← all cardsTopic 11 · Wave Motion · Definitions
Displacement, y
● VERBATIM — examined word-for-wordThe distance in a specified direction of a particle from its equilibrium position. Vector — can be negative. Unlike y₀, λ, f, T and v (all constants of the wave), displacement differs from particle to particle at an instant.
What this actually means
Displacement is a per-particle, per-instant quantity: this particle, right now, sits 2 mm above its rest position. It is a vector, so below equilibrium counts as negative.
'In a specified direction' and 'from its equilibrium position' are the mark words. Distance from where it started, or from another particle, is wrong.
Contrast with the wave's constants: y₀, λ, f, T and v describe the whole wave and are the same everywhere. Displacement is the one quantity that varies from particle to particle at a single instant.
Prove it — watch it be true
- Pick two particles P and Q at the same instant
- Their y values differ, while amplitude, wavelength and frequency are shared
- Step time forward: each particle's y changes; the wave's constants do not