ASVAB General Science — Physics, Forces, and Energy

Speed of light in vacuum (c) = 299,792,458 m/s ≈ 3 × 10⁸ m/s ≈ 300,000 km/s ≈ 186,000 mi/s. This is the universal speed limit per Einstein's special relativity — no information or matter can exceed it. Light slows in matter (water n=1.33, glass n=1.5, diamond n=2.4 — refractive index); the ratio c/v in matter is the index of refraction. Light is electromagnetic radiation, part of the electromagnetic spectrum: (1) Radio waves — longest wavelength, lowest frequency, lowest energy; (2) Microwaves; (3) Infrared (heat); (4) Visible light (the narrow band our eyes detect: 400-700 nm; red longest wavelength, violet shortest); (5) Ultraviolet; (6) X-rays; (7) Gamma rays — shortest wavelength, highest frequency, highest energy. All travel at c in vacuum. Wave equation: c = λf (speed = wavelength × frequency). Light from the Sun takes ~8 minutes to reach Earth (~150 million km). Light-year: distance light travels in 1 year (~9.46 trillion km), a unit of cosmic distance. Nearest star (Proxima Centauri): 4.24 light-years.

Electric current is measured in Amperes (A or 'amps'). One ampere = one coulomb of charge per second. Key electrical quantities and units: (1) Current (I) — flow of charge — Amperes (A); (2) Voltage (V) — electrical potential difference — Volts (V); (3) Resistance (R) — opposition to current flow — Ohms (Ω); (4) Power (P) — rate of energy transfer — Watts (W); (5) Charge (Q) — electric charge — Coulombs (C); (6) Energy — Joules (J). Ohm's Law: V = IR (voltage = current × resistance). Solve for any variable: I = V/R, R = V/I. Power: P = VI = I²R = V²/R. Series circuits: same current through all components; voltages add; total resistance = sum of resistances. Parallel circuits: same voltage across all components; currents add; 1/R_total = 1/R₁ + 1/R₂ + ... Direct Current (DC): constant direction; batteries, solar cells. Alternating Current (AC): direction reverses periodically; household electricity (60 Hz in US, 50 Hz in much of world). Conductors (low resistance): metals, especially copper, silver, gold. Insulators (high resistance): rubber, plastic, glass, dry wood. Semiconductors (intermediate): silicon, germanium — used in electronics. Electricity safety: avoid wet locations, GFCI outlets, breakers and fuses prevent overcurrent.

Mass vs Weight — commonly confused: MASS: amount of matter in an object; measured in kilograms (kg) or grams (g); constant — same on Earth, Moon, or in space; measured using a balance (compares to known masses). WEIGHT: gravitational force on an object; measured in Newtons (N); varies with gravitational field strength; measured using a scale (measures force). Weight = mass × gravitational acceleration. On Earth: g = 9.8 m/s². Weight of 1 kg on Earth ≈ 9.8 N. Common bathroom scale measures weight (in pounds, which is a force unit), but displays it as if it were mass — works because Earth's gravity is constant on the surface. On other bodies: Moon's gravity ≈ 1/6 Earth's, so a 60 kg person weighs ~98 N on Moon (vs ~588 N on Earth) but still has 60 kg of mass. In free fall or orbit: 'weightless' — gravity acts but no normal force; mass unchanged. The pound is sometimes used as a mass unit (avoirdupois) and sometimes as a force unit (pound-force) — confusion arises in everyday language. SI strictly: kilogram = mass; Newton = force; pound-mass and pound-force are separate.

The Ohm (Ω) is the unit of electrical resistance. Ampere = current; Volt = voltage; Watt = power.

NEWTON'S THREE LAWS: First Law (Inertia): Objects at rest stay at rest; objects in motion stay in motion, unless acted on by a net force. Second Law (F=ma): Force = Mass × Acceleration; more force = more acceleration; more mass = less acceleration for the same force. Third Law (Action-Reaction): For every action force, there is an equal and opposite reaction force. EXAMPLES of Third Law: Rocket propulsion (exhaust pushes back, rocket pushes forward); walking (foot pushes back on ground, ground pushes foot forward); recoil when firing a weapon (bullet goes forward, weapon kicks backward).

WOOD BURNING is a CHEMICAL CHANGE — it produces new substances (ash, gases) and cannot be reversed; the chemical composition changes. ASVAB General Science distinguishes physical vs chemical changes. PHYSICAL CHANGES (no new substance, often reversible): melting ice, boiling water, tearing paper, dissolving salt — the substance changes form/state but remains the same chemically. CHEMICAL CHANGES (new substances form): burning, rusting, cooking, digestion. Burning wood is a classic chemical change (combustion); the others listed are physical changes.

GRAVITY is the force that pulls objects toward the center of the Earth (and that attracts masses to each other generally). ASVAB General Science/physics covers basic forces. Near Earth's surface, gravity accelerates falling objects at about 9.8 m/s². Other concepts: FRICTION (resists motion between surfaces); MAGNETISM (attraction/repulsion of magnetic materials); INERTIA (an object's resistance to a change in motion — Newton's first law). Gravity as the force pulling objects toward Earth is a fundamental, commonly tested concept.

Newton's third law of motion states that for every action there is an equal and opposite reaction — when one object exerts a force on a second, the second exerts an equal force in the opposite direction on the first. A rocket pushing exhaust down and being pushed up is a classic example. Newton's first law is the law of inertia (objects keep their motion unless acted on by a force); the second law relates force, mass, and acceleration (F = ma). Knowing all three laws and an example of each is common General Science content.

Conduction is the transfer of heat through direct contact between materials, as when a metal spoon heats up in a hot pot of soup. Convection is heat transfer through the movement of fluids (liquids or gases), such as warm air rising. Radiation transfers heat through electromagnetic waves and needs no medium, like the warmth you feel from the sun. Insulation is a material that slows heat transfer, not a method of transfer. Distinguishing the three modes of heat transfer — conduction, convection, and radiation — is a frequent physics topic.