Science required practicals
How do you do the force and extension required practical?
Hang a spring from a clamp beside a vertical ruler and note where its bottom sits. Add masses one at a time and record the new position each time. Extension is the new length minus the original length. Work out each force from the weight (W = m g), then plot force against extension. The straight part of the graph shows F = k e, and its gradient is the spring constant k, in N/m.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the force and extension practical investigate?
It investigates how far a spring stretches as you increase the force on it. Up to a point, the extension is directly proportional to the force, which is what F = k e says. Stretch the spring past its limit of proportionality and the graph stops being a straight line, and the spring may not go back to its original length.
It is a required practical on AQA GCSE Physics and on AQA GCSE Combined Science: Trilogy.
What equipment do you need?
The set-up is a spring hanging from a clamp stand, with a ruler beside it to measure how far it stretches as you add masses.
- A clamp stand with two bosses and clamps
- A spring
- A mass hanger and slotted masses, often 100 g each
- A metre ruler, clamped upright beside the spring
- A pointer taped to the bottom of the spring (a fiducial marker)
- A G-clamp or heavy weight, so the stand cannot tip over
- Eye protection
What is the method?
You read the pointer's position with no load, then again after each mass is added. The extensions come from those readings.
- Clamp the stand to the bench, or weigh its base down, so it cannot tip.
- Hang the spring from one clamp and clamp the metre ruler upright next to it.
- Tape the pointer to the bottom of the spring so its tip lies against the ruler's scale.
- Record the pointer's position with nothing hanging on the spring.
- Hang the mass hanger on the spring and record the new position. The hanger counts as part of the load.
- Add masses one at a time, recording the position after each one.
- Work out each extension: the new position minus the starting position.
- Take the masses off one at a time and check the pointer goes back to where it started.
What are the variables?
You change the force by adding masses, and you measure the extension each one causes.
| Variable | In this practical |
|---|---|
| Independent variable | The force on the spring (the weight of the masses hanging on it) |
| Dependent variable | The extension of the spring |
| Control variable | The spring itself: the same one for every reading |
| Control variable | The ruler's position, clamped so it cannot move between readings |
How do you work out and present the results?
Turn each mass into a force with W = m g, using the mass in kilograms and g = 9.8 N/kg. Then use F = k e, rearranged to k = F ÷ e, with the force in newtons and the extension in metres.
Worked example: a 300 g load has a weight of 0.300 × 9.8 = 2.94 N. The pointer moves from 5.0 cm to 11.0 cm, so e = 6.0 cm = 0.060 m. Then k = 2.94 ÷ 0.060 = 49 N/m.
Plot force on the y-axis against extension on the x-axis. The first part is a straight line through the origin, and its gradient is the spring constant. Where the line starts to curve, the spring has passed its limit of proportionality, so find k from the straight part only. If a graph puts extension on the y-axis instead, its gradient is 1 ÷ k.
How do you make it accurate and safe?
The pointer is your fiducial marker. It gives you one fixed point to read every time, instead of guessing where the curled end of the spring meets the scale. Get your eye level with the pointer before you read it, because looking from above or below shifts the reading (parallax). Let the spring stop bouncing before each reading, and check the ruler is truly upright.
On safety: wear eye protection, because a spring can slip off or snap back. Keep your feet out from under the masses. Stop adding masses if the spring suddenly stretches much more than it did for the last one.
What do exam questions ask about it?
You may be asked to calculate a spring constant, or to read one from a graph and spot the limit of proportionality. Method questions often ask why the pointer is used. Some questions go on to the energy stored in the stretched spring.
Here is one we wrote in the exam style: "A spring stretches from 4.0 cm to 7.5 cm when a force of 1.4 N is applied. Calculate the spring constant in N/m. (3 marks)" The extension is 3.5 cm, which is 0.035 m, so k = 1.4 ÷ 0.035 = 40 N/m.
Common mistakes
- Using the total length instead of the extension. Subtract the original length first.
- Leaving the extension in centimetres. For k in N/m, it has to be in metres.
- Using the mass as the force. A 200 g mass is not 200 N: its weight is 0.2 × 9.8 = 1.96 N.
- Finding k from points past the limit of proportionality. Use the straight part of the graph only.
- Reading the scale from an angle. Line your eye up with the pointer.