Wave Motion: Mechanical Waves and the Need for a Medium

A wave is a disturbance that travels through a medium (or, for electromagnetic waves, through empty space) carrying energy and momentum from one point to another without any net displacement of the medium's own material. A mechanical wave -- the only kind this chapter deals with -- specifically requires a material medium to travel through, and that medium must possess two properties together: elasticity, which supplies the restoring force that pulls a displaced particle back towards its mean position once disturbed, and inertia, which supplies the mass that causes the particle to overshoot its mean position and carry the disturbance onward to its neighbour rather than simply snapping back and stopping. A stretched string, the column of air inside a pipe, and the bulk of water in a pond are all such elastic, inertial media, which is why transverse waves can run along a string and longitudinal (sound) waves can run through air or water; a truly rigid, massless, or non-elastic medium could support no mechanical wave at all. As a wave passes any one point of the medium, that point's own particle is set into oscillation (very often, for the small disturbances this syllabus considers, into simple harmonic motion) about its undisturbed mean position, and it is precisely this handi …