Physics · Ch 11 — Waves
Longitudinal Wave Motion
Longitudinal Wave Motion
In longitudinal wave motion, the individual particles that make up the medium oscillate or vibrate about their own fixed mean (equilibrium) positions in a direction that is parallel to the direction the wave itself propagates. Sound waves travelling through air are the standard example: as demonstrated by the vibrating tuning fork in an earlier subsection, each air molecule oscillates back and forth along the same line the sound wave is travelling, alternately being pushed closer to its neighbours (forming a compression, a region of locally raised density and pressure) and then pulled farther from them (forming a rarefaction, a region of locally lowered density and pressure). Unlike transverse waves, which require an elastic medium capable of resisting shearing forces, longitudinal waves can propagate through solids, liquids, and gases alike, since all that is required is that the medium be compressible -- able to be alternately squeez …
What this figure shows. A row of closely spaced dots representing air molecules is drawn along a horizontal line, with the pattern showing alternating regions labelled "Compressed" (where the dots are bunched noticeably closer together than average) and "Stretched" (where the dots are spread noticeably farther apart than average), repeating periodically along the direction of wave travel. A horizontal double-headed arrow labelled "Motion" indicates that the wave itself, and the back-and-forth oscillation of each individual molecule, both lie along this same horizontal line -- unlike the transverse case, here there is no perpendicular displacement at all. The figure makes visually clear that a longitudinal wave is nothing more than this alternating compressed/stretched density pattern advancing steadily along the dire …