Science

Motion & Kinematics

Distance vs. displacement, speed vs. velocity

• Scalar = magnitude only (distance, speed, mass, time, energy). Vector = magnitude and direction (displacement, velocity, acceleration, force, momentum). learn more
• Distance is the total path length; displacement is the straight-line change in position (start to finish, with direction). Run one lap of a 400 m track: distance 400 m, displacement 0. learn more
• Average speed = distance / time; average velocity v = Δx / Δt (displacement / time). Often written simply v = d/t. SI unit: m/s. speed · velocity
• Acceleration is the rate of change of velocity: a = Δv / t = (v − v₀)/t, unit m/s². Speeding up, slowing down (“deceleration”), or changing direction are all accelerations. learn more
• Graphs: the slope of a position–time graph is velocity; the slope of a velocity–time graph is acceleration; the area under a velocity–time graph is displacement. learn more

Speed is not velocity. A car going around a curve at a steady 60 mph has constant speed but changing velocity — so it is accelerating. Likewise, a ball at the top of its flight has zero velocity but still has acceleration g (downward). learn more

Constant-acceleration (“SUVAT”) equations

For constant acceleration a, initial velocity v₀, final velocity v, displacement d (or Δx), time t. Each equation leaves out one variable — pick the one missing the quantity you don't know and don't need. learn more

(Textbooks using the SUVAT letters write s = displacement, u = initial velocity, v = final velocity, a, t: v = u + at, s = ut + ½at², v² = u² + 2as, s = ½(u + v)t.)

Free fall

Near Earth's surface, ignoring air resistance, every object falls with the same acceleration g ≈ 9.8 m/s² (≈ 32 ft/s²; standard value 9.80665 m/s²) regardless of its mass — Galileo's insight. On the Moon g is about 1.6 m/s², roughly ⅙ of Earth's. learn more · standard gravity

With air resistance, a falling object eventually stops accelerating at its terminal velocity (about 55 m/s ≈ 120 mph for a skydiver belly-down). Apollo 15 astronaut David Scott dropped a hammer and a feather on the airless Moon in 1971 — they landed together. terminal velocity · hammer & feather

Projectile motion

• Horizontal and vertical motion are independent: horizontal velocity stays constant (no air resistance); vertical motion is free fall. The path is a parabola. learn more
• A bullet fired horizontally and a bullet dropped from the same height hit level ground at the same time. learn more
• Launched at speed v₀ and angle θ over level ground: components vₓ = v₀ cos θ, v_y = v₀ sin θ; time of flight 2v₀ sin θ / g; range R = v₀² sin 2θ / g. Maximum range at 45°; complementary angles (30° and 60°) give equal ranges. learn more

One-liners to do in your head: a car going from 0 to 30 m/s in 6 s has a = 5 m/s². A car at 20 m/s for 15 s goes 300 m. 60 mph = 88 ft/s ≈ 27 m/s; 1 m/s ≈ 2.24 mph. learn more

Forces & Newton's Laws

Newton's three laws of motion

Published in the Philosophiæ Naturalis Principia Mathematica (“the Principia”), 1687. Force is measured in newtons: 1 N = 1 kg·m/s² (about the weight of a small apple). learn more · Principia

Weight, normal force & friction

• Weight is the gravitational force on a mass: W = mg. A 50 kg student weighs 50 × 9.8 = 490 N (≈ 110 lb). learn more
• The normal force is the perpendicular push of a surface (equals mg on level ground). learn more
• Friction: f = μN (μ = coefficient of friction, unitless). Static friction (not yet sliding) is usually greater than kinetic (sliding) friction — that's why it's harder to start a box moving than to keep it moving. Rolling friction is smaller still. learn more

Mass vs. weight: mass (kg) is the amount of matter and is the same everywhere; weight (N, or pounds) is a force and changes with gravity. On the Moon your mass is unchanged but your weight is about ⅙. The pound is a unit of force; the kilogram is a unit of mass. (The rarely used English mass unit is the slug.) learn more

Momentum & impulse

• Momentum p = mv (kg·m/s), a vector. learn more
• Impulse J = FΔt = Δp. Spreading a stop over more time lowers the force — air bags, crumple zones, bending your knees when you land, “following through” in sports. learn more
• Conservation of momentum: in a closed system with no external force, total momentum before = total momentum after. learn more
• Elastic collisions conserve momentum and kinetic energy (billiard balls, approximately); inelastic collisions conserve only momentum; perfectly inelastic = objects stick together. Example: 2 kg cart at 3 m/s hits a stationary 1 kg cart and they stick → v = 6/3 = 2 m/s. learn more · elastic

Hooke's law (springs)

F = −kx: the restoring force of a spring is proportional to its stretch or compression x (k = spring constant, N/m; the minus sign means it opposes the displacement). Robert Hooke published it in 1678 as the Latin anagram “ceiiinosssttuv” = ut tensio, sic vis (“as the extension, so the force”). A mass on a spring has period T = 2π√(m/k); a simple pendulum T = 2π√(L/g) — independent of mass and (for small swings) amplitude. learn more · pendulum

Universal gravitation

Newton's law of universal gravitation is an inverse-square law: double the distance → ¼ the force; triple → ⅑. G was first measured by Henry Cavendish (1797–98) with a torsion balance — the experiment called “weighing the Earth.” Acceleration due to gravity at a planet's surface is g = GM/r². learn more · G

Big G (6.674 × 10⁻¹¹, the universal constant) is not little g (9.8 m/s², Earth's surface gravity). Astronauts on the ISS are not “weightless” because gravity is gone — gravity there is still about 90% of surface strength; they are in continuous free fall. learn more

Circular motion

An object moving in a circle at constant speed accelerates toward the center: centripetal acceleration a_c = v²/r, requiring a centripetal force F_c = mv²/r (supplied by tension, friction, gravity…). “Centrifugal force” is the apparent outward push felt in a rotating frame — not a real force in an inertial frame. learn more · centrifugal

Torque & rotation

Torque is a twisting force: τ = rF sin θ (lever arm × force), unit N·m. That's why door handles are far from the hinges and long wrenches loosen tight bolts. Rotational analog of F = ma is τ = Iα (I = moment of inertia). Angular momentum L = Iω is conserved — a spinning figure skater pulls in her arms and spins faster. learn more · angular momentum

Pressure, density & fluids

• Pressure P = F/A; SI unit the pascal (1 Pa = 1 N/m²). Snowshoes spread force over a large area; a knife edge concentrates it. Atmospheric pressure at sea level: 1 atm = 101,325 Pa = 101.3 kPa = 760 mmHg (torr) = 14.7 psi ≈ 1.013 bar. learn more · atm
• Fluid pressure increases with depth: P = ρgh. About every 10 m of seawater adds 1 atm. learn more
• Density ρ = m/V. Water ≈ 1 g/cm³ = 1 g/mL = 1000 kg/m³ (max density at about 4 °C — ice floats because it is ~9% less dense than liquid water). Densest element: osmium (≈ 22.6 g/cm³, with iridium a near tie); least dense: hydrogen. Densest planet: Earth; least dense: Saturn. learn more · osmium
• Archimedes' principle: the buoyant force on a submerged object equals the weight of the fluid it displaces. An object floats if it is less dense than the fluid. Legend: Archimedes shouted “Eureka!” (“I have found it!”) in his bath when he realized how to test King Hiero II's gold crown. learn more · Eureka
• Pascal's principle: pressure applied to an enclosed fluid is transmitted undiminished throughout it — the basis of hydraulic lifts and brakes (F₁/A₁ = F₂/A₂). learn more
• Bernoulli's principle (1738): where a fluid flows faster, its pressure is lower. Used to explain lift on wings, curveballs, atomizers, and shower curtains blowing inward. learn more

“Eureka,” “crown,” “displaced water” → Archimedes / buoyancy. “Hydraulic press” → Pascal. “Airplane wing lift,” “fast fluid = low pressure” → Bernoulli. learn more

Work, Energy & Power

Work

W = Fd cos θ, where θ is the angle between force and displacement. Unit: the joule (1 J = 1 N·m). No displacement → no work (pushing a wall, holding a heavy box still). A force perpendicular to motion does no work (θ = 90°, cos 90° = 0) — e.g. gravity on a satellite in circular orbit. learn more

Lifting a 10 kg box 2 m: W = mgh = 10 × 9.8 × 2 = 196 J. Work–energy theorem: net work = change in kinetic energy. learn more

Kinetic & potential energy

Conservation of energy & efficiency

Energy cannot be created or destroyed, only transformed (first law of thermodynamics). A pendulum or roller coaster trades PE for KE: a frictionless object dropped from height h reaches v = √(2gh) (from 5 m: v = √98 ≈ 9.9 m/s). learn more

Efficiency = useful energy (or work) out ÷ energy in × 100%. No real machine is 100% efficient — some energy always goes to heat via friction. Incandescent bulbs are only a few % efficient at making light; large electric motors exceed 90%. learn more

A “perpetual motion machine” is impossible — it would violate the first (or second) law of thermodynamics. learn more

Power

P = W/t (also P = Fv). Unit: the watt (1 W = 1 J/s), named for James Watt. 1 horsepower ≈ 746 W — a unit Watt himself coined to market steam engines. Climbing 5 m of stairs (60 kg) in 6 s: P = (60 × 9.8 × 5)/6 ≈ 490 W. learn more · horsepower

Energy units

The kilowatt-hour measures energy; the kilowatt measures power (energy per time). A 100 W bulb left on for 10 hours uses 1 kWh. learn more

Simple machines & mechanical advantage

The six classical simple machines (Renaissance list): lever, wheel and axle, pulley, inclined plane, wedge, screw. (The wedge and screw are variations of the inclined plane.) Machines trade force for distance — they never multiply work. learn more

• Mechanical advantage MA = output force / input force. Ideal MA = input distance / output distance. Inclined plane: IMA = length / height. Pulley system: IMA ≈ number of supporting rope segments. Wheel and axle: IMA = wheel radius / axle radius. learn more
• Archimedes on the lever: “Give me a place to stand, and I shall move the Earth.” learn more

Mnemonic “F-L-E, 1-2-3”: what's in the middle — Fulcrum, Load, Effort — gives the class. “Archimedes' screw” raises water; the screw is “an inclined plane wrapped around a cylinder.” Archimedes' screw

Waves, Sound & Light

Wave basics

• Frequency is measured in hertz (Hz = cycles per second), named for Heinrich Hertz. Example: a 340 Hz sound in air (343 m/s) has λ ≈ 1 m. learn more · frequency
• Transverse waves vibrate perpendicular to travel (light, waves on a string, S-waves). Longitudinal (compression) waves vibrate parallel to travel (sound, P-waves) — made of compressions and rarefactions. transverse · longitudinal
• Parts: crest, trough, amplitude (height — relates to energy/loudness/brightness), wavelength (crest to crest). learn more
• Wave behaviors: reflection (bounce), refraction (bend on changing speed), diffraction (spread around obstacles/through openings), interference (constructive/destructive), standing waves (nodes & antinodes), resonance (Tacoma Narrows Bridge collapse, 1940, is the classic — though imperfect — example). interference · diffraction · resonance

Sound

• Sound is a mechanical, longitudinal wave — it needs a medium and cannot travel through a vacuum. learn more
• Speed of sound: about 343 m/s in dry air at 20 °C (≈ 767 mph, ≈ 1125 ft/s), faster in warmer air; about 1,480 m/s in water and about 5,000–6,000 m/s in steel. Generally solids > liquids > gases. learn more
• Lightning rule of thumb: count seconds from flash to thunder; ÷ 3 ≈ km, ÷ 5 ≈ miles. learn more
• Pitch ↔ frequency; loudness ↔ amplitude, measured in decibels (dB, a log scale named for Alexander Graham Bell: +10 dB = 10× intensity). Human hearing: about 20 Hz to 20,000 Hz; below = infrasound (elephants, whales), above = ultrasound (bats, dolphins, medical imaging). decibel · hearing range
• Mach number = speed ÷ speed of sound (Ernst Mach). Chuck Yeager first broke the sound barrier in level flight in the Bell X-1 on Oct 14, 1947; a sonic boom is the shock wave. Mach · Bell X-1
• Echolocation/sonar; the musical note A above middle C = 440 Hz (concert pitch); doubling frequency raises pitch one octave. A440

Doppler effect

The observed frequency rises when source and observer approach and falls when they separate — the falling pitch of a passing siren or train horn. Described by Austrian physicist Christian Doppler (1842). In light: approaching objects are blueshifted, receding ones redshifted — Hubble used galaxy redshifts to show the universe is expanding. Also used in radar guns and Doppler weather radar. learn more · redshift

Electromagnetic spectrum

All EM waves travel in vacuum at c = 299,792,458 m/s ≈ 3.00 × 10⁸ m/s (≈ 186,000 mi/s) — exact by definition since 1983. Listed from longest wavelength / lowest frequency / lowest energy to shortest/highest. Boundaries are approximate. learn more · speed of light

Order mnemonic (long → short): “Rabbits Mate In Very Unusual eXpensive Gardens” — Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma. learn more

Higher frequency = shorter wavelength = more energy per photon. Radio waves are light too (EM radiation), not sound. learn more

Visible light & color

• ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. Red has the longest wavelength and lowest energy; violet the shortest and highest. “Roy G. Biv.” Newton split sunlight with a prism and named seven colors (adding indigo to match the seven notes of the scale). learn more
• Additive primaries (light, screens): red, green, blue → together make white. Subtractive primaries (ink, pigment): cyan, magenta, yellow (+ black = CMYK). additive · subtractive
• The sky is blue because of Rayleigh scattering (short wavelengths scatter more, ∝ 1/λ⁴); sunsets are red for the same reason. learn more
• Rainbows: refraction + dispersion + internal reflection in raindrops; red on the outside of the primary bow; seen with the Sun behind you at ~42°. learn more
• Polarization (light vibrating in one plane) proves light is a transverse wave; used in sunglasses and LCD screens. learn more

Reflection, refraction & Snell's law

• Law of reflection: angle of incidence = angle of reflection (measured from the normal). learn more
• Index of refraction n = c/v: vacuum 1 (air ≈ 1.0003), water 1.33, glass ~1.5, diamond 2.42 (why diamonds sparkle). learn more
• Snell's law: n₁ sin θ₁ = n₂ sin θ₂. Light bends toward the normal entering a slower (higher-n) medium. Named for Dutch astronomer Willebrord Snellius (1621); first described by Ibn Sahl (984). learn more
• Total internal reflection occurs when light going from high n to low n exceeds the critical angle (sin θ_c = n₂/n₁; about 49° for water→air) — fiber optics, diamond brilliance, mirages are related. learn more
• Dispersion: n depends on wavelength, so a prism spreads white light into colors (violet bends most). learn more

Lenses & mirrors

Thin-lens / mirror equation. f = focal length, d_o = object distance, d_i = image distance (positive = real image, negative = virtual). Example: object 30 cm from a lens with f = 10 cm → 1/d_i = 1/10 − 1/30 = 2/30 → d_i = 15 cm, m = −0.5 (real, inverted, half size). Lens power in diopters = 1/f (in meters). learn more · curved mirrors

Convex lens converges, but convex mirror diverges — the shapes swap roles. Plane mirrors form virtual, upright, same-size, left-right reversed images. plane mirror

Light as particles: photons

E = hf = hc/λ, with Planck's constant h = 6.626 × 10⁻³⁴ J·s. Planck proposed energy quanta in 1900; Einstein applied them to light in 1905 (photoelectric effect). Light shows wave–particle duality. learn more · Planck constant · duality

Newton favored a corpuscular (particle) theory of light; Huygens a wave theory; Young's double slit (c. 1801) backed waves; Einstein's photons brought particles back. A laser (“light amplification by stimulated emission of radiation”) — first built by Theodore Maiman, 1960. corpuscular theory · laser

Electricity & Magnetism

Charge & Coulomb's law

• Like charges repel, opposite charges attract. Charge is conserved and quantized in units of the elementary charge e = 1.602 × 10⁻¹⁹ C (electron −e, proton +e). Measured by Millikan's oil-drop experiment (1909). learn more · elementary charge
• Benjamin Franklin named the charges positive and negative; because of his choice, electrons are negative and conventional current flows opposite to electron flow. learn more
• Coulomb's law (1785): F = k q₁q₂ / r², k ≈ 8.99 × 10⁹ N·m²/C² — another inverse-square law, measured with a torsion balance. learn more
• Electron mass ≈ 9.109 × 10⁻³¹ kg; a proton is about 1836× heavier. learn more
• Conductors (metals — silver is the best, copper common) vs. insulators (rubber, glass, plastic) vs. semiconductors (silicon, germanium). Superconductors have zero resistance below a critical temperature (discovered by Heike Kamerlingh Onnes in mercury, 1911). A Faraday cage shields its interior from external electric fields. conductors · superconductivity · Faraday cage

Circuits: Ohm's law & electric power

• Ohm's law (Georg Ohm, 1827): voltage = current × resistance. Example: a 12 V battery across 4 Ω → I = 3 A, P = 36 W. learn more
• Electric power P = IV. A 1200 W hair dryer on 120 V draws 10 A. learn more
• Resistance of a wire R = ρL/A: longer and thinner wires resist more; resistance usually rises with temperature in metals. learn more

Capacitors combine the opposite way from resistors: capacitors in parallel add directly; in series they add as reciprocals. Kirchhoff's laws: currents into a junction = currents out; voltages around a loop sum to zero. capacitors

Electrical & magnetic units

Magnetism & electromagnetism

• Magnets have north and south poles; like poles repel. Cutting a magnet makes two smaller magnets — no isolated magnetic monopole has been found. Ferromagnetic metals: iron, nickel, cobalt (and some rare earths like neodymium, gadolinium). Earth's geographic North Pole is near a magnetic south pole (that's why a compass's north end points there). William Gilbert, De Magnete (1600): Earth is a giant magnet. learn more · Earth's field
• Hans Christian Ørsted (1820) noticed an electric current deflects a compass needle — the first link between electricity and magnetism. learn more
• Right-hand rule: point your right thumb along conventional current, and your fingers curl in the direction of the magnetic field around the wire. For force on a moving positive charge (F = qvB sin θ, the Lorentz force), fingers along v, curl toward B, thumb gives F (reverse for electrons). learn more · Lorentz force
• Faraday's law of induction (Michael Faraday, 1831; Joseph Henry independently): a changing magnetic flux induces a voltage (EMF), ε = −N ΔΦ/Δt. Basis of generators, transformers, induction stoves. learn more
• Lenz's law (Heinrich Lenz, 1834): the induced current opposes the change that produced it (the minus sign in Faraday's law). learn more
• Motors turn electrical energy into motion; generators (dynamos) turn motion into electricity. Faraday built the first electric motor (1821) and the first dynamo (Faraday disk, 1831). motor · generator
• Maxwell's equations (James Clerk Maxwell, 1860s) unify electricity, magnetism, and light: light is an electromagnetic wave. learn more

Transformers

V_s/V_p = N_s/N_p (secondary/primary voltage = ratio of coil turns). A step-up transformer has more secondary turns (raises voltage, lowers current); step-down the reverse. Ideal transformer: V_pI_p = V_sI_s. Power lines use very high voltage to reduce I²R losses. Transformers work only with AC. Example: 120 V primary with 100 turns, 500 secondary turns → 600 V. learn more

AC vs. DC — the “War of the Currents”

• DC (direct current) flows one way — batteries, electronics, solar cells. Thomas Edison championed DC (Pearl Street Station, New York, 1882). DC · Pearl Street
• AC (alternating current) reverses direction — 60 Hz, ~120 V in U.S. homes (50 Hz, 230 V in much of the world). Nikola Tesla's polyphase AC system and induction motor were backed by George Westinghouse. AC · mains electricity
• The War of the Currents (late 1880s–1890s): Edison's camp publicized AC's dangers (including electrocuting animals, and AC was used in the first electric chair, 1890). AC won — Westinghouse lit the 1893 Chicago World's Columbian Exposition and built the Niagara Falls hydroelectric plant (1895–96). learn more

Batteries were invented by Volta (voltaic pile, 1800), not Franklin or Galvani. Galvani's frog-leg experiments (1780s, “animal electricity”) inspired Volta — and gave us the word “galvanize.” voltaic pile · Galvani

Heat & Thermodynamics

Temperature scales

• Fahrenheit — Daniel Gabriel Fahrenheit (1724), also invented the mercury thermometer (1714). Celsius — Anders Celsius (1742); his original scale was inverted (100 = freezing). Kelvin — William Thomson, Lord Kelvin (1848); the absolute scale uses no degree sign (“300 kelvins,” not “300 °K”). Rankine = absolute Fahrenheit. Celsius · kelvin · Rankine
• A change of 1 °C = a change of 1 K = a change of 1.8 °F. learn more

Quick checks: 25 °C = 77 °F; 30 °C = 86 °F; 100 °F ≈ 37.8 °C; −40 is the same on both scales. Absolute zero can be approached but never reached. learn more

Heat, specific heat & latent heat

• Specific heat c = heat needed to raise 1 g by 1 °C. Water: 4.184 J/(g·°C) = 1 cal/(g·°C) — unusually high, which is why oceans moderate climate and coastal cities have milder weather. Metals are low (iron ≈ 0.45 J/g·°C). Example: heating 200 g of water from 20 °C to 70 °C takes 200 × 4.184 × 50 ≈ 41,800 J. learn more
• Latent heat is absorbed or released during a phase change at constant temperature. Water: heat of fusion 334 J/g (80 cal/g); heat of vaporization 2,260 J/g (540 cal/g). Steam burns are worse than boiling-water burns because condensing steam releases this latent heat. Joseph Black discovered latent heat (c. 1760s). learn more
• A calorimeter measures heat flow; heat lost by the hot object = heat gained by the cold one. learn more

Heat vs. temperature: temperature measures the average kinetic energy of particles; heat is energy transferred because of a temperature difference (in joules). A bathtub of warm water holds more thermal energy than a cup of boiling water, but has a lower temperature. learn more

Heat transfer

A thermos (Dewar flask, James Dewar, 1892) blocks all three: vacuum stops conduction and convection, silvered walls reflect radiation. Radiated power rises as T⁴ (Stefan–Boltzmann law). learn more · Stefan–Boltzmann

Laws of thermodynamics

Joke version: (1) You can't win. (2) You can't break even. (3) You can't get out of the game. Entropy (S, units J/K) measures disorder / the number of microstates; the word was coined by Rudolf Clausius (1865). Ludwig Boltzmann's S = k log W is engraved on his tombstone in Vienna. learn more

Atomic, Nuclear & Modern Physics

Atomic models, in order

J. J. Thomson (electron, plum pudding, Nobel 1906 — showed the electron is a particle) is the father of G. P. Thomson (Nobel 1937 — electron diffraction showed the electron is a wave). Neither is spelled “Thompson.” Kelvin was William Thomson — no relation. G. P. Thomson

Quantum ideas

• Planck (1900): energy comes in quanta E = hf — solved the blackbody “ultraviolet catastrophe.” Considered the birth of quantum theory. learn more
• Photoelectric effect: light above a threshold frequency ejects electrons from metal; brighter light gives more electrons, higher frequency gives faster ones. KE_max = hf − φ (φ = work function). Explained by Einstein (1905) using photons — the work cited for his 1921 Nobel Prize, not relativity. learn more
• de Broglie (1924): matter has waves too, λ = h/p = h/mv. Confirmed by electron diffraction (Davisson–Germer, 1927). learn more
• Heisenberg uncertainty principle (1927): position and momentum can't both be known exactly, Δx·Δp ≥ ħ/2 (ħ = h/2π). learn more
• Schrödinger equation (1926) governs wave functions; Schrödinger's cat (1935) thought experiment mocks superposition. learn more
• Pauli exclusion principle (1925): no two electrons in an atom share all four quantum numbers (n, l, m_l, m_s) → at most 2 electrons per orbital, with opposite spins. learn more

Radioactivity & decay

Discovered by Henri Becquerel (1896, uranium salts fogging photographic plates); the word “radioactivity” was coined by Marie Curie, who with Pierre discovered polonium and radium (1898). Rutherford named alpha and beta rays (1899); Paul Villard found gamma rays (1900). learn more

Half-life

Example: 80 g of a substance with a 5-day half-life → after 15 days (3 half-lives) 10 g remain. Rutherford introduced the half-life concept (1907). learn more

Fission vs. fusion

Critical mass, control rods (absorb neutrons — boron, cadmium), moderator (slows neutrons — water, graphite) → fission reactor. Major accidents: Three Mile Island (1979, PA), Chernobyl (1986, Ukrainian SSR), Fukushima (2011, Japan). learn more

The four fundamental forces

Electromagnetic and weak forces are unified as the electroweak force (Glashow, Salam, Weinberg; Nobel 1979). learn more

The Standard Model

• 6 quarks: up, down, charm, strange, top, bottom. Proton = uud (charge +1), neutron = udd (charge 0). Up = +⅔, down = −⅓. Named by Murray Gell-Mann (1964) from James Joyce's Finnegans Wake (“Three quarks for Muster Mark!”). Top quark (heaviest) found at Fermilab, 1995. learn more
• 6 leptons: electron, muon, tau, and their three neutrinos. Pauli predicted the neutrino (1930); Fermi named it (“little neutral one”). learn more · neutrino
• Gauge bosons (force carriers): photon, gluon, W and Z. Higgs boson gives particles mass — predicted 1964, discovered at CERN's Large Hadron Collider in 2012 (Nobel 2013 to Peter Higgs & François Englert); nicknamed the “God particle.” Higgs · Standard Model
• Fermions (half-integer spin; obey Pauli exclusion; named for Fermi) vs. bosons (integer spin; named for Satyendra Nath Bose). Hadrons are made of quarks: baryons (3 quarks) and mesons (quark + antiquark). fermion · hadron
• Antimatter: Paul Dirac predicted it (1928); Carl Anderson found the positron (1932). Matter + antimatter annihilate into energy. learn more

Relativity

• Special relativity (Einstein, 1905 — his “miracle year,” annus mirabilis). Postulates: (1) the laws of physics are the same in all inertial frames; (2) the speed of light in vacuum is the same for all observers. Consequences: time dilation (moving clocks run slow), length contraction, relativity of simultaneity, E = mc², nothing with mass reaches c. Lorentz factor γ = 1/√(1 − v²/c²). Twin paradox. learn more · 1905 papers
• General relativity (1915): gravity is the curvature of spacetime by mass and energy. Equivalence principle (gravity ≡ acceleration). Evidence: precession of Mercury's perihelion; starlight bending observed by Arthur Eddington in the 1919 solar eclipse; gravitational redshift; GPS satellite clock corrections; black holes; gravitational waves detected by LIGO on Sept 14, 2015 (Nobel 2017). learn more · LIGO
• The 1887 Michelson–Morley experiment found no “luminiferous aether,” setting the stage for special relativity. learn more

“Annus mirabilis,” “1905,” “Brownian motion,” “photoelectric,” “E = mc²” → Einstein. Einstein's Nobel (1921) was for the photoelectric effect. learn more

Units, Prefixes & Constants

The seven SI base units

The International System of Units (SI, from French Système international, 1960). Since the 2019 redefinition, all seven are defined by fixed constants — the kilogram via Planck's constant, retiring the platinum–iridium cylinder “Le Grand K” kept near Paris. learn more · 2019 revision

Mnemonic for the 7 base units: “Some Mean Kids Always Kick My Cat” — second, meter, kilogram, ampere, kelvin, mole, candela. Everything else (newton, joule, watt, volt…) is a derived unit. Only three countries haven't fully adopted metric: the U.S., Liberia, and Myanmar. learn more · U.S. metrication

Metric prefixes

Beyond: peta- (10¹⁵), exa- (10¹⁸), zetta-, yotta-, and (added 2022) ronna- (10²⁷) and quetta- (10³⁰); femto- (10⁻¹⁵), atto- (10⁻¹⁸), zepto-, yocto-, ronto- (10⁻²⁷), quecto- (10⁻³⁰). Mnemonic (kilo → milli): “King Henry Died By Drinking Chocolate Milk” (kilo, hecto, deca, base, deci, centi, milli). learn more

Capital M = mega (10⁶) but lowercase m = milli (10⁻³). 1 mL = 1 cm³ = 1 cc. 1 L of water has a mass of about 1 kg. learn more

Key physical constants

Units named after scientists

SI rule: the unit name is lowercase (newton), but the symbol is capitalized (N). learn more

Curie units: the curie (Ci) is a large, older unit of radioactivity; the SI unit is the becquerel (Bq, one decay per second), so 1 Ci = 3.7 × 10¹⁰ Bq. Don't confuse either with the Curie temperature (Pierre Curie — where a magnet loses its ferromagnetism) or the element curium (Cm, element 96). The roentgen, gray, and sievert measure dose, not activity. Curie temperature · curium

Atoms & the Periodic Table

Atomic structure

• Atomic number (Z) = number of protons — it defines the element. Henry Moseley (1913) showed X-ray spectra track Z, and the table was reordered by it. learn more
• Mass number (A) = protons + neutrons (a whole number). Neutrons = A − Z. Notation: ^A_ZX, e.g. ¹⁴₆C has 6 p, 8 n. learn more
• Isotopes: same element (same Z), different numbers of neutrons. Hydrogen's three: protium (¹H), deuterium (²H, “heavy hydrogen,” found by Harold Urey 1931), tritium (³H, radioactive). Term coined by Frederick Soddy (1913). learn more · hydrogen isotopes
• Atomic mass on the periodic table is a weighted average of isotopes (why chlorine is 35.45, not a whole number: ~76% Cl-35, ~24% Cl-37). learn more
• Ions: atoms that gained (anion, −) or lost (cation, +) electrons. Valence electrons (outer shell) control bonding; the octet rule says atoms tend toward 8. ion · octet rule
• Electron configuration fills by the Aufbau principle (lowest energy first: 1s 2s 2p 3s 3p 4s 3d 4p…), Hund's rule (spread out in equal-energy orbitals before pairing), and the Pauli exclusion principle. Orbital capacities: s = 2, p = 6, d = 10, f = 14. Example: oxygen = 1s² 2s² 2p⁴. learn more

Mass number (count of nucleons, always an integer) ≠ atomic mass (weighted average, usually a decimal). Changing protons changes the element; changing neutrons gives an isotope; changing electrons gives an ion. learn more

Organization of the periodic table

Dmitri Mendeleev (Russia, 1869) arranged elements by atomic mass and properties, leaving gaps and predicting undiscovered elements — eka-aluminium (gallium, 1875), eka-boron (scandium, 1879), eka-silicon (germanium, 1886). Today: 118 elements, 7 periods (rows) and 18 groups (columns). Elements in a group share valence electrons and similar chemistry. learn more · Mendeleev's predictions

Metals (left, ~¾ of elements: shiny, malleable, ductile, conductive) vs. nonmetals (upper right) separated by a staircase of metalloids (B, Si, Ge, As, Sb, Te — semiconductors). Most abundant element in the universe: hydrogen (then helium); in Earth's crust: oxygen (then silicon); in the human body by mass: oxygen; in air: nitrogen. metalloids · abundance

Periodic trends

Electronegativity, ionization energy and nonmetal reactivity all rise toward the upper right (fluorine corner; noble gases usually excluded from electronegativity). Atomic radius and metallic character rise toward the lower left. learn more

Symbols that don't match English names

Chemical symbols of one or two letters were introduced by Jöns Jacob Berzelius (1810s). learn more

The first 20 elements

Note argon (39.95) is heavier than potassium (39.10) even though it comes first — one of the “pair reversals” that convinced chemists to order by atomic number, not mass. Mnemonic for 1–20: “Happy Henry Likes Beer But Could Not Obtain Food. Next Nancy Might Also Sit Peacefully Sipping Claret Around Kansas Capitals.” atomic number order

Elements named after people & places

States of matter & phase changes

Solid (fixed shape & volume), liquid (fixed volume), gas (neither), and plasma (ionized gas — stars, lightning, neon signs; the most common state of visible matter in the universe). A Bose–Einstein condensate (predicted 1924–25; first made by Eric Cornell & Carl Wieman, 1995) forms near absolute zero. learn more · BEC

A phase diagram shows the triple point (all three phases coexist; water: 0.01 °C) and the critical point (beyond which liquid and gas are indistinguishable — a supercritical fluid). Water's solid–liquid line slopes backward, so pressure can melt ice. learn more · triple point

Physical change (shape, state — no new substance) vs. chemical change (new substance: gas produced, color change, precipitate, light/heat, odor). Exothermic releases heat (combustion, freezing, condensing); endothermic absorbs it (melting, evaporating, cold packs, photosynthesis). learn more

Balancing Equations & Reaction Types

Conservation of mass

In a chemical reaction, mass is neither created nor destroyed — the same atoms rearrange. Established by Antoine Lavoisier through careful weighing in sealed vessels (summarized in his Elementary Treatise of Chemistry, 1789); he is called the “father of modern chemistry.” A balanced equation has the same number of each kind of atom on both sides. learn more · Lavoisier

Step-by-step method

1. Write correct formulas for every reactant and product (never change these afterward).
2. Count atoms of each element on each side.
3. Balance one element at a time by changing coefficients — start with the element in the most complex molecule; save H and O for near the end, and lone elements (like O₂ or Fe) for last.
4. Treat unchanged polyatomic ions (SO₄²⁻, NO₃⁻) as single units.
5. Recount; reduce coefficients to the smallest whole numbers. (If you get a fraction like 7/2 O₂, multiply everything by 2.)

learn more

Worked examples

Coefficients vs. subscripts: balance only by changing the big numbers in front (coefficients). Changing a subscript changes the substance itself — H₂O is water, H₂O₂ is hydrogen peroxide. Coefficients multiply the whole formula: 3H₂O contains 6 H and 3 O. learn more

Reaction types

OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Or LEO the lion says GER: Lose Electrons = Oxidation, Gain Electrons = Reduction. The reducing agent is itself oxidized; the oxidizing agent is reduced. In a cell, oxidation happens at the anode, reduction at the cathode (“An Ox, Red Cat”). learn more · anode

Reaction rates increase with temperature, concentration, surface area, and catalysts (which lower activation energy and aren't consumed; enzymes are biological catalysts). Le Chatelier's principle: a system at equilibrium shifts to counteract a stress (added reactant, pressure, heat). rates · Le Chatelier

Moles, Stoichiometry & Gas Laws

The mole

• One mole = 6.022 × 10²³ particles (Avogadro's number) — the chemist's “dozen.” Named for Amedeo Avogadro (the number itself was named and estimated later, notably by Jean Perrin, Nobel 1926). Mole Day is October 23, from 6:02 a.m. to 6:02 p.m. learn more · Mole Day
• Molar mass (g/mol) = sum of atomic masses in the formula. H₂O = 2(1.008) + 16.00 = 18.02 g/mol; CO₂ = 44.01; NaCl = 58.44; glucose C₆H₁₂O₆ = 180.16. learn more
• Conversions: moles = grams ÷ molar mass; particles = moles × 6.022 × 10²³. Example: 36 g of water = 2 mol = 1.2 × 10²⁴ molecules. learn more
• Percent composition = (mass of element in formula ÷ molar mass) × 100. Water: H = 2.016/18.02 ≈ 11.2%, O ≈ 88.8%. Glucose is 40.0% carbon. Empirical formula = simplest ratio (glucose C₆H₁₂O₆ → CH₂O); molecular formula = actual count. learn more · empirical formula

Stoichiometry

Use the balanced equation's coefficients as mole ratios: grams A → moles A → (ratio) → moles B → grams B. Example: in 2H₂ + O₂ → 2H₂O, 4 g H₂ (2 mol) makes 2 mol = 36 g of water. The limiting reactant runs out first and sets the amount of product; the other is in excess. Percent yield = actual ÷ theoretical × 100. learn more · limiting reagent

Concentration

• Molarity M = moles of solute / liters of solution. 0.5 mol NaCl in 250 mL → 2 M. 58.44 g NaCl in 1 L of solution → 1 M. learn more
• Dilution: M₁V₁ = M₂V₂. 100 mL of 6 M acid diluted to 600 mL → 1 M. learn more
• Molality (m) = moles solute per kg of solvent. Colligative properties (depend on number of particles, not identity): boiling-point elevation, freezing-point depression (salting icy roads; antifreeze), osmotic pressure. learn more

Gas laws

Always use kelvins for temperature in gas laws. learn more

Charles vs. Boyle: Boyle = pressure–volume (inverse, “Boyle's PV”); Charles = volume–temperature (direct, “Charles is Cool with Temperature”); Gay-Lussac = pressure–temperature. Jacques Charles also flew the first hydrogen balloon (1783). Doubling Celsius temperature does not double volume — convert to kelvins. learn more

STP & the kinetic molecular theory

• STP (standard temperature and pressure), classic textbook definition: 0 °C (273.15 K) and 1 atm — one mole of any ideal gas occupies 22.4 L. (IUPAC since 1982 uses 100 kPa, giving 22.7 L; some tests still use 22.4.) learn more
• Kinetic molecular theory: gas particles are tiny, in constant random motion, have elastic collisions, and no attractions; average KE ∝ kelvin temperature. Real gases deviate at high pressure and low temperature (van der Waals equation). learn more · van der Waals

Acids, Bases, Solutions & Bonding

The pH scale

Scale runs roughly 0–14: below 7 acidic, 7 neutral, above 7 basic (alkaline). It's logarithmic — each step is a factor of 10, so pH 3 is 100× more acidic than pH 5. [H⁺] = 10⁻⁴ M → pH 4. Introduced by Danish chemist Søren Sørensen at the Carlsberg Laboratory (1909). learn more

Common acids & bases

The common strong acids (ionize completely): HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄ (perchloric). Strong bases: group 1 hydroxides plus Ca, Sr, Ba hydroxides. learn more

“Strong” ≠ “concentrated.” Strength is how completely an acid ionizes; concentration is how much is dissolved. Hydrofluoric acid (HF) is a weak acid yet extremely dangerous. HF

Three definitions of acids & bases

Water is amphoteric (amphiprotic) — it can act as acid or base. A buffer (weak acid + its conjugate base, like the carbonic acid/bicarbonate system in blood) resists pH change. learn more

Indicators & neutralization

Neutralization: acid + base → salt + water (HCl + NaOH → NaCl + H₂O). A titration adds a solution of known concentration until the equivalence point, shown by an indicator's color change. Antacids neutralize stomach acid. learn more

Litmus mnemonic: “Base turns red litmus Blue.” Acids taste sour and react with metals to make H₂; bases taste bitter and feel slippery. learn more · base

Chemical bonds & intermolecular forces

Lewis dot structures (G. N. Lewis, 1916) show valence electrons. “Like dissolves like”: polar solvents (water — the “universal solvent”) dissolve polar and ionic solutes; nonpolar solvents dissolve nonpolar solutes (oil and water don't mix). Soap and detergents have a polar head and nonpolar tail, forming micelles. Lewis structures · solubility · soap

Solution vocabulary: solute dissolves in solvent; saturated holds the max; supersaturated holds more (unstable — rock candy, hand warmers); suspension settles out; colloid (milk, fog, gelatin) shows the Tyndall effect (scatters a light beam). Gas solubility decreases with temperature (warm soda goes flat). solutions · Tyndall

VSEPR molecular shapes

Valence Shell Electron Pair Repulsion (Gillespie & Nyholm, 1957): electron domains around a central atom spread as far apart as possible; lone pairs repel more and squeeze bond angles. learn more

CO₂ has polar bonds but is a nonpolar molecule (linear, dipoles cancel); H₂O is polar because it is bent. Molecular polarity depends on both bond polarity and shape. learn more

Organic Chemistry & Biomolecules

Organic chemistry is the chemistry of carbon compounds. Carbon forms 4 bonds and makes chains and rings. In 1828 Friedrich Wöhler synthesized urea (an organic compound) from inorganic ammonium cyanate, undermining vitalism (the idea that organic compounds need a “life force”). learn more · Wöhler synthesis

Hydrocarbons & naming prefixes

Mnemonic for meth/eth/prop/but: “Monkeys Eat Peeled Bananas.” From pent- on, the prefixes are Greek numbers. numeric prefixes

• Alkanes (single bonds, saturated): C_nH_2n+2, suffix -ane. Alkenes (one C=C double bond): C_nH_2n, -ene (ethene = ethylene, ripens fruit). Alkynes (C≡C triple bond): C_nH_2n−2, -yne (ethyne = acetylene, welding torches). alkanes · alkenes · alkynes
• Aromatic compounds contain a benzene ring (C₆H₆). August Kekulé said the ring structure came to him (1865) in a dream of a snake biting its own tail (the ouroboros). Benzene was first isolated by Michael Faraday (1825). learn more
• Isomers: same formula, different structure (butane vs. isobutane). Mirror-image isomers are enantiomers (chirality — discovered by Pasteur in tartaric acid crystals, 1848). learn more · chirality
• Crude oil is separated by fractional distillation; fossil fuels are coal, oil, natural gas. learn more

Functional groups

Polymers

Polymers are long chains of repeating monomers (the term “polymer” was coined by Berzelius). Made by addition or condensation (releases water) polymerization. learn more

The four classes of biomolecules

Enzymes end in -ase, sugars in -ose. Enzymes work by the “lock and key” (Emil Fischer, 1894) or “induced fit” models and can be denatured by heat or pH. CHNOPS = the six most common elements of life (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur). learn more

Biology

Cell theory

(1) All living things are made of cells; (2) the cell is the basic unit of life; (3) all cells come from pre-existing cells. Robert Hooke coined “cell” looking at cork in Micrographia (1665); Antonie van Leeuwenhoek first saw living microbes (“animalcules”) in the 1670s; Matthias Schleiden (plants, 1838) and Theodor Schwann (animals, 1839) proposed cell theory; Rudolf Virchow added omnis cellula e cellula (“every cell from a cell,” 1855). learn more

Cell parts & functions

Endosymbiotic theory (championed by Lynn Margulis, 1967): mitochondria and chloroplasts descend from free-living bacteria engulfed by ancestral cells — evidence: their own circular DNA, double membranes, and bacteria-like ribosomes. learn more

Transport: diffusion (high → low concentration), osmosis (diffusion of water across a membrane), facilitated diffusion (via proteins) — all passive; active transport uses ATP to go against the gradient (sodium–potassium pump). A cell in a hypotonic solution swells; in a hypertonic solution it shrinks. osmosis · active transport

Prokaryotes vs. eukaryotes; plant vs. animal cells

Plant cells have a cell wall, chloroplasts, and a large central vacuole, and are usually boxy; animal cells lack these but have centrioles and more lysosomes. Viruses are not cells — they need a host to reproduce. plant cell · virus

Mitosis vs. meiosis

The cell cycle: interphase (G₁ growth, S = DNA synthesis/replication, G₂) → mitosis → cytokinesis. Mitosis phases: PMAT — learn more

1. Prophase — chromosomes condense, nuclear envelope breaks down, spindle forms.
2. Metaphase — chromosomes line up at the middle (metaphase plate).
3. Anaphase — sister chromatids pulled apart to opposite poles.
4. Telophase — two new nuclei form; then cytokinesis splits the cytoplasm (cleavage furrow in animals, cell plate in plants).

Mitosis vs. meiosis: miTosis = Two identical cells; mEiosis = sEx cells, four different haploid cells. Humans have 23 pairs (46 total) — 22 pairs of autosomes + 1 pair of sex chromosomes (XX or XY). Cancer is uncontrolled mitosis. learn more

Photosynthesis & cellular respiration

• Photosynthesis has light-dependent reactions (thylakoid membranes; split water and release O₂; make ATP and NADPH) and the Calvin cycle (light-independent, in the stroma; fixes CO₂ into sugar using the enzyme RuBisCO — likely the most abundant protein on Earth). Melvin Calvin, Nobel 1961. The O₂ we breathe comes from the water, not the CO₂. learn more · Calvin cycle
• Respiration stages: glycolysis (cytoplasm; glucose → 2 pyruvate; net 2 ATP; anaerobic) → Krebs (citric acid) cycle (mitochondrial matrix; Hans Krebs, 1937) → electron transport chain (inner mitochondrial membrane; most ATP; O₂ is the final electron acceptor). Total ≈ 30–38 ATP per glucose (textbooks often say 36–38). learn more · Krebs cycle
• Without oxygen: fermentation — lactic acid (muscle cramps, yogurt) or alcoholic (yeast → ethanol + CO₂: bread, beer). Only 2 ATP. learn more
• History: Jan Ingenhousz (1779) showed plants need light to “restore” air; Joseph Priestley's mint-and-mouse experiment (1771). learn more

The two equations are reverses of each other. ATP (adenosine triphosphate) is the cell's energy currency. Plants do both photosynthesis and respiration. ATP

DNA, RNA & the genetic code

• Base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds); in RNA, A–U. Purines (A, G — two rings) pair with pyrimidines (C, T, U — one ring). Chargaff's rules (Erwin Chargaff, ~1950): %A = %T and %G = %C. learn more · Chargaff
• Double helix (1953): James Watson and Francis Crick (Cambridge) built the model, using X-ray diffraction data from Rosalind Franklin — notably “Photo 51,” taken by her student Raymond Gosling in 1952 and shown to Watson by Maurice Wilkins (King's College London) without her knowledge. Watson, Crick and Wilkins shared the 1962 Nobel in Physiology or Medicine; Franklin had died in 1958, and Nobels are not awarded posthumously. Watson's memoir: The Double Helix (1968). 1953 paper · Photo 51
• Replication is semiconservative (each new helix keeps one old strand — Meselson–Stahl, 1958), using DNA polymerase, helicase (unzips), and ligase. learn more
• Central dogma (Crick, 1958): DNA → RNA → protein. Transcription (DNA → mRNA, in the nucleus, by RNA polymerase); translation (mRNA → protein, at ribosomes; tRNA anticodons match mRNA codons). Exception: retroviruses like HIV use reverse transcriptase (RNA → DNA). learn more
• Codons are 3 bases long; 4³ = 64 codons code for 20 amino acids (the code is redundant). Start codon AUG (methionine); stop codons UAA, UAG, UGA. First cracked by Marshall Nirenberg & Heinrich Matthaei (1961: UUU = phenylalanine). learn more
• Mutations: point (substitution — sickle-cell anemia is a single base change), insertions/deletions (frameshift). learn more
• Human Genome Project completed 2003: ~3 billion base pairs, ~20,000 protein-coding genes. PCR (Kary Mullis, 1983) copies DNA; CRISPR-Cas9 (Doudna & Charpentier, 2012) edits it. HGP · CRISPR

“Transforming principle” → Griffith (1928) / Avery–MacLeod–McCarty (1944) showed DNA is the genetic material; Hershey–Chase (1952) confirmed it with radioactively labeled viruses. Friedrich Miescher first isolated DNA (“nuclein”) in 1869. learn more

Mendelian genetics & Punnett squares

Gregor Mendel, an Augustinian friar in Brno (now Czech Republic), bred pea plants (c. 1856–1863) and published in 1866; his work was ignored until rediscovered in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak. He studied seven traits (seed shape, seed color, flower color, pod shape, pod color, flower position, stem height). learn more · Mendelian inheritance

• Law of segregation: each parent passes one of its two alleles per gene to each gamete. Law of independent assortment: genes for different traits sort independently (true for unlinked genes). Law of dominance. learn more
• Vocabulary: allele (gene version), genotype (Bb) vs. phenotype (brown eyes), homozygous (BB, bb) vs. heterozygous (Bb), dominant (capital) vs. recessive (lowercase). learn more
• Punnett square — named for Reginald Punnett. Monohybrid cross Bb × Bb → genotypes 1 BB : 2 Bb : 1 bb, phenotypes 3 dominant : 1 recessive. Dihybrid cross (AaBb × AaBb) → phenotypes 9 : 3 : 3 : 1. A test cross with a homozygous recessive reveals an unknown genotype. learn more

→ ¼ BB, ½ Bb, ¼ bb; ¾ show the dominant trait. learn more

• Incomplete dominance: blended phenotype (red × white snapdragons → pink). Codominance: both show (AB blood type, roan cattle). Polygenic traits (height, skin color) vary continuously. learn more
• Sex-linked (X-linked) traits — red-green color blindness, hemophilia (Queen Victoria's descendants) — show up more in males (XY). Thomas Hunt Morgan's fruit flies (Drosophila, white eyes) proved genes ride on chromosomes (Nobel 1933). learn more
• Hardy–Weinberg equilibrium: p² + 2pq + q² = 1, p + q = 1. learn more

Taxonomy & classification

Carl Linnaeus (Sweden) introduced binomial nomenclature — Genus species, italicized, genus capitalized (Homo sapiens, “wise man,” a name he coined) — in Systema Naturae (1st ed. 1735; 10th ed. 1758 is the starting point for animal names) and Species Plantarum (1753). learn more · ranks

“Dear King Philip Came Over For Good Soup.” learn more

• Three domains (Carl Woese, 1977/1990, from ribosomal RNA): Bacteria, Archaea (extremophiles — hot springs, salt lakes), Eukarya. learn more
• Six kingdoms (common U.S. textbook system): Animalia, Plantae, Fungi, Protista, Archaebacteria, Eubacteria. Robert Whittaker's earlier five-kingdom system (1969) lumped bacteria as Monera. Fungi are more closely related to animals than to plants. learn more

Evolution

• Charles Darwin: voyage of HMS Beagle (1831–1836), including the Galápagos Islands (finches, tortoises, mockingbirds); On the Origin of Species (Nov 24, 1859); The Descent of Man (1871). Influenced by Thomas Malthus (population) and Charles Lyell (geology). learn more · Beagle voyage
• Alfred Russel Wallace independently conceived natural selection in the Malay Archipelago; their joint papers were read at the Linnean Society in 1858. learn more
• Natural selection: variation + inheritance + overproduction + competition → individuals with favorable traits survive and reproduce more. “Survival of the fittest” was coined by Herbert Spencer (1864). learn more
• Jean-Baptiste Lamarck (1809): inheritance of acquired characteristics — the giraffe stretching its neck — now rejected. learn more
• Evidence: fossils, homologous structures (same origin — human arm, bat wing, whale flipper), analogous structures (same function, different origin — bird vs. insect wings), vestigial structures (appendix, whale hip bones), embryology, DNA comparisons. Other ideas: genetic drift, gene flow, punctuated equilibrium (Eldredge & Gould, 1972), convergent evolution, the peppered moth in industrial England. learn more
• The Scopes “Monkey” Trial (Dayton, Tennessee, 1925) tested a ban on teaching evolution. learn more

Darwin did not say humans descended from modern apes — humans and apes share a common ancestor. And Lamarck, not Darwin, proposed inheriting traits acquired during life. learn more

Ecology

• Organism → population → community → ecosystem → biome → biosphere. “Ecology” was coined by Ernst Haeckel (1866). learn more
• Trophic levels: producers (autotrophs — plants, algae) → primary consumers (herbivores) → secondary consumers → tertiary consumers / apex predators; decomposers (fungi, bacteria) recycle nutrients. Food chain vs. food web. learn more
• 10% rule: only about 10% of energy passes to the next trophic level (the rest is lost mostly as heat) — why energy pyramids are narrow at the top (Raymond Lindeman, 1942). 10,000 kcal of grass → ~1,000 in grasshoppers → ~100 in frogs → ~10 in snakes. learn more
• Symbiosis: mutualism (+/+, bees & flowers, lichens), commensalism (+/0, barnacles on whales), parasitism (+/−, ticks). learn more
• Succession: primary (bare rock; pioneer species like lichens) vs. secondary (after fire/plowing; soil remains). Keystone species (sea otters, wolves in Yellowstone, prairie dogs on the plains). succession · keystone
• Biogeochemical cycles: carbon, nitrogen (nitrogen-fixing bacteria on legume roots), water, phosphorus. nitrogen cycle
• Rachel Carson's Silent Spring (1962) exposed DDT's harm (eggshell thinning in birds), spurring the modern environmental movement; DDT was banned in the U.S. in 1972. learn more

Human body systems

Blood & blood types

• Blood components: red blood cells (erythrocytes — hemoglobin carries O₂; no nucleus; live ~120 days), white blood cells (leukocytes — fight infection), platelets (thrombocytes — clotting), plasma (~55%, mostly water). learn more
• ABO system discovered by Karl Landsteiner (1901; Nobel 1930); he and Alexander Wiener later found the Rh factor (1937–40, named for rhesus monkeys). learn more · Rh

O+ is the most common type in the U.S.; AB− the rarest. For plasma the roles flip: AB is the universal plasma donor. Charles Drew pioneered large-scale blood plasma banking (1940s). distribution · Drew

Vitamins & deficiency diseases

Casimir Funk coined “vitamine” (1912). (Kansas) Elmer McCollum, born near Fort Scott and a University of Kansas graduate, co-discovered vitamin A (1913) and named vitamin D (1922), starting the letter-naming system. Fat-soluble vitamins: A, D, E, K (stored in fat; can build up); the rest (B's, C) are water-soluble. learn more · McCollum

Earth Science & Weather

Earth's layers

• Boundaries: Mohorovičić discontinuity (“Moho,” crust–mantle; Andrija Mohorovičić, 1909); Gutenberg discontinuity (mantle–core; Beno Gutenberg, 1913); Lehmann discontinuity. Moho · core–mantle boundary
• Seismic waves reveal the interior: P-waves (primary, compressional, fastest, travel through solids and liquids) and S-waves (secondary, shear, slower, cannot pass through liquid — the S-wave “shadow zone” proved the outer core is liquid). Surface waves (Love and Rayleigh) cause the most damage. learn more
• Earth is about 4.54 billion years old (radiometric dating of meteorites — Clair Patterson, 1956). learn more

Plate tectonics

• Alfred Wegener (German meteorologist) proposed continental drift (1912; book The Origin of Continents and Oceans, 1915): all continents were once one supercontinent, Pangaea (“all lands”), surrounded by the ocean Panthalassa; it later split into Laurasia (north) and Gondwana (south). Evidence: jigsaw fit of South America and Africa, matching fossils (Mesosaurus, Glossopteris), rock formations, glacial scratches. He couldn't explain the mechanism and was largely rejected; he died on a Greenland expedition in 1930. continental drift · Pangaea
• Seafloor spreading (Harry Hess, early 1960s), magnetic striping on the seafloor (Vine–Matthews–Morley, 1963), and Marie Tharp's maps of the Mid-Atlantic Ridge led to the theory of plate tectonics by the late 1960s. plate tectonics · seafloor spreading

The Ring of Fire rings the Pacific with ~75% of the world's volcanoes. Hot spots (mantle plumes) form volcano chains away from boundaries: Hawaii, Yellowstone. Volcano types: shield (Mauna Loa — gentle basalt flows), stratovolcano/composite (Fuji, St. Helens — explosive), cinder cone. Ring of Fire · hot spots · volcanoes

Rock types & the rock cycle

The rock cycle links all three: any rock can become any other. James Hutton (“father of modern geology”) proposed deep time and uniformitarianism — “the present is the key to the past” (Hutton's 1788 paper; phrase popularized later); Charles Lyell's Principles of Geology (1830–33) spread it. Nicolas Steno (1669): law of superposition — lower layers are older. rock cycle · uniformitarianism · superposition

(Kansas) Western Kansas was covered by the Western Interior Seaway in the Cretaceous: its Niobrara Chalk forms Monument Rocks and Castle Rock and yields famous fossils — Pteranodon (Kansas state flying fossil), the mosasaur Tylosaurus (state marine fossil), and the “fish-within-a-fish” Xiphactinus at the Sternberg Museum in Hays. The Flint Hills are Permian limestone with chert (flint) — too rocky to plow, which preserved the prairie. Hutchinson sits on thick Permian rock salt beds (Strataca underground salt museum). Niobrara · Monument Rocks · Western Interior Seaway

Mohs hardness scale

Devised by German mineralogist Friedrich Mohs (1812): a harder mineral scratches a softer one. It is ordinal, not linear — diamond (10) is far more than twice as hard as apatite (5). learn more

Mnemonic: “Tall Giraffes Can Fight Anyone Other Than Carnivorous Dinosaurs.” Other mineral ID tests: streak (powder color), luster, cleavage vs. fracture, specific gravity. Hematite has a red-brown streak; pyrite (“fool's gold”) a greenish-black one. learn more

Earthquakes & their scales

• The focus (hypocenter) is where rupture starts underground; the epicenter is the point on the surface directly above. Locate the epicenter by triangulation from three seismograph stations using the P–S arrival gap. learn more
• Richter scale (Charles Richter with Beno Gutenberg, Caltech, 1935): logarithmic — each whole number = 10× the wave amplitude and about 32× the energy. learn more
• Moment magnitude scale (M_w; Hanks & Kanamori, 1979) has replaced Richter for large quakes and is what the USGS reports. learn more
• Modified Mercalli intensity scale (Roman numerals I–XII) rates shaking and damage at a location, not energy. learn more
• Largest recorded: 1960 Valdivia, Chile (M_w ≈ 9.5). Others: 1964 Alaska (9.2, largest in U.S.), 2004 Indian Ocean (≈ 9.1–9.3, tsunami), 2011 Tōhoku, Japan (≈ 9.0–9.1, Fukushima), 1906 San Francisco (≈ 7.9). learn more · lists

Magnitude (Richter/moment) measures the energy released at the source — one number per quake. Intensity (Mercalli) varies by place. A magnitude 7 releases about 1,000× (32²) the energy of a magnitude 5. learn more

Geologic time scale

Eons: Hadean → Archean → Proterozoic (together often called “Precambrian,” ~88% of Earth history) → Phanerozoic (“visible life,” the last ~539 million years). Phanerozoic eras: Paleozoic (“ancient life”), Mesozoic (“middle life,” age of reptiles/dinosaurs), Cenozoic (“recent life,” age of mammals). Ma = millions of years ago (approximate, ICS chart). learn more

Period mnemonic: “Camels Often Sit Down Carefully; Perhaps Their Joints Creak — Possibly Next Quarter.” The “Big Five” mass extinctions: end-Ordovician, Late Devonian, end-Permian, end-Triassic, end-Cretaceous. learn more

Atmosphere

Composition of dry air: nitrogen ~78%, oxygen ~21%, argon ~0.93%, carbon dioxide ~0.04% (over 420 ppm and rising; measured at Mauna Loa since 1958 — the Keeling Curve). learn more · Keeling Curve

Mnemonic: “The Strong Man's Triceps Explode.” The Kármán line (100 km) is the conventional edge of space. The 1987 Montreal Protocol phased out ozone-destroying CFCs (Molina, Rowland & Crutzen, Nobel 1995). Kármán line · Montreal Protocol

Clouds

Latin cloud names were proposed by English pharmacist Luke Howard (1802–03). learn more

Weather & severe storms

• Fronts: cold front (blue line with triangles — cold air pushes under warm, fast; thunderstorms, then cooler), warm front (red line with semicircles — slow, long steady rain), stationary, occluded (purple). High pressure → generally fair; low pressure → storms. learn more
• Coriolis effect (Gaspard-Gustave de Coriolis): Earth's rotation deflects winds right in the Northern Hemisphere — hurricanes spin counterclockwise there (clockwise in the Southern). learn more
• Instruments: barometer (pressure — Torricelli, 1643), anemometer (wind speed), wind vane, hygrometer/psychrometer (humidity), rain gauge, thermometer, Doppler radar. Dew point = temperature at which air becomes saturated. barometer · anemometer
• Climate classification: Köppen system (Wladimir Köppen, 1884/1900). learn more

Tornadoes — the (Enhanced) Fujita scale

Created by Tetsuya “Ted” Fujita (University of Chicago, “Mr. Tornado”) in 1971; replaced in the U.S. by the Enhanced Fujita (EF) scale on Feb 1, 2007. Ratings are assigned after the storm from damage surveys (28 damage indicators), giving estimated 3-second gust speeds. EF scale · original F scale

(Kansas) Kansas sits in Tornado Alley. Notable Kansas tornadoes: Udall, May 25, 1955 (F5; deadliest in state history, 80 killed); Topeka, June 8, 1966 (F5; tore through downtown and Washburn University, then the costliest U.S. tornado); Greensburg, May 4, 2007 (the first EF5 under the new scale; ~1.7 miles wide; destroyed about 95% of the town, which rebuilt as a model “green” town). A tornado watch means conditions are favorable; a warning means one is occurring or detected — take shelter. Udall · Topeka · Greensburg · Tornado Alley

The Fujita scales rate tornadoes by damage (winds are estimated from it); the Saffir–Simpson scale rates hurricanes by measured sustained wind. Don't swap them. learn more

Hurricanes — the Saffir–Simpson scale

Developed by engineer Herbert Saffir and National Hurricane Center director Robert Simpson (early 1970s); based on 1-minute sustained winds. A tropical storm (39–73 mph) gets a name; at 74 mph it becomes a hurricane (called a typhoon in the western Pacific, a cyclone in the Indian Ocean). The calm center is the eye. learn more · tropical cyclones

The water cycle

Processes: evaporation (mostly from oceans), transpiration (water vapor released by plants; together “evapotranspiration”), condensation (clouds), precipitation, infiltration (soaking into ground → groundwater/aquifers), runoff, collection; also sublimation from snow and ice. The Sun powers it. About 97% of Earth's water is salty ocean; of the ~3% fresh water, about two-thirds is locked in ice caps and glaciers. learn more · water distribution

(Kansas) Western Kansas farms irrigate from the Ogallala (High Plains) Aquifer, one of the world's largest — but it is being pumped far faster than rain recharges it. learn more

Astronomy & Space

The planets

Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — “My Very Educated Mother Just Served Us Nachos.” The four inner terrestrial (rocky) planets are separated from the outer gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune) by the asteroid belt. Moon counts below are as of mid-2026 and keep rising as small moons are found. learn more · Solar System

Venus, not Mercury, is the hottest planet. Venus and Uranus spin “backward.” Saturn has the most known moons (the lead has flipped with Jupiter as discoveries are announced — check the date of any “how many moons” clue). Titan is the only moon with a thick atmosphere. retrograde rotation

Earth's Moon

About ¼ Earth's diameter, ~384,000 km away; tidally locked (we always see the same side). Phases: new → waxing crescent → first quarter → waxing gibbous → full → waning gibbous → third quarter → waning crescent (~29.5-day synodic month). Likely formed by the giant-impact hypothesis (a Mars-sized body, “Theia,” hit early Earth). Tides are caused mainly by the Moon's gravity (spring tides at new/full moon, neap tides at quarters). A solar eclipse happens at new moon; a lunar eclipse at full moon. learn more · phases · giant impact

Dwarf planets & small bodies

• The IAU (meeting in Prague, Aug 24, 2006) defined a planet as a body that (1) orbits the Sun, (2) is round (hydrostatic equilibrium), and (3) has cleared its orbital neighborhood. Pluto fails #3 and was reclassified as a dwarf planet. The 2005 discovery of Eris (Mike Brown's team — “How I Killed Pluto”) — slightly smaller than Pluto but more massive — forced the decision. learn more
• The five recognized dwarf planets: Ceres (largest asteroid-belt object; first found, by Giuseppe Piazzi, Jan 1, 1801; Dawn mission), Pluto, Haumea (football-shaped, fast spin), Makemake, Eris (moon Dysnomia). learn more
• Pluto: discovered Feb 18, 1930 by Clyde Tombaugh at Lowell Observatory (Flagstaff, AZ) using a blink comparator; named by 11-year-old Venetia Burney of Oxford, England. Five moons: Charon (half Pluto's size, found 1978), Nix, Hydra, Kerberos, Styx. New Horizons flew by on July 14, 2015, revealing the heart-shaped Tombaugh Regio. learn more · Tombaugh Regio
• Asteroid belt (between Mars and Jupiter); Kuiper belt (beyond Neptune; Pluto, Arrokoth); Oort cloud (distant shell, source of long-period comets). Comets: icy “dirty snowballs” with tails pointing away from the Sun; Halley's Comet returns every ~76 years (last 1986, next 2061). Meteoroid (in space) → meteor (streak in sky) → meteorite (hits the ground). Kuiper belt · Halley's Comet · meteoroid

(Kansas) Clyde Tombaugh (1906–1997) was born in Illinois but grew up on a farm near Burdett, Kansas, where he built telescopes from farm-machinery parts and sent his drawings of Jupiter and Mars to Lowell Observatory, which hired him. After discovering Pluto he earned his degrees at the University of Kansas. Some of his ashes flew past Pluto aboard New Horizons. Kansas is also rich in meteorites — the Brenham pallasite field in Kiowa County is famous. Tombaugh · Brenham meteorite

The Sun

• A G2V main-sequence star (“yellow dwarf”), ~4.6 billion years old, about halfway through its main-sequence life. Holds ~99.86% of the solar system's mass; diameter ~109 Earths; mass ~333,000 Earths. learn more
• Powered by nuclear fusion of hydrogen into helium (proton–proton chain) in the core (~15 million °C). Visible surface (photosphere) ~5,500 °C (5,772 K); the corona is over 1 million °C (the “coronal heating problem”). p–p chain
• Sunlight takes about 8 minutes 20 seconds to reach Earth. Sunspots (cooler, magnetic regions) follow an ~11-year cycle; solar flares and the solar wind cause auroras. sunspots · solar cycle
• NASA's Parker Solar Probe (launched 2018) made the closest-ever approach to the Sun on Dec 24, 2024, becoming the fastest human-made object. learn more

Stars

• Hertzsprung–Russell (H–R) diagram (Ejnar Hertzsprung & Henry Norris Russell, c. 1910–13) plots luminosity vs. temperature (hot on the left). Most stars lie on the diagonal main sequence; giants and supergiants at upper right; white dwarfs lower left. learn more
• Spectral classes: O B A F G K M, hottest (blue) to coolest (red) — “Oh Be A Fine Girl/Guy, Kiss Me.” Scheme finalized by Annie Jump Cannon at Harvard. The Sun is G2. learn more
• Magnitude: smaller (even negative) numbers are brighter (from Hipparchus). Apparent magnitude = how bright it looks; absolute magnitude = brightness at 10 parsecs. learn more
• Notable stars: Sirius (brightest in the night sky, “Dog Star,” Canis Major); Proxima Centauri (nearest star after the Sun, ~4.24 ly; part of the Alpha Centauri system); Polaris (North Star, end of the Little Dipper's handle — not especially bright); Betelgeuse (red supergiant in Orion's shoulder); Rigel (blue supergiant, Orion's foot); Vega. Sirius · Proxima · Polaris · Betelgeuse

Stellar life cycle

• Chandrasekhar limit ≈ 1.4 solar masses — the maximum mass of a white dwarf. learn more
• A black hole's boundary of no return is the event horizon (radius = Schwarzschild radius). The Event Horizon Telescope released the first image of one (M87*) in April 2019, then Sagittarius A* (2022). John Wheeler popularized the term “black hole” (1967). learn more
• The Crab Nebula (M1) is the remnant of a supernova recorded by Chinese astronomers in 1054. learn more

Kepler's laws of planetary motion

Derived by Johannes Kepler from Tycho Brahe's precise naked-eye observations of Mars; later explained by Newton's gravity. learn more

Distances

• Astronomical unit (AU): mean Earth–Sun distance ≈ 150 million km (93 million miles). learn more
• Light-year: the distance light travels in a year ≈ 9.46 × 10¹² km ≈ 5.88 trillion miles ≈ 63,000 AU. learn more
• Parsec: distance at which a star shows a parallax of one arcsecond ≈ 3.26 light-years. learn more
• “Standard candles” for measuring distance: Cepheid variable stars (Henrietta Leavitt's period–luminosity relation, 1912) and Type Ia supernovae. learn more

A light-year is a unit of distance, not time. learn more

Galaxies

• The Milky Way is a barred spiral galaxy ~100,000 light-years across with roughly 100–400 billion stars; the Sun is ~26,000 ly from the center, in the Orion Arm. At its center is the supermassive black hole Sagittarius A* (~4 million solar masses; Nobel 2020 to Reinhard Genzel and Andrea Ghez). learn more · Sgr A*
• Andromeda Galaxy (M31): nearest large spiral galaxy, ~2.5 million ly away; the most distant object easily visible to the naked eye; approaching the Milky Way and may merge with it in several billion years. Edwin Hubble proved it lies outside our galaxy (1920s), ending the “Great Debate” (Shapley vs. Curtis, 1920). learn more
• The Milky Way, Andromeda, Triangulum and dozens of dwarf galaxies (including the Large and Small Magellanic Clouds) form the Local Group. Hubble's tuning fork classifies galaxies as elliptical, spiral, barred spiral, (lenticular) and irregular. Local Group · Hubble sequence

The Big Bang & cosmology

• The universe began ~13.8 billion years ago. Belgian priest-physicist Georges Lemaître proposed the expanding universe and the “primeval atom” (1927–31). The name “Big Bang” was coined by rival Fred Hoyle (a steady-state supporter) on BBC radio in 1949. learn more
• Hubble's law (1929; now Hubble–Lemaître law): galaxies recede at speeds proportional to their distance, v = H₀d, H₀ ≈ 70 km/s per megaparsec (the exact value is disputed — the “Hubble tension”). learn more
• Cosmic microwave background (CMB): afterglow of the Big Bang at ~2.7 K, discovered accidentally by Arno Penzias and Robert Wilson at Bell Labs (1964–65; Nobel 1978) — they first suspected pigeon droppings in their antenna. Mapped by COBE, WMAP, Planck. learn more
• The universe is roughly 5% ordinary matter, 27% dark matter, 68% dark energy. Dark matter: Fritz Zwicky (1933), Vera Rubin's galaxy rotation curves (1970s). Dark energy: the expansion is accelerating (1998; Nobel 2011 to Perlmutter, Schmidt, Riess). dark matter · dark energy

Space exploration milestones

(Kansas) The Cosmosphere in Hutchinson (a Smithsonian affiliate) holds one of the world's largest space-artifact collections: the Apollo 13 command module Odyssey, Gus Grissom's recovered Mercury capsule Liberty Bell 7, an SR-71 Blackbird, and the largest collection of Soviet space hardware outside Russia. learn more

Scientists

About 25 of the most-asked names in each field, with the facts that show up as clues: laws, units, equations, discoveries, and inventions named after them. Women scientists from every field are collected in their own table. learn more

Physicists

Chemists

Biologists & medicine

Earth & space scientists

Inventors & engineers

Women in science

Many “overlooked women” clues end with the man who got the credit: Franklin → Watson & Crick; Meitner → Hahn; Wu → Lee & Yang; Bell Burnell → Hewish; Leavitt → Hubble. If a clue stresses a missed Nobel, think of these pairs. Matilda effect

Famous Experiments

Listed roughly in chronological order. Tossups often describe the apparatus (“oil droplets between charged plates,” “a flask with a curved neck”) and ask for the scientist or the result. learn more

Rutherford directed the gold foil experiment but Geiger and Marsden performed it. Millikan measured the electron's charge; Thomson measured its charge-to-mass ratio. Pasteur (swan-neck flask, 1859) and Redi (meat and flies, 1668) both attacked spontaneous generation — Redi for maggots, Pasteur for microbes. spontaneous generation

Nobel Prizes & Named Laws

The Nobel Prizes

• Founded by the will (1895) of Alfred Nobel (1833–1896), the Swedish chemist who invented dynamite. First awarded in 1901; presented each year on December 10, the anniversary of his death. learn more
• Five original categories: Physics, Chemistry, Physiology or Medicine, Literature, and Peace. The Peace Prize is awarded in Oslo, Norway; the rest in Stockholm, Sweden. The Economics prize (Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel) was created in 1968 and first given in 1969 — technically not a “Nobel Prize.” There is no Nobel for mathematics (see the Fields Medal). economics
• Firsts (1901): Physics — Wilhelm Röntgen (X-rays); Chemistry — Jacobus van 't Hoff; Medicine — Emil von Behring (diphtheria antitoxin); Peace — Henry Dunant (Red Cross founder) and Frédéric Passy. Prizes are not awarded posthumously (since 1974) and at most three people may share one. list of laureates

Rutherford's Nobel was in Chemistry, Pavlov's was for digestion, and Einstein's was for the photoelectric effect — three favorite trick clues. Rutherford · Pavlov

Laws & principles named after people

If a clue says “this man's law” and mentions a gas, think Boyle (P–V), Charles (V–T), Gay-Lussac (P–T), Avogadro (V–n), Dalton (partial pressures), Graham (effusion), or Henry (solubility). If it mentions electrons in orbitals, think Aufbau, Hund, or Pauli. Aufbau