Physics Glossary

The rate of change of velocity with time, measured in m/s². Acceleration occurs whenever speed, direction, or both change — a car speeding up, slowing down, or turning a corner at constant speed are all accelerating.

The maximum displacement of a wave from its equilibrium (rest) position. For sound, larger amplitude means louder; for light, larger amplitude means brighter.

The rotational equivalent of linear momentum, L = Iω, where I is moment of inertia and ω is angular velocity. It is conserved in any system with no external torque — the reason a spinning ice skater speeds up when pulling their arms in.

The basic unit of matter, consisting of a dense positively-charged nucleus (protons and neutrons) surrounded by a cloud of negatively-charged electrons occupying discrete quantised energy levels.

A curved section of road or track tilted at an angle so part of the normal force supplies the centripetal force needed to turn, reducing reliance on friction alone. Racetracks and highway on-ramps are banked for exactly this reason.

As the speed of a moving fluid increases, its pressure decreases (and vice versa) — the reason airplane wings generate lift and why a shower curtain gets pulled inward.

The electromagnetic radiation emitted by an idealised object that absorbs all radiation falling on it, with a spectrum and intensity determined entirely by its temperature — the basis of the Stefan-Boltzmann law.

An early quantum model of the atom in which electrons orbit the nucleus only at specific, quantised energy levels, absorbing or emitting photons when jumping between them — superseded by quantum mechanics but still useful for hydrogen-like atoms.

The interplay of tension, compression, and torque that keeps a bridge standing under load. Every structural member is either being stretched (tension) or squeezed (compression), and the whole structure must remain in rotational and translational equilibrium.

The upward force a fluid exerts on a submerged or floating object, equal to the weight of fluid displaced (Archimedes' Principle). An object floats if this force can equal its weight before it's fully submerged.

A measure of a component’s ability to store electric charge per unit voltage, C = Q/V, measured in farads. A capacitor with higher capacitance stores more charge at a given voltage.

An electrical component that stores energy in an electric field between two conductive plates separated by an insulator, charging and discharging to smooth voltage, filter signals, or store energy briefly.

An idealised heat engine that achieves the maximum possible efficiency permitted by the laws of thermodynamics for given hot and cold reservoir temperatures — a theoretical benchmark no real engine can exceed.

The single point where an object’s entire weight can be considered to act. For a uniform gravitational field it coincides with the centre of mass, and it determines whether an object tips over or stays balanced.

The average position of all the mass in a system, weighted by how much mass is at each point. In projectile motion, the centre of mass always follows a perfect parabola even if the object is spinning or irregularly shaped.

The apparent outward force felt inside a rotating reference frame — not a real force in the Newtonian sense, but a consequence of inertia as observed from a frame that is itself accelerating.

The net force directed toward the centre of a circular path that keeps an object moving in that circle, F = mv²/r. It isn’t a new fundamental force — it’s whatever force (tension, gravity, friction, normal force) happens to be doing the job.

Movement of an object along a circular path at constant radius. Even at constant speed, an object in circular motion is continuously accelerating because its direction is constantly changing.

A relationship combining Boyle’s, Charles’s, and Gay-Lussac’s laws — (P₁V₁)/T₁ = (P₂V₂)/T₂ — describing how pressure, volume, and temperature of a fixed gas sample relate as conditions change.

The principle that the total energy of an isolated system remains constant — energy can be converted between forms (kinetic, potential, thermal, etc.) but never created or destroyed.

In any collision or interaction with no external force, the total momentum of a system before equals the total momentum after. This single principle governs everything from billiard balls to rocket propulsion.

The force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them, F = kq₁q₂/r² — the electrostatic analogue of Newton’s law of gravitation.

The wavelength associated with any moving particle, λ = h/p, revealing that matter — not just light — exhibits wave-like behaviour. The effect is only noticeable for extremely small masses.

Mass per unit volume of a substance, ρ = m/V, measured in kg/m³. Density determines whether an object floats or sinks in a given fluid.

The bending and spreading of waves as they pass through an opening or around an obstacle, most pronounced when the opening size is comparable to the wavelength.

The straight-line distance and direction from an object’s starting point to its final position — a vector quantity, unlike distance, which is a scalar that only tracks total path length travelled.

The change in observed frequency of a wave caused by relative motion between the source and observer — a rising pitch as an ambulance approaches, falling as it passes, or redshift in light from receding galaxies.

A collision in which both momentum and kinetic energy are conserved — no energy is lost to heat, sound, or deformation. Collisions between hard billiard balls or gas molecules approximate this closely.

A fundamental property of matter that causes it to experience a force in an electromagnetic field. Charge comes in two types, positive and negative, and is always conserved — never created or destroyed, only transferred.

A closed loop of conductive material through which electric current can flow, typically containing a power source and components like resistors, capacitors, or bulbs.

The rate of flow of electric charge past a point, measured in amperes (1 A = 1 coulomb/second). Conventional current is defined as flowing from positive to negative terminal.

The region around a charged object where another charge would experience a force, quantified as force per unit charge (N/C). Field lines point away from positive charges and toward negative ones.

The electric potential energy per unit charge at a point in an electric field, measured in volts. The difference in electric potential between two points is what we commonly call voltage.

The energy a charged object possesses due to its position within an electric field, analogous to gravitational potential energy but arising from electric rather than gravitational forces.

The rate of energy transfer in an electrical circuit, P = VI = I²R = V²/R, measured in watts — used to calculate energy consumption and heat dissipation in circuit components.

The generation of an electric current in a conductor caused by a changing magnetic field — the fundamental principle behind electric generators, transformers, and induction charging.

The full continuous range of electromagnetic radiation, from long-wavelength radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, to short-wavelength gamma rays — all travelling at the speed of light.

Self-propagating waves of oscillating electric and magnetic fields, requiring no medium to travel through — they can cross the vacuum of space at the speed of light.

A subatomic particle with a negative electric charge and negligible mass compared to protons or neutrons, occupying quantised energy levels around an atomic nucleus and responsible for chemical bonding and electric current.

The capacity to do work or cause change, existing in many interchangeable forms — kinetic, potential, thermal, electrical, chemical, nuclear. Total energy in an isolated system is always conserved.

A measure of disorder or the number of ways a system’s energy can be arranged. The second law of thermodynamics states entropy in an isolated system never decreases over time.

The state of an object when the net force and net torque acting on it are both zero, so it has no linear or rotational acceleration. An object in equilibrium is either at rest or moving at constant velocity.

The minimum speed an object needs to break free from a gravitational field without further propulsion, v = √(2GM/r). Escape velocity depends on the mass and radius of the body being escaped.

The induced EMF in a circuit equals the negative rate of change of magnetic flux through it, EMF = −dΦ/dt — the quantitative law behind electromagnetic induction.

A statement of energy conservation for thermal systems: the change in internal energy of a system equals heat added minus work done by the system, ΔU = Q − W.

Motion under gravity alone, with no other forces (like air resistance) acting. In free fall, every object accelerates at the same rate regardless of mass — roughly 9.8 m/s² near Earth’s surface.

The number of complete wave cycles passing a fixed point per second, measured in hertz (Hz). Frequency and wavelength are inversely related for waves travelling at a fixed speed.

The force that resists relative motion between two surfaces in contact, acting opposite to the direction of motion (or attempted motion). Static friction prevents motion from starting; kinetic friction opposes motion already underway.

The region of space around a mass where another mass would feel a gravitational force, quantified as force per unit mass (N/kg) — numerically equal to the local gravitational acceleration g.

The attractive force that every object with mass exerts on every other object with mass, described by Newton’s law of universal gravitation F = Gm₁m₂/r². It is always attractive and follows an inverse-square law.

The energy an object possesses due to its position in a gravitational field, GPE = mgh near a planet’s surface. Raising an object converts work done against gravity into stored potential energy.

The time required for half of the radioactive nuclei in a sample to decay. Half-life is constant for a given isotope regardless of sample size — a statistical property of the decay process itself.

Thermal energy transferred between objects or systems due to a temperature difference — heat always flows from the hotter to the cooler object until thermal equilibrium is reached.

The movement of thermal energy between objects or regions by conduction (direct contact), convection (fluid movement), or radiation (electromagnetic waves) — often occurring by all three simultaneously.

A fundamental limit stating that the position and momentum of a particle cannot both be known with arbitrary precision simultaneously, Δx·Δp ≥ ℏ/2 — a property of nature itself, not a limitation of measurement technique.

The force needed to stretch or compress a spring is proportional to the displacement, F = kx, where k is the spring constant. It holds only within a material’s elastic limit.

The equation of state PV = nRT relating pressure, volume, amount, and temperature of an idealised gas, accurately describing real gases under most everyday conditions.

The change in momentum produced by a force acting over a time interval, J = FΔt = Δp. A small force over a long time can produce the same impulse as a large force over a short time — the principle behind car airbags.

A collision in which momentum is conserved but kinetic energy is not — some energy converts to heat, sound, or permanent deformation. A perfectly inelastic collision is one where the objects stick together afterward.

An object's tendency to resist changes in its state of motion — to keep doing whatever it's already doing unless acted on by a net force. Mass is the quantitative measure of inertia.

The combination of two or more overlapping waves to form a resultant wave — constructive interference occurs where waves reinforce each other, destructive where they cancel out.

One of two or more forms of the same chemical element whose atoms have the same number of protons but a different number of neutrons — giving them the same chemistry but different nuclear stability and mass.

The energy an object possesses due to its motion, KE = ½mv². Because velocity is squared, doubling an object’s speed quadruples its kinetic energy.

A model explaining macroscopic gas properties (pressure, temperature) as the statistical result of countless microscopic gas molecules in constant, random motion, colliding with each other and their container.

Two rules governing circuit analysis: the current law states current into a junction equals current out; the voltage law states the sum of voltages around any closed loop is zero.

The energy absorbed or released during a phase change (melting, boiling, freezing, condensing) at constant temperature — energy that goes into breaking or forming molecular bonds rather than raising temperature.

The relationship 1/f = 1/v + 1/u connecting a lens or mirror’s focal length (f) to object distance (u) and image distance (v), used to predict where and how an image will form.

An induced current always flows in the direction that opposes the change in magnetic flux that created it — a direct consequence of energy conservation applied to electromagnetic induction.

A wave in which particles of the medium oscillate parallel to the direction of wave travel, creating alternating compressions and rarefactions — sound waves are the classic example.

The region around a magnet or current-carrying conductor where a magnetic force can be detected, represented by field lines running from north to south pole outside the source.

A measure of the total magnetic field passing through a given area, Φ = BA cos θ — the quantity whose rate of change drives electromagnetic induction.

The force exerted on a moving charge or current-carrying wire within a magnetic field, F = qvB (on a charge) or F = BIL (on a wire) — always perpendicular to both the field and the motion.

A measure of the amount of matter in an object and its resistance to acceleration (inertia). Unlike weight, mass does not change with location — an object has the same mass on Earth, the Moon, or in deep space.

Einstein's relation E = mc², showing that mass and energy are interchangeable — a small amount of mass corresponds to an enormous amount of energy, since c² is such a large number.

The sum of an object’s kinetic and potential energy. In the absence of friction or other dissipative forces, total mechanical energy is conserved even as it converts between kinetic and potential forms.

The product of an object’s mass and velocity, p = mv — a vector quantity that measures "quantity of motion." Momentum is conserved in any closed system, making it one of physics’ most powerful problem-solving tools.

The vector sum of every individual force acting on an object. Only the net force determines an object’s acceleration (F = ma) — individual forces can cancel out even while each is substantial on its own.

An electrically neutral subatomic particle found alongside protons in the atomic nucleus, contributing to mass and nuclear stability without affecting an atom’s chemical identity.

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. Also known as the law of inertia.

The net force on an object equals its mass times its acceleration, F = ma. This is the equation that lets you calculate exactly how an object will accelerate once you know every force acting on it.

For every action force, there is an equal and opposite reaction force. The two forces act on different objects, so they never cancel each other out for either individual object.

The contact force a surface exerts perpendicular to itself, preventing objects from passing through it. It equals an object’s weight only on a flat, level surface — it changes on slopes, in lifts, and during circular motion.

The energy required to completely separate a nucleus into its individual protons and neutrons — equivalently, the energy released when they bind together, accounting for the small "mass defect" via E = mc².

The splitting of a heavy atomic nucleus into two or more lighter nuclei, releasing substantial energy — the process powering nuclear reactors and fission weapons.

The combining of two light atomic nuclei into a heavier nucleus, releasing enormous energy — the process that powers stars, including the Sun.

The dense, positively-charged core of an atom, containing protons and neutrons bound together by the strong nuclear force — accounting for nearly all of an atom’s mass despite occupying a tiny fraction of its volume.

The voltage across a conductor is directly proportional to the current flowing through it, V = IR, with resistance as the constant of proportionality — the foundational relationship of circuit analysis.

The physics governing how objects move under gravitational influence — orbits, launch windows, and gravitational slingshots are all predictable applications of Newtonian gravity and circular/elliptical motion.

A mass suspended from a pivot that swings back and forth under gravity. For small swing angles, a pendulum exhibits simple harmonic motion, with period depending only on its length and local gravity — not its mass or amplitude.

The time taken to complete one full cycle of oscillation or wave motion, measured in seconds. Period and frequency are reciprocals of each other: T = 1/f.

The emission of electrons from a material when light of sufficient frequency shines on it — evidence that light behaves as discrete packets of energy (photons) rather than a continuous wave, as explained by Einstein.

A discrete packet (quantum) of electromagnetic energy, E = hf, with zero rest mass but carrying both energy and momentum — the particle-like nature of light.

The restriction of a transverse wave’s oscillations to a single plane. Unpolarised light vibrates in every direction perpendicular to its travel; polarising filters block all but one orientation.

Stored energy an object has due to its position or configuration — gravitational (height), elastic (compression/stretch), or electrical (charge separation) are the most common forms encountered in mechanics.

The rate at which work is done or energy is transferred, measured in watts (1 W = 1 J/s). The same amount of work done faster requires more power.

Force applied per unit area, P = F/A, measured in pascals. In fluids, pressure increases with depth and acts equally in all directions at a given point.

The curved path of an object launched into the air and subject only to gravity, which can be analysed as independent horizontal (constant velocity) and vertical (constant acceleration) components.

A subatomic particle with a positive electric charge, found in the atomic nucleus. The number of protons in an atom (its atomic number) determines which chemical element it is.

A set of four numbers that uniquely describe the quantum state of an electron in an atom — principal, angular momentum, magnetic, and spin — collectively determining its energy, shape, orientation, and spin.

A quantum phenomenon in which a particle passes through an energy barrier it classically shouldn’t have enough energy to cross, due to the wave nature of matter — essential to nuclear fusion in stars and modern electronics.

Energy emitted as either electromagnetic waves (light, X-rays, gamma rays) or as particles (alpha, beta particles from radioactive decay), travelling outward from its source.

The spontaneous transformation of an unstable atomic nucleus, emitting radiation (alpha, beta, or gamma) as it transitions toward a more stable configuration.

The bouncing back of a wave when it meets a boundary, with the angle of incidence equal to the angle of reflection, measured from the normal.

The bending of a wave as it passes from one medium into another at an angle, caused by a change in wave speed — the reason a straw looks bent when placed in water.

A measure of how much a material slows down and bends light compared to a vacuum, n = c/v. A higher refractive index means light travels slower and bends more sharply in that material.

The velocity of one object as measured from the reference frame of another moving object — found by vector subtraction of their individual velocities relative to a common frame.

A measure of how strongly a material opposes the flow of electric current, measured in ohms (Ω). Resistance depends on a material’s resistivity, length, cross-sectional area, and temperature.

Resistors in series add directly (R_total = R₁ + R₂ + ...), increasing total resistance; resistors in parallel combine reciprocally (1/R_total = 1/R₁ + 1/R₂ + ...), always giving a total lower than the smallest individual resistor.

The dramatic amplification of oscillation amplitude that occurs when a system is driven at its natural frequency, allowing even small periodic forces to build up large-amplitude motion over time.

A quantity described by magnitude alone, with no direction — such as mass, temperature, speed, or energy. Scalars add using ordinary arithmetic, unlike vectors.

Heat flows spontaneously from hotter to colder objects, never the reverse, and the total entropy of an isolated system can never decrease — the principle that gives time its arrow.

Oscillatory motion where the restoring force is directly proportional to displacement from equilibrium and directed toward it, producing smooth, repeating sinusoidal motion — seen in springs, pendulums, and vibrating molecules.

The mathematical relationship governing refraction, n₁ sin θ₁ = n₂ sin θ₂, relating the angles and refractive indices of two media at the boundary where a wave bends.

Einstein's theory describing how space and time are relative to an observer's motion, built on the postulates that physics laws are the same in all inertial frames and the speed of light is constant for all observers.

The amount of heat energy required to raise the temperature of 1 kg of a substance by 1°C, Q = mcΔT. Water has an unusually high specific heat capacity, which is why it moderates coastal climates.

The rate at which an object covers distance, a scalar quantity with magnitude only (no direction) — distinct from velocity, which also specifies direction.

The universal constant c ≈ 3 × 10⁸ m/s at which light and all electromagnetic radiation travels through a vacuum — the same value for every observer regardless of their own motion, and the ultimate speed limit of the universe.

A measure of a spring’s stiffness (k in Hooke’s Law), representing the force required per unit of extension or compression. A stiffer spring has a higher spring constant.

A wave pattern that appears stationary, formed by the interference of two identical waves travelling in opposite directions, with fixed points of zero displacement (nodes) and maximum displacement (antinodes).

Stress is force applied per unit cross-sectional area on a material; strain is the resulting fractional deformation. Their ratio, within the elastic limit, defines a material’s Young’s modulus.

The principle that when two or more waves overlap at a point, the resultant displacement is the vector sum of each individual wave’s displacement at that point.

Five equations relating displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t) for motion at constant acceleration — the core toolkit for solving kinematics problems.

A measure of the average kinetic energy of the particles in a substance. Unlike heat, temperature is not a form of energy itself, but an indicator of how hot or cold an object is.

The pulling force transmitted through a string, rope, cable, or similar object when it is stretched taut by forces acting on its ends. Tension pulls inward along the length of the object at every point.

The maximum constant speed a falling object reaches once air resistance (which increases with speed) grows large enough to exactly balance the object’s weight, resulting in zero net acceleration.

The total kinetic energy of all the randomly moving particles within a substance — the internal energy associated with temperature.

The tendency of matter to increase in length, area, or volume as its temperature rises, as increased particle motion pushes neighbouring particles further apart on average.

The rotational equivalent of force — a measure of how much a force causes an object to rotate about an axis, τ = rF sin θ. The further from the pivot a force is applied, the more torque it produces.

The complete reflection of light at a boundary between two media, occurring when light travels from a denser to a less-dense medium at an angle beyond the critical angle — the principle behind fibre optics.

A wave in which particles of the medium oscillate perpendicular to the direction of wave travel — light, water ripples, and waves on a rope are all transverse.

A quantity with both magnitude and direction, such as velocity, force, or displacement — as opposed to a scalar, which has magnitude only. Vectors combine using vector addition, not simple arithmetic.

The rate of change of displacement with time — a vector quantity that specifies both speed and direction, unlike speed alone.

The difference in electric potential between two points, driving current to flow through a conductor when a circuit is closed — measured in volts.

A disturbance that transfers energy from one place to another without transferring matter, characterised by amplitude, wavelength, frequency, and speed. Waves can be transverse or longitudinal, and mechanical (requiring a medium) or electromagnetic (not requiring one).

The speed at which a wave pattern propagates through a medium, related to frequency and wavelength by v = fλ. Wave speed depends on the medium, not the source that created the wave.

The principle that all quantum objects — light and matter alike — exhibit both wave-like and particle-like properties, with which behaviour dominates depending on how the object is observed or measured.

The distance between two consecutive identical points on a wave — crest to crest, or trough to trough — measured in metres.

The gravitational force exerted on an object by a nearby massive body, W = mg. Unlike mass, weight varies with location — the same object weighs less on the Moon than on Earth despite having identical mass.

The energy transferred to or from an object by a force acting over a distance, W = Fd cos θ. Work is done only when a force causes displacement in the direction the force acts.

The principle that the net work done on an object equals its change in kinetic energy — a direct bridge between the concepts of force-over-distance and energy of motion.

A landmark experiment in which light passing through two closely-spaced slits produces an interference pattern of bright and dark fringes, providing definitive evidence that light behaves as a wave.

A measure of a material’s stiffness — the ratio of stress to strain within its elastic region, E = stress/strain. A higher Young’s modulus means a stiffer, less stretchy material.