Elastic potential energy ideal spring and the spring"s displacement. Ideal spring: any spring that obeys hooke"s law; it does not experience any internal or external friction. Hooke"s law: the amount of force exerted by a spring is directly proportional to the displacement of the spring force exerted by the spring on whatever stretches it: displacement of the spring from its upstretched, equilibrium position. Spring constant ( ): the constant of variation between the force exerted by an. Amplitude ( ): the maximum displacement of a wave. Elastic potential energy: potential energy due to the stretching or compressing of an elastic material. Simple harmonic motion: periodic motion in which the acceleration of the moving object is proportional to its displacement. Perpetual motion: machine a machine that can operate forever without restarting or refueling. Damped harmonic motion: periodic motion affected by friction.