Physics Glossary: Key Terms & Definitions A–Z

A

Absolute Zero: The lowest possible temperature, 0 K (−273.15 °C), at which particle thermal motion is at a minimum.
Acceleration Learn more: The rate at which velocity changes over time, measured in m/s². A vector quantity.
Amplitude: The maximum displacement of a wave or oscillation from its equilibrium (rest) position.
Angular Velocity Calculator: The rate at which an object rotates about an axis, measured in radians per second.
Atom: The smallest unit of an element that keeps its chemical identity, made of protons, neutrons, and electrons.

B

Boyle's Law Calculator: For a fixed amount of gas at constant temperature, pressure is inversely proportional to volume.
Buoyancy Calculator: The upward force a fluid exerts on an immersed object, equal to the weight of fluid it displaces.

C

Capacitance: A measure of how much electric charge a component stores per unit voltage, in farads.
Centripetal Force Calculator: The net inward force that keeps an object moving along a circular path.
Conduction: Heat transfer through direct contact between particles, with no bulk movement of matter.
Conservation of Energy Learn more: Energy cannot be created or destroyed, only transformed from one form to another.
Conservation of Momentum Calculator: The total momentum of an isolated system stays constant when no external force acts.
Convection: Heat transfer by the bulk movement of a heated fluid (a liquid or gas).
Coulomb's Law Learn more: The electrostatic force between two charges is proportional to their product and inversely proportional to the square of the distance between them.
Current: The rate of flow of electric charge through a conductor, measured in amperes (A).

D

Density: The mass per unit volume of a substance, measured in kg/m³.
Diffraction: The bending and spreading of waves as they pass an edge or move through an opening.
Displacement Learn more: The straight-line distance and direction from start point to end point; a vector quantity.
Doppler Effect Calculator: The change in observed frequency of a wave when the source and observer move relative to each other.

E

Elastic Collision: A collision in which both total momentum and total kinetic energy are conserved.
Electric Field: A region around a charge in which another charge feels a force; measured in N/C or V/m.
Energy Learn more: The capacity to do work, measured in joules (J). It comes in many forms but is always conserved.
Entropy: A measure of disorder, or of energy unavailable for work, in a system; it always increases in an isolated system.
Equilibrium: A state in which all forces and torques on an object are balanced, so there is no net acceleration.
Escape Velocity Calculator: The minimum speed an object needs to escape a body's gravity without further propulsion.

F

Force Learn more: A push or pull that can change an object's motion, measured in newtons (N).
Free Fall Calculator: Motion under the influence of gravity alone, with no other forces such as air resistance.
Frequency Learn more: The number of wave cycles or oscillations passing a point per second, measured in hertz (Hz).
Friction Learn more: A force that opposes the relative motion or attempted motion of surfaces in contact.

G

Gravitational Potential Energy Calculator: Stored energy due to an object's height in a gravitational field, U = mgh.
Gravity: The attractive force between any two masses. Near Earth's surface it gives an acceleration g ≈ 9.8 m/s².

H

Half-Life: The time it takes for half the nuclei in a radioactive sample to decay.
Heat Learn more: Energy transferred between objects because of a temperature difference between them.
Hooke's Law Calculator: The extension of a spring is proportional to the force applied, F = kx (within its elastic limit).

I

Ideal Gas Law Calculator: Relates the pressure, volume, and temperature of a gas: PV = nRT.
Impulse Calculator: The product of a force and the time over which it acts; it equals the change in momentum.
Inertia Learn more: An object's resistance to changes in its state of motion, related to its mass.
Interference: The combining of two or more waves to produce a resultant of greater, lower, or equal amplitude.

J

Joule: The SI unit of energy and work, equal to one newton-metre (N·m).

K

Kinematics Calculator: The study of motion — position, velocity, and acceleration — without reference to the forces that cause it.
Kinetic Energy Learn more: The energy an object has because of its motion, KE = ½mv².

L

Latent Heat: The energy absorbed or released during a change of state (phase change) at constant temperature.
Longitudinal Wave: A wave in which particle motion is parallel to the direction of energy travel, such as sound.

M

Magnetic Field: A region in which a magnetic force acts on moving charges and magnetic materials.
Mass: The amount of matter in an object and a measure of its inertia, in kilograms (kg).
Momentum Calculator: The product of an object's mass and velocity (p = mv); a conserved vector quantity.

N

Newton: The SI unit of force — the force that gives a 1 kg mass an acceleration of 1 m/s².
Newton's First Law Learn more: An object stays at rest, or moves at constant velocity, unless acted on by a net external force.
Newton's Second Law Learn more: The net force on an object equals its mass times its acceleration, F = ma.
Newton's Third Law Learn more: For every action there is an equal and opposite reaction.
Normal Force: The support force a surface exerts perpendicular to an object resting on it.

O

Ohm's Law Calculator: For many conductors, voltage equals current times resistance, V = IR.
Optics: The branch of physics dealing with the behaviour and properties of light.

P

Period Calculator: The time taken for one complete cycle of an oscillation or wave, measured in seconds.
Photon: A quantum (particle) of electromagnetic radiation, carrying energy E = hf.
Potential Energy Learn more: Stored energy due to an object's position or configuration.
Power Calculator: The rate at which work is done or energy is transferred, measured in watts (W).
Pressure: Force applied per unit area, measured in pascals (Pa).
Projectile Motion Learn more: The curved, parabolic path of an object launched into the air and moving under gravity.

Q

Quantum: The smallest discrete amount of a physical quantity, such as a single packet of energy.

R

Radiation: The transfer of energy as electromagnetic waves or particles, requiring no medium.
Reflection: The bouncing back of a wave when it meets a boundary between two media.
Refraction Calculator: The bending of a wave as it passes from one medium into another of different density.
Resistance Calculator: A measure of how strongly a material opposes electric current, measured in ohms (Ω).
Resonance: A large-amplitude response that occurs when a system is driven at its natural frequency.

S

Scalar: A quantity that has magnitude but no direction, such as mass, time, or temperature.
Snell's Law Calculator: Relates the angles of incidence and refraction to the refractive indices of two media.
Special Relativity Learn more: Einstein's theory describing space, time, and motion for objects moving at high speeds.
Specific Heat Capacity Learn more: The energy needed to raise the temperature of 1 kg of a substance by 1 °C, Q = mcΔT.
Speed Learn more: How fast an object moves regardless of direction; a scalar quantity.
Static Electricity: An imbalance of electric charge built up on the surface of a material.

T

Temperature Learn more: A measure of the average kinetic energy of the particles in a substance.
Tension: The pulling force transmitted along a string, rope, cable, or similar object.
Thermal Expansion Calculator: The tendency of matter to change in size in response to a change in temperature.
Torque Calculator: The rotational equivalent of force — the turning effect produced about an axis.
Transverse Wave: A wave in which particle motion is perpendicular to the direction of travel, such as light.

U

Uniform Motion: Movement at a constant velocity — constant speed in a straight line.

V

Vector: A quantity that has both magnitude and direction, such as velocity, force, or displacement.
Velocity Learn more: The rate of change of displacement; speed together with a direction. A vector quantity.
Voltage Calculator: The electric potential difference that drives current through a circuit, measured in volts (V).

W

Wavelength Calculator: The distance between successive identical points on a wave, such as crest to crest.
Wave Speed Calculator: How fast a wave travels through a medium, given by v = fλ.
Weight: The gravitational force acting on an object, W = mg, measured in newtons (N).
Work Calculator: Energy transferred when a force moves an object through a distance, W = Fd.

X

X-ray: A form of high-energy electromagnetic radiation with a very short wavelength.

Y

Young's Modulus: A measure of a material's stiffness — the ratio of stress to strain in the elastic region.

Z

Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third, then they are in equilibrium with each other.