a hypothetical pendulum suspended by a weightless frictionless thread of constant length

a linear system only when the oscillations of the pendulum are constrained to be quite small (as they are in introductory physics classes)

a mechanical system that displays oscillatory motion

a point (small) mass suspended by a light (of negligible mass) string

a mass swinging on the end of a massless string under the influence of gravity.

a simple machine based on the effect of gravity