AQA GCSE Physics · Forces
Force and extension required practical
Investigate the relationship between force and extension for a spring.
You hang masses on a spring and measure how much it stretches. Up to the limit of proportionality, extension is directly proportional to force, and the gradient of the graph gives the spring constant.
Equipment
- Spring and clamp stand with two clamps and bosses
- Metre ruler, clamped vertically
- Mass hanger and 100 g slotted masses (each adds about 1 N)
- Pointer (e.g. a splint taped to the bottom of the spring)
- Heavy weight to stop the stand tipping
Variables
- Independent you change
- Force (weight) on the spring
- Dependent you measure
- Extension of the spring
- Control you keep the same
- Same spring
- Read the ruler at eye level
- Same pointer position
Method
- 1
Hang the spring from the clamp with a pointer at its bottom. Clamp a ruler vertically next to it.
- 2
Record the pointer’s reading on the ruler with no masses: the original length.
- 3
Add a 100 g mass (about 1 N) to the hanger and record the new pointer reading.
- 4
Keep adding masses one at a time, recording the reading each time.
- 5
Calculate extension = new reading − original reading.
- 6
Plot force against extension and find the gradient to get the spring constant.
Worked example
Spring readings, with extension converted to metres.
| Force (N) | Extension (cm) | Extension (m) |
|---|---|---|
| 0 | 0.0 | 0.000 |
| 1.0 | 4.0 | 0.040 |
| 2.0 | 8.1 | 0.081 |
| 3.0 | 12.0 | 0.120 |
| 4.0 | 15.9 | 0.159 |
| 5.0 | 20.1 | 0.201 |
Working
Spring constant k = F ÷ e (the gradient of the line)
k = 4.0 N ÷ 0.159 m = 25 N/m
Conclusion
The graph is a straight line through the origin, so extension is directly proportional to force (Hooke’s law), with a spring constant of about 25 N/m. Adding much more force would pass the limit of proportionality, and the line would start to curve.
Risks
- Masses could fall on your feetStand up while working, and keep feet clear of the masses.
- Clamp stand could tip overPut a heavy weight on its base.
- Spring could snap back if overstretchedWear eye protection and don’t overload it.
Common mistakes
- Plotting the total length instead of extension. Extension = new length − original length.
- Reading the ruler at an angle (parallax error). Read it at eye level.
- Forgetting to convert cm to m before calculating k in N/m.
Better results
- Use a pointer on the spring so readings are taken from the same point each time.
- Wait for the spring to stop bouncing before taking a reading.
- Repeat the test, unloading as well as loading, and take a mean.
Launching November 2026
7 exam questions on this practical.
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