English: Illustration of Hooke's law for a helical spring. The top part is a plot of the linear relation between force F and elongation X according to Hooke's law (solid line). The red half correspond to positive elongation and pulling force, the blue part to negative elongaton (compression) and pushing force. The slope of the curve is Hooke's constant "k".

The bottom part has schematic pictures of the spring states corresponding to some points of the plot; the arrows represent the forces that need to be applied to the left (free) end to obtain each state. The central picture represents the spring's relaxed state, when no force is applied. Again, red arrows are used for elongation, blue arrows for compression.

The dashed line on the plot is a schematic illustration of what the actual force vs elongation plot would look like for a real spring. Note that beyond certain limits, the spring becomes much stiffer and very large increases in the force will be needed for small increases in the deformation. Some of the deformation may be plastic (permanent) rather than elastic (self-restoring). Needless to say, at very large elongations the spring will snap and the force will become zero.).

The illustration was generated by me with gnuplot, through the script make-hookes-fig.sh, presently available at http://www.ic.unicamp.br/~stolfi/EXPORT/projects/wikipedia-figs/hookes-law/