Final answer:
A mass-spring system capable of simple harmonic motion stores energy as elastic potential energy when the spring is compressed or extended, which is then converted to kinetic energy as the system oscillates.
Step-by-step explanation:
A mass-spring system can oscillate with simple harmonic motion because a compressed or stretched spring has elastic potential energy. This form of energy is related to the displacement from the equilibrium position according to Hooke's law, which states F = -kx, with F representing the restoring force, x the displacement, and k the force constant of the spring.
In a simple harmonic oscillator, the energy oscillates between kinetic energy (K = 1/2mv²) and the said elastic potential energy (U = 1/2kx²). As the system oscillates without any dissipative forces, such as friction, the total energy remains constant. At maximum compression or extension (the amplitude of the motion), all of the system's energy is stored as potential energy in the spring. As the mass moves through the equilibrium position, this potential energy is converted into kinetic energy.