 In the above example the spring has been extended by a value of x and therefore some potential energy (formula_6) is stored in the spring.Once released, the spring tends to return to its un-stretched state (which is the minimum potential energy state) and in the process accelerates the mass.At the point where the spring has reached its un-stretched state all the potential energy that we supplied by stretching it has been transformed into kinetic energy (formula_7).