Force and extension (Hooke’s law) — required practical
AQA GCSE Physics
Investigate the relationship between the force applied to a spring and its extension.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Method
- Hang a spring from a clamp and measure its natural length.
- Add masses one at a time, recording the new length each time (extension = new length − natural length).
- Continue beyond the point where the line stops being straight if asked to find the limit of proportionality.
Variables
- Independent variable: Force applied (weight of the masses, F = mg)
- Dependent variable: Extension of the spring
- Control variables: The spring used
Results & analysis
Force = spring constant × extension (F = k e). A force–extension graph is a straight line through the origin up to the limit of proportionality, then it curves. The gradient of the straight part = the spring constant k.
Risk assessment
- Wear eye protection and stand back — a spring can fly off if overstretched; place a mat under the masses.
Improving accuracy & reliability
- Measure length at eye level to avoid parallax.
- Do not exceed the limit of proportionality if testing Hooke’s law.
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