The words you need to know

A push or pull that changes, or tends to change, the motion of an object. Measured in newtons. Newton's second law: force equals mass times acceleration (F = ma). A larger force produces greater acceleration; a larger mass requires greater force to accelerate. Forces can act in contact (friction, tension, normal force) or at a distance (gravity, magnetism). When forces are balanced, there is no change in motion. Unbalanced forces cause acceleration.

Mass multiplied by velocity. A heavy object moving slowly and a light object moving fast can have the same momentum. Momentum is conserved: in any closed system, the total momentum before a collision equals the total momentum after. This is why a gun recoils when fired: the forward momentum of the bullet is matched by equal and opposite backward momentum of the gun. Conservation of momentum is one of physics' most fundamental principles.

The foundations of classical mechanics. First: an object at rest stays at rest, and an object in motion stays in motion, unless acted on by an external force (inertia). Second: force equals mass times acceleration. Third: every action has an equal and opposite reaction. These three laws describe almost all motion in everyday experience with extraordinary accuracy, from falling apples to orbital mechanics. They break down only at very high speeds (approaching light) or very small scales (quantum), where relativity and quantum mechanics take over.

The attractive force between objects with mass. Newton described it as a force proportional to mass and inversely proportional to the square of the distance between objects. Einstein redescribed it as the curvature of spacetime caused by mass and energy: objects follow the straightest possible path through curved spacetime, which we perceive as gravitational attraction. Both descriptions are accurate within their domains. Newton's is simpler and works for most practical purposes. Einstein's is more fundamental and necessary for GPS, gravitational waves, and black holes.

A force that opposes relative motion between surfaces in contact. Static friction prevents an object from starting to move. Kinetic friction opposes motion that has already started. Friction converts kinetic energy into heat. Without friction, you could not walk, drive, or hold anything. With too much friction, machines are inefficient and components wear out. Lubrication reduces friction by separating surfaces with a thin film of fluid. The coefficient of friction is a measure of how grippy two surfaces are against each other.

The capacity to do work. It takes many forms: kinetic (motion), potential (stored position or configuration), thermal (heat), chemical, nuclear, electromagnetic. Energy is conserved: it cannot be created or destroyed, only converted from one form to another. A falling object converts potential energy to kinetic. A burning log converts chemical energy to heat and light. An inefficient engine converts most of its fuel energy to waste heat rather than useful work. The total energy in a closed system remains constant.

Kinetic energy is the energy of motion: half of mass times velocity squared (½mv²). Double the speed and you quadruple the kinetic energy, which is why car crash severity increases dramatically with speed. Potential energy is stored energy, waiting to be converted. Gravitational potential energy depends on height and mass: a raised hammer has potential energy that becomes kinetic as it falls. Spring potential energy is stored in elastic deformation. Chemical potential energy is stored in molecular bonds.

The rate at which energy is transferred or work is done. Measured in watts (joules per second). A 100-watt light bulb uses 100 joules of electrical energy every second. A car engine producing 100 kilowatts delivers 100,000 joules of energy per second. Power is not energy: a very powerful engine running briefly uses less total energy than a less powerful engine running for a long time. Confusing power and energy is one of the most common errors in energy discussions.

The study of heat, energy, and their relationship to work. The first law: energy is conserved. The second law: the total entropy of an isolated system always increases or stays the same. Heat flows spontaneously from hot to cold, never the reverse. No engine can convert heat energy to work with 100% efficiency. The second law explains why your coffee cools down (not up), why you cannot unscramble an egg, and why the universe is moving towards increasing disorder.

A measure of disorder or the number of possible microscopic configurations of a system. High entropy means high disorder. The second law of thermodynamics says entropy in a closed system increases over time. A clean room left alone becomes messy; a messy room does not spontaneously become clean. An ice cube melts in warm water because the disordered liquid state has higher entropy than the ordered solid. Life maintains local low entropy at the cost of generating larger entropy elsewhere (burning food, releasing heat).

The theoretical minimum temperature, at which all thermal motion ceases: minus 273.15 degrees Celsius, or 0 kelvin. You cannot actually reach absolute zero; you can only approach it asymptotically. The kelvin scale starts at absolute zero, making it the natural temperature scale for science. At temperatures close to absolute zero, quantum effects dominate: superconductivity (zero electrical resistance) and superfluidity (zero viscosity) emerge, along with exotic states of matter that do not exist at everyday temperatures.

A disturbance that transfers energy without transferring matter. Characterised by wavelength (the distance between successive peaks), frequency (the number of peaks passing a point per second), and amplitude (the height of the peaks). Frequency and wavelength are inversely related: high frequency means short wavelength and vice versa. Waves can be mechanical (sound, water waves, requiring a medium) or electromagnetic (light, radio waves, travelling through vacuum at the speed of light).

The full range of electromagnetic radiation, ordered by frequency or wavelength. From lowest frequency to highest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. All travel at the speed of light in vacuum. Visible light is a tiny sliver of the spectrum: the part our eyes evolved to detect because the sun emits most strongly in that range. Radio waves pass through buildings. X-rays pass through soft tissue but not bone. Gamma rays carry enough energy to damage DNA.

299,792,458 metres per second in vacuum. Denoted c. The universal speed limit: nothing with mass can reach it. As an object with mass approaches the speed of light, it requires increasingly more energy to accelerate, approaching infinite energy at c. Light from the sun takes about eight minutes to reach Earth. The nearest star is four light-years away. The observable universe is about 93 billion light-years across. The speed of light is not just fast: it is a fundamental constant woven into the structure of spacetime.

Current is the flow of electric charge, measured in amperes. Voltage (potential difference) is the force driving that flow, measured in volts. Resistance opposes the flow, measured in ohms. Ohm's law: voltage equals current times resistance (V = IR). Higher voltage pushes more current through a given resistance. Higher resistance reduces current for a given voltage. These three quantities and their relationship govern every electrical circuit, from a torch to a power grid.

The unified theory of electricity and magnetism, one of the four fundamental forces. Electric fields and magnetic fields are aspects of the same phenomenon. A moving electric charge creates a magnetic field. A changing magnetic field creates an electric current. This relationship is the basis of generators (mechanical motion produces electricity) and motors (electricity produces mechanical motion). James Clerk Maxwell's equations, published in 1865, unified electricity and magnetism and predicted the existence of electromagnetic waves, later confirmed to be light.

Einstein's 1905 theory describing how space and time behave at high speeds. Two key postulates: the laws of physics are the same in all inertial frames, and the speed of light is the same for all observers regardless of their motion. The consequences: time passes more slowly for moving objects (time dilation), moving objects appear shorter in the direction of motion (length contraction), and mass and energy are equivalent (E = mc²). Special relativity has been confirmed to extraordinary precision. GPS satellites must account for relativistic time dilation to remain accurate.

Einstein's 1915 theory of gravity, extending special relativity to include accelerated frames and gravitation. Mass and energy curve spacetime. Objects follow the straightest possible paths (geodesics) through curved spacetime, which we observe as gravitational attraction. General relativity predicted: the bending of light around massive objects (confirmed 1919), gravitational waves (detected 2015), black holes, and the expanding universe. It replaced Newton's theory of gravity for all situations where precision matters and has passed every experimental test.

The four-dimensional framework combining three dimensions of space with one dimension of time. In Newtonian physics, space and time are separate and absolute. In relativity, they are woven together: how much time passes and how space is measured both depend on the observer's motion and the local gravitational field. Events that are simultaneous in one frame of reference may not be simultaneous in another. Spacetime is not a metaphor: it is the actual structure of the universe, which curves in the presence of mass and energy.

Energy equals mass times the speed of light squared. Because c is very large (nearly 300 million metres per second), even a tiny amount of mass corresponds to an enormous amount of energy. One gram of matter, if completely converted, would yield about 90 trillion joules: equivalent to roughly 21 kilotons of TNT. Nuclear reactions convert a small fraction of mass to energy. The sun loses about 4 million tonnes of mass per second as it converts hydrogen to helium. This is where all that heat and light comes from.

The minimum discrete unit of something. Energy is not continuous: it comes in discrete packets called quanta. The quantum of light is the photon. The insight that energy is quantised (not continuous) was Max Planck's 1900 contribution and marks the birth of quantum theory. At the scale of everyday objects, the quantisation is so fine it appears continuous. At atomic and subatomic scales, the discrete nature of energy and other properties determines all behaviour.

The property of quantum objects (electrons, photons, and others) of exhibiting both wave-like and particle-like behaviour depending on how they are measured. The double-slit experiment: fire electrons one at a time at a barrier with two slits, and an interference pattern builds up on the screen behind, as if each electron passed through both slits as a wave. But when you detect which slit the electron goes through, the interference pattern disappears and the electron behaves as a particle. The act of measurement changes the result.

The quantum property of existing in multiple states simultaneously until measured. A quantum particle can have a superposition of spin states: both up and down at the same time. When measured, the superposition collapses to one definite value. Schrödinger's cat is the famous illustration (and reductio ad absurdum): a cat in a sealed box with a quantum trigger is simultaneously alive and dead until observed. In practice, superposition is disrupted by interaction with the environment (decoherence) before it can exist at macroscopic scales.

A quantum correlation between two or more particles in which measuring one instantly determines properties of the other, regardless of the distance between them. Einstein called it "spooky action at a distance" and believed it indicated quantum mechanics was incomplete. Bell's theorem and subsequent experiments showed the correlations are real and cannot be explained by any local hidden variable theory. Entanglement does not allow faster-than-light communication (the measurement results are random), but it is the resource underlying quantum computing and quantum cryptography.

Heisenberg's principle that certain pairs of physical properties cannot both be known precisely at the same time. The more precisely you know a particle's position, the less precisely you can know its momentum, and vice versa. This is not a limitation of instruments: it is built into the fabric of reality. The uncertainty principle has practical consequences: it sets the minimum size of electron orbitals in atoms (electrons cannot be confined to a point without acquiring infinite momentum) and contributes to radioactive decay.

Splitting a heavy atomic nucleus into lighter ones, releasing energy. When uranium-235 or plutonium-239 absorbs a neutron, it splits, releasing more neutrons, which can trigger further splits: a chain reaction. Nuclear power plants control this reaction to generate heat, which drives turbines. Nuclear weapons allow it to proceed uncontrolled. The energy released comes from E = mc²: the products weigh slightly less than the original nucleus, and the missing mass becomes energy.

Combining light atomic nuclei to form heavier ones, releasing energy. The process that powers the sun and all stars: hydrogen nuclei fuse to form helium, releasing enormous energy. Fusion produces more energy per kilogram of fuel than fission, leaves much less radioactive waste, and uses hydrogen isotopes available from seawater. The challenge: fusion requires extreme temperatures (100 million degrees or more) to overcome the repulsion between positively charged nuclei. Controlled fusion has been the target of research for 70 years and remains commercially unproven.

The spontaneous emission of particles or energy from unstable atomic nuclei as they decay towards more stable configurations. Alpha decay emits a helium nucleus. Beta decay emits an electron (or positron) and changes the element. Gamma decay emits high-energy electromagnetic radiation. Radioactivity is random at the individual atom level but statistically predictable: the half-life is the time for half of a sample to decay. Carbon-14's half-life (5,730 years) makes radiocarbon dating possible. Uranium-238's half-life (4.5 billion years) means it is still present from the formation of the solar system.

Matter that does not interact with electromagnetic radiation (it neither emits nor absorbs light) but exerts gravitational effects. Its existence is inferred from the motion of galaxies (they rotate too fast for their visible matter to account for), the bending of light around galaxy clusters, and the large-scale structure of the universe. Dark matter comprises about 27% of the total mass-energy content of the universe. Nobody knows what it is. It is not antimatter, not black holes, not neutrinos. The leading candidates are hypothetical particles that have not yet been detected.

A hypothetical form of energy permeating all of space, responsible for the observed acceleration of the expansion of the universe. The universe is not just expanding: it is expanding faster and faster. Something is driving this acceleration. That something is called dark energy. It comprises about 68% of the total mass-energy content of the universe. Its nature is entirely unknown. The cosmological constant (Einstein's "greatest blunder," which he added to his equations and later removed) is the simplest candidate. The discrepancy between the predicted and observed value of dark energy is one of physics' largest unsolved problems.