Every Physics concept in the middle school Science Bowl study set (grades 6-8), with a short definition for each. Read through them here, then drill them as flashcards or practise real Physics questions.
1st (Inertia): object stays at rest or in uniform motion without net force. 2nd: F = ma — net force equals mass × acceleration. 3rd: every action has an equal and opposite reaction. Weight W = mg (g ≈ 9.8 m/s²). Normal force is perpendicular to a surface.
Equations (constant acceleration): v = v₀ + at; x = v₀t + ½at²; v² = v₀² + 2ax. Free fall: a = g = 9.8 m/s² downward. Projectile motion: horizontal velocity is constant; vertical component experiences g independently.
Work W = Fd·cosθ (Joules). Kinetic energy KE = ½mv². Gravitational PE = mgh. Work-energy theorem: net work = ΔKE. Conservation of mechanical energy (no friction): KE + PE = constant. Power P = W/t = Fv (Watts). 1 hp = 746 W.
Momentum p = mv (kg·m/s). Impulse J = FΔt = Δp. Conservation of momentum: total momentum conserved in closed system. Elastic collision: KE conserved. Inelastic: KE not conserved. Perfectly inelastic: objects stick together.
Transverse: displacement perpendicular to propagation (light, EM). Longitudinal: displacement parallel (sound). Wave speed v = fλ. Amplitude: maximum displacement. Constructive interference: in phase. Destructive: out of phase. Standing waves: nodes (zero displacement) and antinodes (max displacement).
Increasing frequency (decreasing λ): Radio → Microwave → Infrared → Visible (ROY G BIV) → UV → X-ray → Gamma. All travel at c = 3×10⁸ m/s in a vacuum. Photon energy E = hf. Higher frequency = more energy = more ionizing radiation.
Ohm's Law: V = IR. Series: same current, voltages add, R_total = ΣRᵢ. Parallel: same voltage, currents add, 1/R_total = Σ(1/Rᵢ). Power P = IV = I²R = V²/R. Kirchhoff's junction rule: currents in = currents out. Loop rule: sum of voltages around a closed loop = 0.
0th Law: thermal equilibrium is transitive. 1st Law: ΔU = Q − W. 2nd Law: entropy of isolated system never decreases; heat flows from hot to cold. 3rd Law: entropy → 0 as T → 0 K. Carnot efficiency (maximum): η = 1 − Tc/Th.
Magnetic force on charge: F = qv×B. Right-hand rule for current direction and field. Faraday's Law: ε = −ΔΦ/Δt (changing flux induces EMF). Lenz's Law: induced current opposes flux change. Transformer: Vp/Vs = Np/Ns. AC generation via rotating coils in magnetic fields.
Photoelectric effect (Einstein): E = hf − work function; proved light's particle nature. de Broglie: matter wavelength λ = h/p. Heisenberg uncertainty: Δx·Δp ≥ ℏ/2. Special relativity: E = mc²; time dilation t' = t/√(1−v²/c²); length contraction. Nuclear binding energy from mass defect.
Centripetal acceleration: aₓ = v²/r (directed toward center). Centripetal force: Fc = mv²/r. Newton's Law of Universal Gravitation: F = Gm₁m₂/r². Kepler's Laws: 1st (elliptical orbits), 2nd (equal areas in equal times), 3rd (T² ∝ r³ — orbital period vs semi-major axis).
Oscillation where restoring force ∝ displacement (F = −kx). Period of spring: T = 2π√(m/k). Period of pendulum: T = 2π√(L/g) — independent of amplitude (for small angles). At equilibrium: max velocity, zero PE stored. At extremes: zero velocity, max PE. Total energy E = ½kA² (constant).
Pressure P = F/A; in a fluid P = ρgh. Archimedes' Principle: buoyant force = weight of displaced fluid. Pascal's Law: pressure applied to enclosed fluid transmits equally. Bernoulli's Equation: P + ½ρv² + ρgh = constant (faster flow → lower pressure). Continuity: A₁v₁ = A₂v₂.
Law of reflection: angle of incidence = angle of reflection. Snell's Law (refraction): n₁sinθ₁ = n₂sinθ₂. Total internal reflection: when angle exceeds critical angle (n₁ > n₂). Lenses: converging (convex) focuses light; diverging (concave) spreads light. Mirror equation: 1/f = 1/dₒ + 1/dᵢ.
Longitudinal mechanical wave; requires medium. Speed in air ≈ 343 m/s (20°C). Intensity measured in decibels (dB): logarithmic scale, 10 dB increase = 10× intensity. Doppler effect: source moving toward observer → higher frequency (pitch). Standing waves in pipes/strings produce harmonics.
Coulomb's Law: F = kq₁q₂/r² (k = 9×10⁹ N·m²/C²). Electric field E = F/q = kq/r² (points away from + charges, toward − charges). Electric potential energy U = kq₁q₂/r. Electric potential V = kq/r (volts). Work done moving charge: W = qΔV. Field lines perpendicular to equipotential surfaces.
Angular velocity ω = Δθ/Δt (rad/s). Torque τ = rF·sinθ (N·m). Moment of inertia I: resistance to angular acceleration. Newton's 2nd Law for rotation: τ = Iα. Angular momentum L = Iω; conserved when net torque = 0 (explains spinning figure skater). Rolling without slipping: v = rω.
Nucleus held together by strong nuclear force (overcomes Coulomb repulsion). Binding energy: energy released when nucleons combine (mass defect × c²). Iron-56 has highest binding energy per nucleon. Radioactive decay: alpha (A−4, Z−2), beta-minus (Z+1), beta-plus (Z−1), gamma (no change in A or Z).
Store electrical energy in an electric field between two conducting plates. Capacitance C = Q/V (Farads). Energy stored: U = ½CV² = Q²/2C. Parallel plate capacitor: C = ε₀A/d. Adding a dielectric increases capacitance. Series: 1/C_total = Σ(1/Cᵢ). Parallel: C_total = ΣCᵢ.
Checking and converting units using known equivalences. SI base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminous intensity). Derived units: Newton = kg·m/s², Joule = N·m = kg·m²/s², Pascal = N/m², Watt = J/s.
Force per unit area: P = F/A (Pascals). Atmospheric pressure at sea level ≈ 101,325 Pa = 1 atm = 760 mmHg = 760 torr. Gauge pressure = absolute pressure − atmospheric pressure. Vacuum gauge reads relative to atmosphere. Manometers and barometers measure pressure differences.
Apparent change in frequency due to relative motion between source and observer. Source moving toward observer: higher observed frequency. Source moving away: lower frequency. f_obs = f_source × (v ± v_obs)/(v ∓ v_source). Applies to sound and light; redshift (galaxies moving away) is the Doppler effect for light.
Electrical power P = IV = I²R = V²/R (Watts). Energy E = Pt (Joules or kWh). Transformers change voltage for efficient long-distance transmission (high V, low I reduces resistive losses P = I²R). AC voltage is described by RMS values: V_rms = V_peak/√2.
Light exhibits wave-particle duality. As a wave: interference, diffraction, polarization. As a particle (photon): energy E = hf = hc/λ. Photoelectric effect confirms particle nature. Double-slit experiment demonstrates wave nature. Photons carry momentum p = h/λ. Speed of light c = 3×10⁸ m/s in vacuum.
Einstein's postulates: laws of physics same in all inertial frames; speed of light c is constant for all observers. Consequences: time dilation (moving clocks run slow), length contraction (moving objects shorter in direction of motion), mass-energy equivalence E = mc². Nothing with mass can reach c.
Gravitational field strength g = GM/r² (N/kg or m/s²). Near Earth's surface, g ≈ 9.8 m/s². Gravitational potential energy: U = −GMm/r (negative, bound system). Escape velocity: v_esc = √(2GM/r). Orbital velocity: v_orb = √(GM/r). Black holes have escape velocity ≥ c.
Static friction: force preventing stationary objects from sliding; fₛ ≤ μₛN. Kinetic friction: force opposing sliding motion; fₖ = μₖN. Rolling friction: much less than sliding. μₛ > μₖ (harder to start motion than sustain it). Normal force N: perpendicular contact force. On inclined plane: N = mg·cosθ; friction force = μmg·cosθ; net force along plane = mg·sinθ − μmg·cosθ. Air resistance is a velocity-dependent drag force.
Two-dimensional motion under gravity; horizontal and vertical components are independent. Horizontal: uniform motion, vₓ = v₀cosθ, x = v₀cosθ · t. Vertical: uniform acceleration, vᵧ = v₀sinθ − gt, y = v₀sinθ · t − ½gt². Range R = v₀²sin(2θ)/g (maximum at 45°). At maximum height: vᵧ = 0. Time of flight: t = 2v₀sinθ/g. Neglects air resistance.
Torque τ = r × F = rF sinθ (N·m); the rotational equivalent of force. For static equilibrium: ΣF = 0 (translational) AND Στ = 0 (rotational). Torque is positive (counterclockwise) or negative (clockwise) about a chosen pivot. Lever arm: perpendicular distance from pivot to line of action of force. Center of mass: point where gravity effectively acts. A uniform rod's center of mass is at its midpoint.
Temperature measures average kinetic energy of particles (K = −273.15°C). Heat Q = mcΔT where c = specific heat capacity (J/kg·K). Water: c = 4186 J/kg·K (highest of common substances — moderates climate). Latent heat: Q = mL; energy for phase change at constant temperature. Lf (fusion) for water = 334 kJ/kg; Lv (vaporization) = 2260 kJ/kg. Calorimetry: heat lost by hot object = heat gained by cold (Q_lost = Q_gained).
Diffraction: bending of waves around obstacles or through slits; significant when slit width ≈ wavelength. Single-slit diffraction: minima at a sinθ = mλ. Double-slit (Young's experiment): constructive interference at d sinθ = mλ; bright fringes spaced Δy = λL/d. Thin film interference: path difference = 2t; phase shift on reflection from denser medium. Diffraction gratings: d sinθ = mλ; bright spots at each order m.
Magnetic flux Φ = BA cosθ (Weber). Faraday's Law: induced EMF ε = −ΔΦ/Δt (rate of flux change). Lenz's Law: induced current opposes the change that caused it (−sign). EMF can be induced by: changing B, changing area, or changing angle. Motional EMF: ε = BLv (conductor of length L moving at speed v through field B). Generators: rotating loop in B field produces sinusoidal AC. Transformers: Vp/Vs = Np/Ns; IpNp = IsNs.
Standing wave: superposition of two identical waves traveling in opposite directions; produces nodes (zero displacement) and antinodes (max displacement). String (fixed both ends): fundamental f₁ = v/2L; harmonics fₙ = nv/2L. Open pipe (open both ends): same as string. Closed pipe (one end closed): only odd harmonics; f₁ = v/4L; fₙ = nv/4L (n = 1, 3, 5…). Resonance: driving frequency matches natural frequency → maximum amplitude.
RC circuit (resistor + capacitor): charging — V_C = V₀(1 − e^(−t/RC)); discharging — V_C = V₀e^(−t/RC). Time constant τ = RC; after one τ, capacitor charges to ~63% of final voltage. Impedance in AC circuits: capacitive reactance Xc = 1/(2πfC); inductive reactance XL = 2πfL. At resonance in LC circuit: f = 1/(2π√(LC)). Phase relationships: current leads voltage in capacitor; lags in inductor.
Bohr model (1913): electrons orbit nucleus at fixed radii; energy quantized. Energy levels: Eₙ = −13.6 eV/n² for hydrogen. Electron absorbs photon → jumps to higher level (absorption spectrum). Electron falls to lower level → emits photon of energy E = hf = ΔE (emission spectrum). Lyman series: transitions to n=1 (UV). Balmer series: to n=2 (visible). Paschen: to n=3 (IR). Rydberg equation: 1/λ = R(1/n₁² − 1/n₂²).
Standard Model: matter made of fermions (quarks + leptons). Quarks: up (u), down (d), charm (c), strange (s), top (t), bottom (b). Proton = uud; neutron = udd. Leptons: electron, muon, tau and their neutrinos. Forces carried by bosons: photon (electromagnetic), W/Z bosons (weak), gluons (strong), graviton (hypothetical, gravity). Antimatter: opposite charge/quantum numbers. Pair production: γ → e⁻ + e⁺.
Scalar: magnitude only (mass, speed, temperature, energy). Vector: magnitude + direction (velocity, force, acceleration, displacement, momentum). Vector addition: tip-to-tail or parallelogram method. Component method: break into x/y components, add separately. Resultant magnitude: R = √(Rₓ² + Rᵧ²); angle: θ = arctan(Rᵧ/Rₓ). Dot product (scalar): A·B = AB cosθ. Cross product (vector): A×B, magnitude AB sinθ, direction by right-hand rule.
Most materials expand when heated. Linear expansion: ΔL = αL₀ΔT (α = coefficient of linear expansion). Volume expansion: ΔV = βV₀ΔT; β ≈ 3α. Water anomaly: maximum density at 4°C; expands on freezing (ice less dense than liquid water — important for aquatic life). Bimetallic strips: two metals with different α bonded together; bend on heating (used in thermostats). Thermal stress: if expansion is constrained, large forces develop.
Archimedes' Principle: buoyant force = weight of fluid displaced (F_b = ρ_fluid · V_submerged · g). Object floats if F_b ≥ weight (density of object ≤ density of fluid). Fully submerged: F_b = ρ_fluid · V_object · g (independent of depth). Apparent weight = actual weight − F_b. Applications: ships (large volume displaces much water), submarines (change buoyancy by flooding/emptying ballast tanks), hot air balloons (less dense than cool air).
Angular momentum L = Iω; conserved when net external torque = 0. Moment of inertia I depends on mass distribution (I = mr² for point mass; I = ½mr² for solid disk; I = ⅔mr² for hollow sphere). Spinning figure skater: pulls arms in → I decreases → ω increases (L constant). Gyroscope: resists changes in orientation. Precession: gyroscope axis rotates due to gravity torque. Conservation of L explains planetary orbits (Kepler's 2nd Law — equal areas in equal times).
Speed = distance / time (m/s). Velocity includes both speed and direction (a vector). Acceleration = change in velocity / time; can mean speeding up, slowing down, or changing direction. Distance-time graphs: slope = speed. Velocity-time graphs: slope = acceleration, area = displacement. Average speed = total distance / total time.
A force is a push or pull measured in Newtons (N). Net force is the sum of all forces on an object. Balanced forces: net force = 0; object stays at rest or moves at constant velocity. Unbalanced forces cause acceleration. Types: gravity, normal force, friction (opposes motion), tension, applied force. Weight = mass × gravity (W = mg).
1st Law (Inertia): an object at rest stays at rest, and an object in motion stays in motion unless acted on by a net force. 2nd Law: F = ma (force = mass × acceleration). 3rd Law: for every action, there is an equal and opposite reaction. Inertia is the resistance to change in motion; greater mass = greater inertia.
Work = Force × distance × cos(angle) (Joules). Work is done only when a force moves an object in the direction of the force. Energy is the ability to do work. Kinetic energy (KE = ½mv²): energy of motion. Potential energy: stored energy (gravitational PE = mgh; elastic PE in springs). Energy can change forms but is always conserved.
Six types: lever, wheel and axle, pulley, inclined plane, wedge, and screw. All reduce the force needed to do work by increasing distance. Mechanical advantage (MA) = output force / input force. Levers have three classes based on the positions of the fulcrum, effort, and load. Pulleys and compound machines combine simple machines.
Waves transfer energy without transferring matter. Transverse waves: particles vibrate perpendicular to wave direction (light, water surface). Longitudinal (compressional) waves: particles vibrate parallel to wave direction (sound). Wavelength (λ): distance between identical points. Frequency (f): waves per second (Hz). Wave speed = frequency × wavelength.
Electric current: flow of electrons through a conductor, measured in amperes (A). Voltage: the 'push' that drives current, measured in volts (V). Resistance: opposition to current flow, measured in ohms (Ω). Ohm's Law: V = IR. Series circuits: one path; all components share the same current. Parallel circuits: multiple paths; voltage is the same across each branch.
Light is a form of electromagnetic radiation that travels in a straight line at about 3×10⁸ m/s in a vacuum. Reflection: light bounces off a surface (angle of incidence = angle of reflection). Refraction: light bends when it passes from one medium to another (lens, prism). White light can be separated into the visible spectrum (ROY G BIV). Opaque objects block light; transparent objects transmit light.
Sound is a longitudinal mechanical wave that requires a medium (solid, liquid, or gas) to travel through. Speed of sound in air ≈ 343 m/s; faster in solids and liquids. Pitch depends on frequency (high pitch = high frequency). Volume (loudness) depends on amplitude. Echoes are reflections of sound. Sound cannot travel in a vacuum.
Distance: total path length traveled; scalar quantity (has magnitude only, no direction). Displacement: straight-line change in position from start to finish; vector quantity (has both magnitude and direction). Example: walking 3 m east then 3 m west → distance = 6 m, displacement = 0 m. Speed = distance / time. Velocity = displacement / time (includes direction). Average speed uses total distance; instantaneous speed is speed at a specific moment.
Gravity is a force of attraction between all objects with mass. On Earth, gravity pulls objects downward toward Earth's center at g ≈ 9.8 m/s² (acceleration due to gravity). Weight = mass × gravity (W = mg); measured in Newtons. Mass: amount of matter in an object; does not change with location. Weight: force due to gravity; changes with location (you weigh less on the Moon). All objects fall at the same rate in a vacuum (Galileo) — gravity accelerates them equally regardless of mass.
Contact forces: act through physical contact. Applied force: push or pull. Tension: pulling force through a rope or string. Compression: squeezing force. Friction: opposes relative motion between surfaces. Normal force: perpendicular support force from surface. Non-contact forces: act at a distance. Gravity: attraction between masses. Magnetic force: between magnets or magnetic materials. Electrostatic force: between charged objects. Net force: vector sum of all forces; determines acceleration (F_net = ma).
Air resistance (drag): force opposing motion through air; depends on speed, shape, and surface area. As an object falls faster, air resistance increases until it equals the gravitational force → net force = 0 → acceleration = 0 → constant speed (terminal velocity). Skydivers reach terminal velocity ~195 km/h spread-eagle; parachute increases surface area → greater air resistance → much lower terminal velocity (~20 km/h). Streamlined shapes reduce air resistance (cars, aircraft, birds).
Work is done when a force moves an object in the direction of the force. Work = Force × distance (W = Fd); unit = Joule (J). No work is done if: no movement occurs, or force is perpendicular to motion. Power: rate at which work is done. Power = Work / time = Force × velocity (P = W/t); unit = Watt (W). 1 Watt = 1 Joule per second. A more powerful machine does the same work in less time. 1 horsepower = 746 Watts.
Mechanical energy = kinetic energy + potential energy. Kinetic energy (KE): energy of motion; KE = ½mv². Gravitational potential energy (GPE): stored energy due to height; GPE = mgh. As an object falls: PE converts to KE (speed increases); as it rises: KE converts to PE. Conservation of mechanical energy (no friction): total mechanical energy stays constant. Examples: pendulum (PE at top, KE at bottom), roller coaster (PE at top of hill, KE at bottom). Energy is measured in Joules.
Temperature: average kinetic energy of particles in a substance; measured in °C, °F, or K. Heat: thermal energy transferred from a hotter to a cooler object due to temperature difference; measured in Joules or calories (1 cal = 4.18 J). Specific heat capacity: amount of heat needed to raise 1 kg by 1°C (water = 4,186 J/kg·°C — high specific heat stabilizes Earth's climate). Thermal equilibrium: when two objects reach the same temperature and heat transfer stops. Temperature conversions: K = °C + 273; °F = (°C × 9/5) + 32.
Pressure = Force / Area (P = F/A); unit = Pascal (Pa). A small force over a small area can create high pressure (e.g., sharp knife). Atmospheric pressure at sea level = 101,325 Pa = 1 atm. Water pressure increases with depth: P = ρgh (density × gravity × depth). Pascal's Law: pressure applied to an enclosed fluid is transmitted equally in all directions (basis of hydraulics — car brakes, hydraulic lifts). Archimedes' Principle: buoyant force = weight of fluid displaced; objects float if buoyant force ≥ weight.
Electric charge: property of matter (positive or negative). Like charges repel; opposite charges attract (Coulomb's Law). Neutral objects have equal positive and negative charges. Static electricity: buildup of electric charge on an insulating surface (rubbing balloon transfers electrons). Conductor: allows electrons to flow freely (metals). Insulator: prevents electron flow (rubber, plastic, glass). Grounding: allows excess charge to flow into Earth. Lightning: static charge buildup in clouds discharges to ground.
Magnets: have north and south poles; opposite poles attract, like poles repel. Magnetic field: invisible force field around a magnet (field lines from N to S outside magnet). Earth acts as a giant magnet (geographic north pole is near magnetic south). All magnets are dipoles — you cannot separate poles. Electromagnets: magnetic field created by electric current flowing through a wire coil; stronger with more coils, more current, or an iron core. Used in electric motors, generators, MRI machines, speakers, and doorbells.
Series circuit: components connected in a single loop; same current flows through all; one component fails → all fail; total resistance adds up (R_total = R₁ + R₂ + …); voltage divides among components. Parallel circuit: components connected in multiple branches; same voltage across all branches; one component fails → others continue; total resistance decreases (1/R_total = 1/R₁ + 1/R₂ + …); current divides among branches. Power P = IV = I²R = V²/R. Fuses and circuit breakers protect against excess current.
Momentum = mass × velocity (p = mv); unit = kg·m/s. Momentum is a vector (has direction). The more massive or faster an object, the more momentum it has. Impulse: change in momentum = force × time (J = FΔt = Δp). Law of Conservation of Momentum: in a closed system (no external forces), total momentum before collision = total momentum after. Applications: car airbags (increase time of impact → reduce force), rocket propulsion, sports (catching a ball, billiards).
Most power plants generate electricity by spinning a turbine connected to a generator. Coal/natural gas plants: burn fuel to heat water → steam → turbine → generator. Nuclear plants: fission heats water → steam → turbine. Hydroelectric: falling water spins turbine directly. Wind: wind spins turbine blades. Solar: photovoltaic cells convert light directly to electricity (no turbine). Geothermal: steam from Earth's heat. All electricity from turbine-based plants involves: chemical/nuclear/mechanical energy → thermal energy → kinetic energy → electrical energy.
Most materials expand when heated and contract when cooled. This happens because particles vibrate more at higher temperatures and push each other farther apart. Applications: thermometers (liquid expands in a narrow tube), bimetallic strips in thermostats (two metals expand at different rates, causing bending), gaps in railroad tracks and bridges (allow for expansion without buckling). Water is unusual: it expands when freezing (ice is less dense than water → ice floats). Gases expand much more than liquids or solids when heated.
An object moving in a circle is constantly changing direction → it is accelerating even at constant speed. Centripetal acceleration: directed toward the center of the circle. Centripetal force: net inward force keeping object in circular path; provided by tension (ball on string), gravity (planets), normal force (roller coaster loop), or friction (car turning). Common misconception: there is no outward 'centrifugal force' — the feeling of being pushed outward is inertia (tendency to continue in straight line). Larger speed or smaller radius → greater centripetal force needed.
Plane (flat) mirror: image is virtual, upright, same size, appears as far behind mirror as object is in front. Concave (converging) mirror: curves inward; can produce real or virtual images depending on object position; used in telescopes, flashlights, satellite dishes. Convex (diverging) mirror: curves outward; always produces smaller, virtual, upright image; wider field of view — used in store security mirrors and car side mirrors. Converging (convex) lens: focuses parallel rays to focal point. Diverging (concave) lens: spreads rays apart. Glasses correct vision by adjusting where image forms on retina.
White light is a mixture of all visible colors (ROY G BIV: red, orange, yellow, green, blue, indigo, violet). A prism separates white light into its spectrum by refraction (different wavelengths bend at different angles). The color of an opaque object is the color of light it reflects (a red apple reflects red, absorbs others). Mixing light colors (additive): red + green + blue = white. Mixing pigments/paints (subtractive): yellow + cyan + magenta = black. Electromagnetic waves include (in order of increasing frequency): radio, microwave, infrared, visible, UV, X-ray, gamma.
A projectile is any object launched into the air that then moves under gravity alone (no engine force). Horizontal and vertical motions are independent. Horizontal: constant velocity (no air resistance). Vertical: constant acceleration downward due to gravity (g ≈ 9.8 m/s²). The path (trajectory) is a parabola. Maximum range achieved at 45° launch angle. At maximum height, vertical velocity = 0. Time in air depends only on vertical component. Examples: kicked soccer ball, bullet fired horizontally, thrown ball.
Waves transfer energy from one place to another without transferring matter. Amount of energy carried by a wave depends on its amplitude (larger amplitude → more energy). Frequency also relates to energy (higher frequency → more energy per photon for electromagnetic waves). Wave reflection: bounces off a barrier. Refraction: changes speed (and direction) when entering a new medium. Absorption: wave energy is absorbed by material and converted to heat. Resonance: object vibrates at its natural frequency when driven by an external frequency matching it (can cause bridges to oscillate dangerously).
Machines change the size or direction of a force. Mechanical advantage (MA) = output force / input force. MA > 1: machine multiplies force (requires moving input through larger distance). MA < 1: machine multiplies speed/distance (requires larger input force). Efficiency = (work output / work input) × 100%; always < 100% due to friction. Lever classes: Class 1 (fulcrum between effort and load — seesaw); Class 2 (load between fulcrum and effort — wheelbarrow); Class 3 (effort between fulcrum and load — tweezers). Pulleys: fixed (changes direction only), movable (doubles force), block-and-tackle (multiple pulleys).
Nuclear energy comes from changes in the nucleus of atoms. Fission: splitting a heavy nucleus (like uranium-235) into smaller nuclei + 2–3 neutrons + large amount of energy. Chain reaction: neutrons released split more nuclei. Used in nuclear power plants (controlled chain reaction; heat produces steam to turn turbines) and nuclear bombs (uncontrolled). Fusion: combining light nuclei (hydrogen isotopes) to form helium → even more energy released; powers the Sun and stars; scientists are working to achieve controlled fusion on Earth. No CO₂ emissions from nuclear power.
Conduction: heat transfer through direct contact; atoms/molecules pass kinetic energy to neighbors; best in solids (metals are best conductors; wood, air are good insulators). Convection: heat transfer by movement of a fluid (liquid or gas); warm fluid rises (less dense), cool fluid sinks → creates convection currents; responsible for ocean currents, wind, boiling water. Radiation: heat transfer by electromagnetic waves (infrared); does not need a medium; how the Sun warms Earth; dark surfaces absorb more radiation than light/shiny surfaces.
Density = mass / volume (D = m/V); units g/cm³ or g/mL. An object floats if it is less dense than the fluid it is placed in; sinks if it is denser. Density of water = 1 g/cm³. Ice (0.92 g/cm³) floats on water. A ship made of steel floats because its average density (including air inside) is less than water. Archimedes' principle: buoyant force = weight of displaced fluid. Liquids of different densities can be layered (e.g., oil floats on water). Gases are much less dense than liquids or solids.
Wavelength (λ): distance between two consecutive crests (or troughs). Frequency (f): number of complete waves passing a point per second; unit = Hertz (Hz). Amplitude: maximum displacement from rest position (related to energy). Wave speed (v): v = fλ (wave speed = frequency × wavelength). Period (T): time for one complete wave; T = 1/f. Crest: highest point; trough: lowest point. For electromagnetic waves (light, radio, etc.): all travel at speed of light (c = 3 × 10⁸ m/s) in a vacuum.
Gravity is a long-range attractive force between all masses. Strength of gravity depends on the masses of the objects and the distance between them (greater mass → stronger gravity; greater distance → weaker gravity). Orbit: a planet's curved path around the Sun is caused by gravity pulling it continuously inward while its inertia carries it forward. Satellites orbit Earth the same way. The Moon is in orbit around Earth; Earth orbits the Sun. Without gravity, an orbiting body would travel in a straight line. Orbital speed must be just right — too slow → falls; too fast → escapes.
Latent heat: energy absorbed or released during a phase change at constant temperature. Melting (solid → liquid): absorbs latent heat of fusion. Freezing (liquid → solid): releases latent heat of fusion. Evaporation/boiling (liquid → gas): absorbs latent heat of vaporization (largest). Condensation (gas → liquid): releases latent heat of vaporization. For water: latent heat of vaporization = 2,260 kJ/kg (much larger than fusion = 334 kJ/kg). Latent heat explains why sweating cools the body and why steam burns are worse than boiling water burns.
Specific heat capacity (c): amount of heat required to raise 1 kg of a substance by 1°C (or 1 K). Q = mcΔT (heat = mass × specific heat × temperature change). Water has a very high specific heat (4,186 J/kg·°C) — it resists temperature changes; this moderates Earth's climate and body temperature. Metals have low specific heat (iron ≈ 450 J/kg·°C) — heat up and cool down quickly. Calorimetry: measuring heat transfer; in an insulated system, heat lost by hot = heat gained by cold (Q_lost = Q_gained). Used to find specific heat of unknown substances.
Short circuit: electricity takes an unintended low-resistance path (can cause overheating and fires). Fuse: thin wire that melts when too much current flows, breaking the circuit. Circuit breaker: switch that trips and opens circuit when current is too high; can be reset. Ground fault circuit interrupter (GFCI): detects tiny current differences and shuts off power in 1/40 second (used near water). Insulation: rubber/plastic coating on wires prevents accidental contact. Never use electrical devices near water (water + electricity → electrocution). High-voltage power lines are dangerous to approach.