Core practical: stretching a spring

1. Hang a spring from a clamp, with a metre rule clamped vertically beside it.
2. Fix a pointer to the bottom of the spring and record its position (the original length).
3. Add a 1 N weight, record the new position and work out the extension (new length − original length).
4. Add weights one at a time up to about 6 N. Read the ruler at eye level to avoid parallax error.
5. Plot force (y-axis) against extension (x-axis).
Safety: wear eye protection in case the spring snaps, and keep feet clear of falling weights.

If the line is straight through the origin, extension is directly proportional to force: F = k × x.
The gradient of the line is the spring constant, k, in N/m.
Beyond the limit of proportionality, the line curves (a non-linear relationship). If stretched too far, the spring will not return to its original length (inelastic deformation).

Stretching the spring does work, which is stored in its elastic potential energy store.
E = ½ × k × x² (only up to the limit of proportionality).
This is the same as the area under the force-extension graph: E = ½ × F × x.