comes with notes, audio, flashcards, a quiz, exam-style questions and a personalised daily revision task built around the topics your teen has covered in school.

If your teen sits their GCSEs next summer, they can have full access to all of it for one payment of £99 that lasts right up to the exams. No monthly fees, and it starts with a 14-day free trial.

Get the £99 until the exams plan here, or see the portal in action.