Elastic Strain Energy
When you stretch a material, you're doing work against the internal forces, and this energy gets stored as elastic strain energy. For materials obeying Hooke's law, the work done equals ½FΔL or ½k(ΔL)².
This stored energy can be released when the material returns to its original shape. Springs in car suspension systems work this way - they store energy when compressed and release it to smooth out bumps.
Conservation of energy applies throughout the stretching process. In oscillating springs, energy constantly converts between elastic strain energy, kinetic energy, and gravitational potential energy, but the total remains constant (ignoring friction).
Practical Application: Understanding elastic strain energy is crucial for designing everything from trampolines to earthquake-resistant buildings.