Reference shelf

Glossary

Every load-bearing term across the 29 lessons, most in the lessons' own words, with the CS analogue where one exists.

Written by Claude Fable 5

Every load-bearing term in the physics spine, defined crisply, most in the lessons' own words. Where the course deliberately meets computer science, the definition names the analogue. Filter to find one fast, or skim it to see how the pieces connect.

Absolute zero: The temperature at which molecular motion is at its minimum: 0 kelvin, about minus 273.15 degrees Celsius. You cannot go below it because there is no less motion than none.
Acceleration: How fast velocity is changing: the change in velocity divided by the time it took. Your body feels this, not speed.
Accelerometer: A sensor that measures acceleration directly. Velocity and position have to be reconstructed from it by adding up its readings over time.
Angular frequency: How fast an oscillation cycles, in radians per second, written omega. Larger omega means a quicker rhythm.
Angular momentum: The spinning twin of momentum: moment of inertia times angular velocity, I times omega. With no outside twist it stays constant, so shrinking I forces omega to grow.
Angular velocity: How fast something rotates: the angle it sweeps per second. The spinning twin of ordinary speed. Written with the Greek letter omega.
Band gap: The energy an electron in a solid must gain in one jump to move from the bound valence band up to the conduction band where it can flow. It sets an LED's color and whether a material conducts at all.
Capacitance: How much charge a conductor holds per volt applied. A pair of nearby conductors that store charge is a capacitor; bringing another conductor near changes how much they can hold. It is how a touchscreen senses your finger.
Carnot limit: The maximum fraction of heat a perfect engine can convert to work, set only by the hot and cold temperatures it runs between: 1 minus the ratio of cold to hot absolute temperature.
Centripetal acceleration: The inward acceleration of anything moving in a circle. For speed v on a circle of radius r it equals v squared over r, and it always points toward the center.
Charge: A property of matter that comes in two kinds, called positive and negative. Like charges repel, opposite charges attract. It is carried by particles like electrons (negative) and protons (positive).
Conductor: A material, like metal, in which some charges (electrons) are free to move throughout it. Charge placed on it spreads out and flows readily.
Conservation of charge: The total charge in an isolated system never changes. Charge can move from one object to another, but the positive and negative amounts always sum to the same total.
Constancy of the speed of light: Light in a vacuum moves at the same speed c for every observer, regardless of how the source or the observer is moving. The one postulate that forces time itself to bend.
Constructive interference: Two waves in step reinforcing each other into a larger wave.
Coulomb's law: The force between two point charges: proportional to each charge, and inversely proportional to the square of the distance between them, F = k q1 q2 / r^2. The same inverse-square shape as gravity.
Critical angle: The shallowest angle at which light can still escape a denser material; beyond it, all of the light reflects back inside.
Current: The rate at which charge flows past a point, measured in amperes (coulombs per second). The analog of how many litres per second flow past a point in a pipe.
Damping: Any drain of energy from an oscillator, such as friction or air resistance, written as gamma. More damping shrinks and widens the resonance peak.
Dennard scaling: The rule that as a transistor shrinks, its voltage and capacitance shrink too, so the power used per unit of chip area stays constant even as you pack in more, faster transistors. Its end, not Moore's law, is why clock speeds stalled.
Destructive interference: Two waves out of step cancelling each other toward zero.
Discrete derivative: The difference between consecutive samples. It approximates a slope from a list of readings instead of a smooth curve. CS analogue: differencing an array, the inverse of a running sum.
Distance: The total length of the path covered, regardless of direction.
Doping: Deliberately sprinkling in a trace of another element to add either loose electrons (n-type) or electron vacancies called holes (p-type). It is what turns silicon into a controllable switch.
Efficiency: The fraction of input energy that comes out as the useful output you wanted. A pure ratio between 0 and 1, with no units. The rest becomes heat.
Elastic collision: A collision that keeps all of its kinetic energy as well as its momentum. Billiard balls and steel bearings come close.
Electric field: The force per unit charge that a charge sets up in the space around it. At each point the field is an arrow: the direction and strength of the push a positive test charge would feel there.
Electromagnetic wave: A self-sustaining ripple of electric and magnetic fields that travels through empty space at the speed of light. Radio, light, and X-rays are all electromagnetic waves at different frequencies.
Entropy: A measure of a macrostate's multiplicity: the logarithm of the number of microscopic arrangements consistent with it, S = k ln W. Larger entropy means vastly more ways to be that way. CS analogue: Shannon entropy, the same log-of-count math measuring the average surprise, and so the compressibility, of a source.
Explicit Euler: The simplest integrator: update both velocity and position using the old values from the start of the step. Simple, and it systematically adds energy to oscillating or orbiting systems.
Faraday's law: The voltage induced around a loop equals minus the rate of change of the magnetic flux through it. No change in flux means no induced voltage. The basis of every generator and wireless charger.
Fast Fourier Transform: The FFT: a fast algorithm that computes a signal's spectrum, turning a waveform into its frequency content in about n log n steps instead of n squared. One of the most used algorithms in computing.
Focal length: The distance from a lens to the point where rays that arrived parallel to the axis are brought together. Shorter focal length bends light more strongly.
Force: A push or a pull, one object acting on another. A force is what changes an object's motion; it does not need to keep acting to keep the object moving.
Free fall: Moving under gravity alone, with nothing pushing back. Everything falls together at the same rate, so there is no sensation of weight. An orbit is continuous free fall.
Frequency: How many full cycles of a wave pass a fixed point each second, measured in hertz.
Frequency domain: The same signal described as how much of each frequency it contains: its spectrum. CS analogue: a change of basis, the same data viewed along different axes.
Friction: The force between two surfaces in contact that resists sliding. It is what lets a tyre grip and a foot push off, and also what quietly slows a coasting puck.
Fundamental: The lowest and usually loudest sine tone in a sound. Its frequency is the pitch you hear.
Geostationary orbit: A circular orbit about 35,800 km up where the orbital period is exactly one day, so the satellite stays over the same spot on Earth. It is still falling, not hovering.
Gravitational time dilation: Clocks tick slower where gravity is stronger (lower down) and faster where it is weaker (higher up). GPS satellites, higher up, run fast and must be slowed.
Harmonics: Sine tones at whole-number multiples of the fundamental frequency: twice it, three times it, and so on.
Heat: Energy in transit from a hotter object to a colder one because of the temperature difference between them. It is a flow, not a stored amount.
Impulse: Force multiplied by the time it acts for. It equals the change in momentum it produces: F times delta-t = delta-p. Why a crumple zone, by stretching the time, lowers the peak force.
Inelastic collision: A collision that keeps momentum but loses kinetic energy to heat, sound, and deformation. Sticking together is the extreme case.
Inertia: An object's resistance to any change in its motion. The more mass it has, the more inertia, and the harder it is to speed up, slow down, or turn.
Insulator: A material, like wood, rubber, or glass, in which charges are locked in place. Charge put on it stays put.
Interference pattern: Alternating bands of many hits and few hits, the fingerprint of two waves adding and canceling. Waves make it; particles going through one slit should not.
Kilowatt-hour: A unit of energy, not power: one kilowatt of power sustained for one hour. Equal to 3.6 million joules. The unit your electricity bill counts.
Kinetic energy: The energy a thing has because it is moving. Equal to one half its mass times its speed squared. Doubling speed quadruples it.
Kinetic friction: The friction between surfaces that are sliding across each other. It is weaker than the maximum static friction, which is why a skid grips less than a roll.
Longitudinal wave: A wave where the medium is squeezed and stretched along the direction of travel, like sound in air.
Macrostate: The bulk description you can actually measure: how many particles are on the left, the temperature, the pressure. Many different microstates share the same macrostate.
Magnetic flux: A measure of how much magnetic field passes through a loop, counting both the field strength and the area it threads. More field, or a bigger loop facing the field, means more flux.
Mass: How much stuff an object is made of, and how strongly it resists being accelerated. Set by the object itself, the same on Earth, the Moon, or in deep space.
Measurement collapse: The act of measuring a quantum system forces its superposition to snap to one definite outcome, destroying the interference that depended on the possibilities coexisting.
Microstate: A complete specification of the system down to every molecule: exactly where each one is and how fast it is moving.
Model: A simplified story about part of the world that makes predictions you can check by measuring. CS analogue: a function, inputs to a prediction you can test.
Moment of inertia: How hard something is to spin up or slow down. It grows with mass and, crucially, with the square of how far that mass sits from the axis. The spinning twin of mass. Written I.
Momentum: Mass times velocity. A vector: it has a direction, so leftward and rightward momentum can cancel. Written p = mv. Conserved in every collision.
Moore's law: The empirical trend that the number of transistors on a chip roughly doubles every two years, as manufacturing learns to print them smaller.
Natural frequency: The rate an oscillator swings at on its own once disturbed, set by its stiffness and mass. Written omega-nought.
Net force: The single force you get by adding up every push and pull on an object, as vectors. Zero net force means constant velocity; a nonzero net force means the motion changes.
Newton's first law: An object keeps its velocity, staying at rest or moving in a straight line at constant speed, unless a net force acts on it. Also called the law of inertia.
Newton's second law: The net force on an object equals its mass times its acceleration: F = ma. Equivalently, acceleration is the net force divided by the mass, a = F/m.
Newton's third law: When object A pushes on object B, object B pushes back on A with a force equal in size and opposite in direction. The two forces always act on different bodies.
Numerical integration: Advancing velocity and position forward in small discrete timesteps using the acceleration, because a computer cannot follow a smooth curve, only take finite steps along it.
Ohm's law: For many materials, current is proportional to voltage: V = I R. Resistance R is the constant that relates them.
Order of magnitude: The nearest power of ten to a quantity: a rough answer to how big, ignoring the exact digits. CS analogue: Big-O, which cares about the power of the input, not the constant.
Period: The time for one complete cycle: down, up, and back to the start. The inverse of frequency.
Photoelectric effect: Light ejects electrons from a metal only if each photon's frequency is above a threshold; brightness below that threshold does nothing. The experiment that proved light comes in lumps.
Photon: A single quantum (lump) of light. Its energy depends only on its frequency, not on how many photons there are, E = h f.
Position: Where something is, measured as a distance from a chosen starting point along a line.
Potential energy: Energy stored by position, here by height in gravity. Equal to mass times gravity times height, mgh. Released as motion when the object falls.
Power: The rate at which energy is transferred: energy divided by time. Measured in watts, where one watt is one joule per second.
Power wall: The point where a chip generates more heat per unit area than any practical cooling can remove, capping how fast a single core can be clocked. Why CPUs went wide (many cores) instead of fast.
Pressure: Force per unit area that a gas exerts on its container walls, produced by the combined drumbeat of countless molecular collisions.
Qubit: A quantum bit: a two-state system that can hold a superposition of 0 and 1 at once, and only becomes a definite 0 or 1 when measured. Where the next course begins.
Refraction: The bending of light when it crosses between materials where it travels at different speeds.
Resistance: How strongly a component opposes current for a given voltage, measured in ohms. The analog of a narrow or rough pipe that throttles the flow.
Resonance: The large response an oscillator gives when it is driven at, or very near, its natural frequency. The wine glass, the swing, the radio dial.
Restoring force: A force that always points back toward the equilibrium position and grows with distance from it. The seed of every oscillation.
Sampling: Recording a continuous signal as a stream of measurements taken at a fixed rate, such as 44,100 times per second for CD audio. CS analogue: discretizing a continuous input into an array.
Second law of thermodynamics: The entropy of an isolated system tends to increase over time, reaching a maximum at equilibrium. Equivalently, systems drift toward macrostates with overwhelmingly more microscopic arrangements. This is why time has a direction.
Semi-implicit Euler: Also called symplectic Euler: update velocity first, then move the position with the just-updated velocity. Almost the same code as explicit Euler, but its energy stays bounded, so orbits and springs remain stable.
Simple harmonic motion: Any motion where the restoring force, and so the acceleration, is proportional to displacement and points back toward equilibrium. Its solution is a cosine.
Snell's law: At a boundary, the sine of the angle to the surface normal, times the material's refractive index, is equal on both sides. CS analogue: a ray tracer, the technique behind film and game graphics, applies this exact rule millions of times per frame.
Spectrum: A plot of how much of each frequency a signal contains: amplitude versus frequency.
Speed: How fast position changes: distance covered divided by the time it took.
Static friction: The friction between surfaces that are not sliding relative to each other. It adjusts up to a maximum grip, and that maximum is stronger than kinetic friction.
Stiffness: How hard a spring pulls back per unit of stretch, written k. A big k is a stiff spring.
Superposition: When waves overlap, their displacements add point by point; afterward each wave carries on unchanged. In quantum mechanics the same word means a state holding several possibilities at once.
Temperature: A measure of the average kinetic energy of the random motion of the molecules in a substance. Higher temperature means faster average jiggling.
Thermal resistance: How strongly a layer opposes heat flow: the temperature difference it takes to drive one watt through it, measured in kelvin per watt. Low resistance conducts heat easily. CS analogue: it is electrical resistance with heat for current, delta-T = P R matching V = I R exactly.
Thermal throttling: A chip deliberately lowering its clock speed and voltage to cut the heat it generates, so its temperature stays under a safe limit even when the cooling cannot keep up.
Timbre: The character or color of a sound, what makes a violin and a flute on the same note sound different. Set by the harmonic mix.
Time dilation: A clock moving relative to you ticks slower than your own, by the factor gamma. Real, and large enough that GPS must correct for it.
Time domain: A signal described as amplitude versus time: the wiggly waveform itself.
Time-reversible: A method whose steps run identically backward and forward: reverse the velocity and it retraces its path exactly. This symmetry is why Verlet barely drifts in energy over long runs.
Torque: The twisting effect of a force: the force multiplied by the lever arm, the perpendicular distance from the pivot to the line of the force. The spinning twin of force.
Total internal reflection: When light hitting a boundary beyond the critical angle is completely reflected, losing none to the far side. It is how an optical fibre pipes the internet across oceans.
Transistor: A switch with no moving parts: a small voltage on one terminal (the gate) decides whether current can flow between the other two. The atom of all digital logic.
Transverse wave: A wave where the medium moves at right angles to the direction of travel, like a shaken rope.
Trilateration: Finding a position from several known distances. Each distance is a sphere around a satellite; the point where the spheres intersect is your location. GPS uses the timing of signals to get those distances.
Unit: An agreed reference amount, like the metre or the second, that turns a vague comparison into a number everyone can reproduce. CS analogue: a type, so that adding metres to seconds is caught the way adding a string to an int is.
Velocity: Speed together with a direction. Moving 5 m/s east and 5 m/s west are the same speed but opposite velocities.
Velocity Verlet: An integrator that advances position with the current velocity and half the acceleration, then updates velocity using the average of the old and new accelerations. Time-reversible and very energy-stable, the standard in molecular dynamics.
Voltage: The energy each unit of charge carries, measured in volts (joules per coulomb). The analog of the pressure a pump provides. Voltage is a difference between two points, which is why it never flows.
Wave: A disturbance that travels through a medium, carrying energy and a pattern from place to place while the medium itself only oscillates in place.
Wave-particle duality: Quantum objects like light and electrons behave as spread-out waves in some experiments and as localized particles in others.
Wavelength: The distance between two neighboring crests of a wave, written as the Greek letter lambda.
Weight: The gravitational force on an object, mass times g. It changes with location: the same mass weighs less on the Moon and momentarily less in a dropping lift.
Work: Force multiplied by the distance moved along that force. Pushing a box two meters does twice the work of pushing it one meter. Measured in joules.