Physics Vocabulary: Fundamental Science Terms Defined

Physics is the most fundamental of the natural sciences—it describes the laws governing matter, energy, space, and time. From the motion of a baseball to the behavior of subatomic particles, from the energy that heats your home to the gravity that keeps planets in orbit, physics vocabulary provides the framework for understanding the universe. This guide defines essential terms across classical mechanics, thermodynamics, electromagnetism, waves, optics, nuclear physics, quantum mechanics, and relativity—equipping students, curious learners, and science vocabulary builders with the language of the physical world.

Mechanics

Mechanics is the branch of physics dealing with motion and forces—the first topics most students encounter.

Force

A push or pull on an object, measured in newtons (N). Force can cause an object to accelerate, decelerate, change direction, or deform.

Mass

The quantity of matter in an object, measured in kilograms (kg). Mass is an intrinsic property that does not change with location, unlike weight.

Weight

The gravitational force acting on an object's mass. Weight = mass × gravitational acceleration (W = mg). On Earth, g ≈ 9.8 m/s².

Velocity

The rate of change of an object's position in a specific direction—a vector quantity with both magnitude (speed) and direction.

Acceleration

The rate of change of velocity over time, measured in meters per second squared (m/s²). An object accelerates when it speeds up, slows down, or changes direction.

Newton's First Law (Inertia)

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force. Inertia is an object's resistance to change in motion.

Newton's Second Law

Force equals mass times acceleration (F = ma). The greater the mass of an object, the more force required to accelerate it.

Newton's Third Law

For every action, there is an equal and opposite reaction. When you push against a wall, the wall pushes back with equal force.

Momentum

The product of an object's mass and velocity (p = mv). Momentum is conserved in collisions—the total momentum before equals the total after.

Friction

A force that opposes the relative motion of two surfaces in contact. Static friction prevents motion from starting; kinetic friction opposes ongoing motion.

Gravity

The fundamental force of attraction between objects with mass. Newton's law of universal gravitation states that gravitational force is proportional to the product of the masses and inversely proportional to the square of the distance between them.

Torque

A rotational force that causes an object to spin or twist around an axis. Torque equals force times the perpendicular distance from the axis of rotation.

Energy and Work

Energy

The capacity to do work. Energy comes in many forms: kinetic, potential, thermal, electrical, chemical, nuclear, and more. The SI unit is the joule (J).

Kinetic Energy

The energy of motion. KE = ½mv², where m is mass and v is velocity.

Potential Energy

Stored energy due to position or configuration. Gravitational potential energy depends on height (PE = mgh); elastic potential energy is stored in stretched or compressed objects.

Work

The transfer of energy when a force moves an object through a distance. W = Fd (force × distance in the direction of force), measured in joules.

Power

The rate at which work is done or energy is transferred. P = W/t, measured in watts (W). One watt equals one joule per second.

Conservation of Energy

A fundamental principle: energy cannot be created or destroyed, only transformed from one form to another. The total energy of an isolated system remains constant.

Thermodynamics

Temperature

A measure of the average kinetic energy of the particles in a substance. Temperature is measured in Celsius (°C), Fahrenheit (°F), or kelvin (K).

Heat

The transfer of thermal energy between objects at different temperatures. Heat flows spontaneously from hotter to cooler bodies.

First Law of Thermodynamics

Energy cannot be created or destroyed in a thermodynamic process—it can only change form. The change in internal energy equals the heat added minus the work done by the system.

Second Law of Thermodynamics

In any natural process, the total entropy of an isolated system always increases. Heat flows spontaneously from hot to cold, never the reverse.

Entropy

A measure of disorder or randomness in a system. Higher entropy means greater disorder. The universe's total entropy is always increasing.

Conduction

Heat transfer through direct contact between particles. Metals are excellent conductors; wood and air are poor conductors (insulators).

Convection

Heat transfer through the movement of fluids (liquids or gases). Warm fluid rises while cooler fluid sinks, creating convection currents.

Radiation

Heat transfer through electromagnetic waves, requiring no medium. The Sun heats Earth primarily through radiation.

Specific Heat Capacity

The amount of heat required to raise the temperature of one kilogram of a substance by one degree Celsius. Water has an unusually high specific heat capacity.

Waves and Sound

Wave

A disturbance that transfers energy through matter or space without permanently displacing the matter itself.

Frequency

The number of complete wave cycles per second, measured in hertz (Hz).

Wavelength

The distance between consecutive identical points on a wave (e.g., crest to crest), usually measured in meters.

Amplitude

The maximum displacement of a wave from its equilibrium position. Greater amplitude means greater energy.

Transverse Wave

A wave in which the displacement is perpendicular to the direction of propagation. Light waves are transverse.

Longitudinal Wave

A wave in which the displacement is parallel to the direction of propagation. Sound waves in air are longitudinal.

Sound

A mechanical wave produced by vibrations that propagates through a medium (air, water, solids). Sound cannot travel through a vacuum.

Doppler Effect

The change in frequency of a wave relative to an observer moving toward or away from the source. An ambulance siren sounds higher-pitched as it approaches and lower as it recedes.

Resonance

The amplification of a wave when it matches the natural frequency of a system. Resonance explains why a singer can shatter a glass and why bridges can oscillate dangerously.

Light and Optics

Light

Electromagnetic radiation visible to the human eye, with wavelengths roughly between 380 nm (violet) and 700 nm (red).

Reflection

The bouncing of light off a surface. The angle of incidence equals the angle of reflection.

Refraction

The bending of light as it passes from one medium to another (e.g., air to water) due to a change in speed.

Diffraction

The bending and spreading of waves around obstacles or through openings, more noticeable when the opening is similar in size to the wavelength.

Spectrum

The range of wavelengths of electromagnetic radiation. Visible light's spectrum spans the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet.

Lens

A transparent object (usually glass) that refracts light to converge or diverge rays. Convex lenses converge light; concave lenses diverge it.

Prism

A transparent object (often triangular glass) that separates white light into its component colors through refraction.

Electromagnetism

Electric Charge

A fundamental property of matter. Charges are positive (protons) or negative (electrons). Like charges repel; opposite charges attract.

Electric Current

The flow of electric charge through a conductor, measured in amperes (A). Current is driven by a voltage difference.

Voltage (Electric Potential Difference)

The "push" that drives electric current through a circuit, measured in volts (V). Voltage is analogous to water pressure in a pipe.

Resistance

The opposition to the flow of electric current, measured in ohms (Ω). Ohm's Law states V = IR (voltage = current × resistance).

Circuit

A closed path through which electric current flows, including a power source, conductors, and one or more loads (devices that consume energy).

Magnetic Field

A region of space where a magnetic force is exerted on moving charges or magnetic materials. Earth itself has a magnetic field generated by its molten iron core.

Electromagnetic Induction

The generation of an electric current by changing the magnetic field in or around a conductor. Electromagnetic induction is the principle behind electric generators and transformers.

Electromagnetic Wave

A wave of oscillating electric and magnetic fields that propagates through space at the speed of light. Light, radio waves, X-rays, and microwaves are all electromagnetic waves.

Nuclear Physics

Radioactivity

The spontaneous emission of particles or energy from unstable atomic nuclei. Types include alpha decay, beta decay, and gamma radiation.

Half-Life

The time required for half of a radioactive substance to decay. Carbon-14 has a half-life of about 5,730 years; radon-222 has a half-life of about 3.8 days.

Nuclear Fission

The splitting of a heavy atomic nucleus into two or more lighter nuclei, releasing enormous energy. Fission powers nuclear reactors and atomic weapons.

Nuclear Fusion

The combining of light atomic nuclei to form a heavier nucleus, releasing even more energy than fission. Fusion powers the Sun and stars.

Chain Reaction

A self-sustaining sequence of nuclear fission reactions in which the neutrons released by one fission event trigger additional fission events.

Quantum Mechanics

Quantum

The smallest discrete unit of a physical quantity. In quantum mechanics, energy, charge, and other properties come in quantized "packets" rather than continuous streams.

Photon

A quantum (particle) of light and other electromagnetic radiation, carrying energy proportional to its frequency (E = hf, where h is Planck's constant).

Wave-Particle Duality

The principle that particles like electrons and photons exhibit both wave-like and particle-like behavior, depending on the experiment.

Uncertainty Principle

Heisenberg's principle stating that certain pairs of physical properties (like position and momentum) cannot both be known with arbitrary precision simultaneously. Greater precision in one means less in the other.

Superposition

The principle that a quantum system can exist in multiple states simultaneously until it is measured, at which point it "collapses" to a single state.

Quantum Entanglement

A phenomenon in which two or more particles become linked so that the quantum state of one instantly influences the state of the other, regardless of distance.

Relativity

Special Relativity

Einstein's 1905 theory establishing that the laws of physics are the same in all inertial (non-accelerating) reference frames and that the speed of light is constant regardless of the observer's motion. Consequences include time dilation and length contraction.

General Relativity

Einstein's 1915 theory describing gravity not as a force but as the curvature of spacetime caused by mass and energy. General relativity predicts black holes, gravitational waves, and the expansion of the universe.

Spacetime

The four-dimensional fabric combining three dimensions of space with one dimension of time. Massive objects curve spacetime, and this curvature is what we experience as gravity.

Time Dilation

The slowing of time for an object moving at high speeds relative to an observer, or in a stronger gravitational field. GPS satellites must account for time dilation to maintain accuracy.

E = mc²

Einstein's mass-energy equivalence equation, stating that energy (E) equals mass (m) times the speed of light squared (c²). This equation reveals the enormous energy locked within matter.

Gravitational Waves

Ripples in spacetime caused by the acceleration of massive objects, such as merging black holes. First detected directly by LIGO in 2015, confirming a prediction of general relativity.

Tips for Learning Physics Vocabulary

• Start with mechanics. Force, mass, acceleration, and energy are the building blocks of all physics vocabulary.
• Study word roots. "Physics" comes from Greek physis (nature). "Thermo-" means heat; "electro-" means electricity; "magneto-" means magnet.
• Solve problems. Physics vocabulary becomes meaningful when you apply it to calculations and real-world scenarios.
• Watch demonstrations. Videos of physics experiments make abstract concepts visible and vocabulary memorable.
• Connect terms to daily life. Friction when you walk, gravity holding you down, electromagnetic waves in your phone—physics is everywhere.
• Build your English vocabulary broadly. Physics connects to math, engineering, and all other sciences.

Physics vocabulary is the language of the universe's fundamental laws. As you master these terms, you gain the ability to understand not just textbook problems but the deep structure of reality itself—from the smallest quantum fluctuation to the largest cosmic structure. Explore more at dictionary.wiki.