What does 'simple harmonic motion' mean? By definition, it describes any periodic motion that is the result of a restoring force that is proportional to displacement. What does 'periodic' mean here? It means recurring at equal intervals of time? Ususally, the example of SHM is virating mass-spring system. But the definition of SHM is sort of according to the formula, so I don't really get its essence. Is simple harmonic motion simply a back-and-forth motion over the same path? In the 'wave', it says that a wave whose source vibrates with simple harmonic motion is called a sine wave. Once again, what does SHM really mean? Thanks.