My question is how we describe a harmonic oscillate. Wikipedia says, "a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x." My question is, how is the harmonic oscillator a "system"? I thought that the HO was the mechanism, such as a spring or a pendulum. If not, does this mean that the HO is spring + the weight, and the pendulum + the bob, not one or the other? Also, as an extension of this, when we talk about Hooke's law, ##F = -kx##, what is being displaced? Does ##x## measure the displacement of the spring itself or the weight at the end? If it describes the weight, then then does this equation refer to two objects, i.e., F refers to the force caused by the spring while x represents the displacement of some weight at the end of a spring? Does the equation ultimately refer to the spring imposing the force, or does it describe the motion of the weight that the spring manipulates? Also, how does all of this relate to physical systems?