THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
velocity
The rate of change of position with respect to time; a vector in 2D and 3D, a signed scalar in 1D.
Venturi effect
The drop in static pressure observed when a fluid flows through a constricted section of pipe.
vis viva equation
v² = GM(2/r − 1/a) — gives the orbital speed at any distance r for an orbit with semi-major axis a.
Viscosity
A fluid's internal resistance to shear. For Newtonian fluids, shear stress = η·(du/dy). Unit: Pa·s.
Volt
The SI unit of electric potential difference. One volt equals one joule of work per coulomb of charge transported. Symbol: V.
Voltage divider
Two resistors in series between a voltage source and ground, with the output voltage taken at the midpoint. V_out = V_in · R₂/(R₁+R₂). The simplest non-trivial circuit.
Vortex
A coherent region of swirling fluid — the basic building block of turbulent and near-turbulent flow.
Wave equation
The linear PDE whose solutions are any right-mover plus any left-mover at speed v.
Wave packet
A localised wave formed by superposing many plane-wave components with a narrow band of wavenumbers.
Waveguide mode
A specific transverse field pattern that propagates along a waveguide without distortion, characterised by a propagation constant β and a cutoff frequency below which the mode is evanescent.
Wavelength
The spatial period of a wave — distance crest to crest, symbol λ.
Wavelength (EM)
The spatial period λ of an EM plane wave, the distance between successive points of equal phase. Related to frequency by λ = c/f in vacuum, λ = c/(nf) in a medium. Ranges from kilometres (longwave radio) to femtometres (gamma).
Wavenumber
k = 2π/λ, the spatial angular frequency of a wave, measured in rad/m. The wavevector k has magnitude k and direction along propagation. In spectroscopy, wavenumber often means k̃ = 1/λ in cm⁻¹.
Weak equivalence principle
The statement that gravitational mass equals inertial mass for any test particle, regardless of composition — equivalently, that all bodies in vacuum fall with identical acceleration. Galileo's Pisa-tower observation, Eötvös's torsion-balance precision result, and the MICROSCOPE 2017 satellite null result confirm it to ≲ 10⁻¹⁵.
Weber
The SI unit of magnetic flux. One weber is the flux through a one-square-metre area of a one-tesla field. Symbol: Wb.
work
The energy transferred to a body by a force acting over a distance: W = F · d · cos θ.
World-line
The locus of events in spacetime traced out by a particle as it moves; a 1D curve in the 4D manifold. Massive particles have timelike world-lines; photons have null world-lines; the proper time elapsed on the particle's clock is the arc length of its world-line in the Minkowski metric.
Yang-Mills equations
The non-abelian generalisation of Maxwell's equations: D_μ F^{aμν} = J^{aν}, where the covariant derivative D_μ = ∂_μ + i g [A_μ, ·] couples the gauge field to itself through the structure constants of the gauge group. The field equations of every non-abelian gauge theory in physics.
Young's interference experiment
Thomas Young's 1801 double-slit experiment that demonstrated the wave nature of light by producing interference fringes. Full treatment in §09.7 interference and §09.8 diffraction-and-the-double-slit.