Physics

Relative density (ratio of densities) Relative density Relative density is the dimensionless ratio between the density of a substance and that of a chosen reference material. It is widely used to compare materials, identify substances,…

Relativity: theories of space, time, motion and gravity Relativity Relativity: the set of physical theories that describe how measurements of space, time, motion and gravity change between observers; includes special and general relativity and the…

Resistance: physical, biological, psychological, and social meanings Resistance An overview of resistance as a concept across physics, engineering, biology, psychology and politics, including types, causes, measurement, and practical significance.

Resonance: principle, types, and practical examples Resonance Resonance is the tendency of systems to oscillate with larger amplitude at certain frequencies. This article explains the mechanism, kinds, examples, history, and practical implica…

Rest energy (mass–energy equivalence) Rest energy Rest energy is the intrinsic energy equivalent of a body's invariant mass, given by E0 = m c². It underlies mass–energy conversion in nuclear and particle processes and differs fro…

Rest mass (invariant mass) in physics Rest mass Rest mass is the invariant mass of a particle or system measured in its rest frame. It is a Lorentz‑invariant property distinct from outdated 'relativistic mass' and central to mas…

Resultant force (net force): definition, calculation and examples Resultant force A resultant (or net) force is the single force that represents the vector sum of all forces on a body. Explains calculation in one and two dimensions, examples, applications and ke…

Revolutions per minute (RPM): definition, uses, and conversions Revolutions per minute Revolutions per minute (RPM) measures how many complete turns an object makes in one minute. This article explains the concept, common conversions, measurement methods, history, an…

RLC Circuit — Theory, Components, Damping, and Applications RLC circuit Comprehensive overview of RLC circuits: series and parallel forms, governing equations, damping and resonance, energy flow, practical applications and distinctions for engineers an…

Rotation (motion about a center or axis) Rotation Rotation is the turning motion of a body around a point or an axis. This article covers geometric and physical views, key quantities (angle, angular velocity, moment of inertia), 2…

Rydberg constant: definition, role in atomic spectra and historical background Rydberg constant The Rydberg constant links atomic spectral lines and fundamental constants. This article explains its definition, the hydrogen vs infinite-mass forms, historical origin, uses in sp…

Saffir–Simpson Hurricane Wind Scale Saffir–Simpson scale A five‑category system that classifies tropical cyclones by sustained wind speed to estimate likely wind damage; used in the Atlantic and northeastern Pacific but does not measure…

Scalar (physics) A scalar in physics is a quantity described solely by magnitude, independent of direction. Examples include mass, temperature, energy and time. Scalars obey simple algebraic rules…

Scalar boson A scalar boson is a spin‑0 quantum particle associated with a scalar field. This article explains its properties, examples (like the Higgs), roles in theory and experiment, and how…

Scale height: characteristic vertical length for exponential decline Scale height Scale height is the characteristic distance over which a quantity (typically atmospheric pressure or density) falls by a factor of e; used in atmospheric physics, planetary science…

Scattering (physics and optics) Scattering Scattering: how waves or particles are redirected by matter. Overview of types, mechanisms, history, atmospheric examples, applications and distinctions from absorption, reflection…

Schrödinger equation: the fundamental wave equation of quantum mechanics Schrödinger equation The central linear equation of nonrelativistic quantum mechanics that governs the wave function, determines allowed energies and probabilities, and underpins atomic and molecular b…

Schrödinger's cat: a thought experiment on quantum superposition and measurement Schrödinger's cat An accessible overview of Schrödinger's cat: the original thought experiment, what it was intended to show about quantum theory, major interpretations, historical background, and m…

Schwarzschild metric — the spherically symmetric vacuum solution in general relativity Schwarzschild metric Metric that describes the spacetime around a non-rotating, uncharged spherical mass. First exact solution of Einstein’s equations; defines event horizon and Schwarzschild radius.

Schwarzschild radius: definition, formula, and significance Schwarzschild radius The Schwarzschild radius is the characteristic radius at which the escape velocity equals the speed of light; it marks the event horizon of a non-rotating black hole and follows fr…

Second law of thermodynamics Principle stating that entropy in an isolated system tends to increase, with formulations by Clausius and Kelvin and consequences for heat engines, irreversibility, and the arrow o…

Shapiro time delay Shapiro delay Extra time taken by light or radio signals to traverse a region of curved spacetime near a massive body, predicted by general relativity and confirmed by radar and spacecraft tests…

Shear modulus (rigidity modulus) Shear modulus Shear modulus (G) quantifies a material's resistance to shape change under shear or torsion. It links shear stress to shear strain and is used in engineering, geophysics, and mater…

Shear stress: definition, formulas, and engineering significance Shear stress Shear stress is the component of stress parallel to a surface. This entry explains its definitions, formulas in solids and fluids, historical notes, engineering examples, and disti…

Shock wave — formation, propagation, effects, and applications Shock wave A shock wave is a rapid, high-amplitude disturbance that travels through a material, produced by sudden energy release; this article explains its physics, examples, effects on mate…

Shrapnel: fragments from explosions, history, effects, and distinctions Shrapnel Fragments produced by explosions or ruptured containers; their formation, historical use, differences from modern fragmentation, hazards, examples, and protective measures.

Skin effect Tendency of alternating current to concentrate near the surface of a conductor, increasing effective resistance at higher frequencies and influencing design in RF and power systems…

Skyrmion: topological quasiparticle and magnetic vortex Skyrmion A skyrmion is a topological soliton—an organized, stable twisting of a field—occurring in particle and condensed-matter systems; magnetic skyrmions are promising for spintronics an…

Slinky: the iconic helical spring toy and its history Slinky Helical spring toy invented by Richard James in 1943. Overview of design, history, variants (Slinky Dog), cultural impact, educational uses, and enduring marketing including a long…

Snell's law (law of refraction) Snell's law Snell's law describes how waves such as light change direction when crossing a boundary between media with different refractive indices; it relates angles to wave speeds and underp…

Solar sail: Propulsion by light — principles, history, and uses Solar sail A solar sail is a spacecraft propulsion concept that uses radiation pressure from sunlight to produce continuous thrust. This article explains the physics, design, history, mission…

Solid mechanics: principles, models, and applications Solid mechanics Solid mechanics studies how solid materials deform and carry loads. It covers stress, strain, constitutive behavior, mathematical models, computational methods, and applications fr…

Sound barrier Aerodynamic and acoustic phenomena that occur as vehicles approach and exceed the speed of sound, including transonic effects, shock waves, sonic booms and design solutions for sup…

Space (physics): the physical extent, dimensions, and spacetime concept Space (physics) Space in physics denotes the arena in which objects have positions and distances. Modern theory treats space together with time as spacetime and links geometry to physical phenomen…

Space-time: the four-dimensional continuum of modern physics Space-time Space-time is the unified, four-dimensional framework combining space and time used in relativity to describe motion, causality, gravity and cosmology.

Span (unit of length) An anthropic unit based on the human hand: the span measures the distance between the thumb and little finger and has varied historically; commonly cited as about 9 inches (0.2286…

Special relativity: the special theory of relativity Special relativity A concise encyclopedia article on special relativity: its principles, historical origin, key consequences (time dilation, length contraction, E=mc²), applications, and distinctions…

Spectral line — atomic and molecular spectroscopy Spectral line A spectral line is a narrow feature in a light spectrum produced when atoms, ions or molecules absorb or emit specific wavelengths. Lines reveal composition, physical conditions, m…

Spectrometer: instrument for measuring the properties of electromagnetic radiation Spectrometer Overview of spectrometers: what they measure, core components, types across the electromagnetic spectrum, history, common applications, and distinctions from related instruments.

Speed (physical quantity) Speed Speed is a scalar measure of how quickly an object changes position, defined as distance traveled per unit time; covers average and instantaneous definitions, units, measurement, u…

Speed of light The speed of light (c) is the fundamental constant that sets the maximum signal speed in the universe, defined as 299,792,458 m/s in vacuum and central to relativity and electromag…

Speed of Sound: Definition, Physics, Typical Values and Applications Speed of sound Overview of the speed of sound: what it is, how it depends on medium and temperature, the basic equations, typical values in air, water and solids, historical notes and key applica…

Spin (rotation, quantum property, sports, and public relations) Spin Spin refers to rotation or angular momentum in physical systems, a quantum property of particles, effects in sports, and a figurative practice of shaping information in public life…

Spin (physics): classical rotation and quantum intrinsic angular momentum Spin (physics) An overview of spin in physics, contrasting classical rotation and quantum intrinsic spin, their properties, measurement, history, and applications such as magnetism and spintronic…

Spin valve (magnetoresistive device) Spin valve A spin valve is a layered magnetic device whose resistance changes with the relative orientation of two ferromagnetic layers; it underpins GMR sensors, hard-disk read heads, and se…

Spontaneous symmetry breaking Phenomenon in which symmetric laws permit asymmetric states; central to phase transitions, collective modes (Goldstone bosons), the Higgs mechanism, and formation of topological de…

Stability: Concepts, Types, and Applications Across Systems Stability A concise overview of stability as a property of physical, biological, and engineered systems, explaining types, measures, history, and practical examples from mechanics to ecology…

Standard Model of particle physics Standard Model Concise, nontechnical overview of the Standard Model: its particles, forces it describes, theoretical foundations, history, applications, and known limitations.