Physics: Practice Questions & Explanations

Newton's three laws of motion: (1) FIRST LAW (Inertia): An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. Objects 'want' to keep doing what they're doing; inertia is the resistance to change in motion. Mass measures inertia — more massive objects are harder to start, stop, or change direction. Example: a book on a table stays still until you push it; a hockey puck on frictionless ice would slide forever. (2) SECOND LAW (F = ma): The net force on an object equals its mass times its acceleration. F = ma. Force is measured in Newtons (kg·m/s²). Acceleration is the change in velocity over time. Mass is constant for an object. Example: more force on the same mass = more acceleration; same force on more mass = less acceleration. (3) THIRD LAW (Action-Reaction): For every action, there is an equal and opposite reaction. When object A exerts force on object B, B exerts equal but opposite force on A. Example: when you push against a wall, the wall pushes back with equal force; a rocket pushes hot gas out the back, the gas pushes the rocket forward. Newton's laws apply to non-relativistic, non-quantum scales — everyday physics. Einstein's relativity modifies them at near-light speeds; quantum mechanics modifies them at atomic scales.

Newton's Second Law: F = ma. Force = mass × acceleration. Units: Force in Newtons (N); mass in kilograms (kg); acceleration in meters per second squared (m/s²). 1 Newton = 1 kg·m/s² (the force needed to accelerate 1 kg at 1 m/s²). Other common physics formulas: (1) Weight = mg (mass × gravitational acceleration; g = 9.8 m/s² on Earth); weight is a force, mass is a quantity of matter; (2) Velocity = displacement/time; (3) Acceleration = change in velocity/time; (4) Kinetic energy = ½mv²; (5) Gravitational potential energy = mgh; (6) Work = force × distance (when in the same direction); (7) Power = work/time; (8) Momentum = mv; (9) Density = mass/volume; (10) Pressure = force/area. Solving F = ma problems: identify what's given (typically 2 of 3), solve for unknown. Example: a 2 kg object accelerates at 5 m/s²; force = 2 × 5 = 10 N. Example: a 50 N force on a 10 kg object; acceleration = 50/10 = 5 m/s². Net force: when multiple forces act, sum them as vectors; F = ma applies to the NET force.

Kinetic energy is the energy of motion. Any moving object has KE: KE = ½mv². Examples: a moving car, falling water, wind, a thrown ball, atoms vibrating (thermal energy), electrons flowing (electricity). Forms of energy: (1) Kinetic — motion energy (mechanical, thermal, sound, electrical); (2) Potential — stored energy (gravitational, elastic, chemical, nuclear); (3) Radiant — electromagnetic waves (light, radio, X-rays). Energy conversions: (1) Falling object: gravitational PE → KE; (2) Pendulum: KE ↔ PE alternately; (3) Burning fuel: chemical PE → thermal KE → light + heat; (4) Battery → electrical current: chemical PE → electrical KE; (5) Photosynthesis: radiant → chemical PE in glucose. Conservation of Energy: energy cannot be created or destroyed, only transformed (First Law of Thermodynamics). The total energy of an isolated system is constant. In real systems, some energy is lost as heat (entropy increase — Second Law). 'Lost' energy isn't gone — it just becomes less useful (dispersed as low-grade thermal energy). Mechanical energy = KE + PE; in conservative systems (no friction), mechanical energy is conserved.

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. A useful way to keep the four straight: the volt pushes, the ampere flows, the ohm resists, and the watt is the rate at which energy is used (volts × amperes). The symbol Ω (omega) is used for ohms on circuit diagrams and multimeters. Resistance is what limits current for a given voltage, per Ohm's Law (V = IR).

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).

The OHM is the basic unit of electrical RESISTANCE. ASVAB General Science covers basic physics/electricity. Key electrical units: VOLT (voltage/electrical potential); AMPERE/amp (current); OHM (resistance); WATT (power). OHM'S LAW relates them: V = I × R (Voltage = Current × Resistance). Knowing which unit measures which quantity (ohm = resistance) is commonly tested on both General Science and Electronics Information.

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.

Sound travels FASTEST through SOLIDS (then liquids, then gases — slowest in gases like air). ASVAB General Science/physics covers wave properties. Sound is a mechanical wave requiring a medium; it travels faster when particles are closer together (denser, more tightly bound), so: SOLID > LIQUID > GAS. Sound CANNOT travel through a VACUUM (no particles to transmit it — which is why space is silent). The fact that sound travels fastest in solids and not at all in a vacuum is commonly tested.

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.

Electric current — the flow of electric charge — is measured in amperes, often shortened to amps (symbol A). The volt is the unit of electric potential difference (voltage), the ohm is the unit of electrical resistance, and the watt is the unit of power. These quantities are related by Ohm's law and the power equation, but the question asks specifically about current, which is measured in amperes. Matching electrical quantities to their units — current/amps, voltage/volts, resistance/ohms, power/watts — appears in both General Science and Electronics content.