MODEL RELEASED. Physics experiment. Schoolgirl holding a mass (black sphere) suspended on the end of a spring. The extension of the spring can be measured as part of an experiment to test Hooke's Law. As applied to a mass on a spring, this law states that the spring's extension is proportional to the mass. If you double the mass, you double the extension of the spring. This law is no longer true when the elastic limit is reached, causing the spring to deform and eventually break.