Physics: Motion, Energy and Waves

⚡ Physics: Motion, Energy and Waves

Part 1 of 7 — Physics on the ASVAB: Units and Metric Prefixes

Topic Overview

Where Physics Shows Up

General Science (GS) is the first subtest on the CAT-ASVAB. You get 15 scored questions in 12 minutes, which works out to about 48 seconds per question. The official description is "knowledge of physical and biological sciences," and physical science (force and motion, energy, fluids, atoms, chemistry) is one of its three big areas, next to life science and earth and space science.

The physics questions are not long calculations. They are mostly "do you know this fact or idea?" questions, plus some one- or two-step problems with friendly numbers. No calculator is allowed, so the numbers are chosen to work out by hand.

Why GS matters: GS does not count toward your AFQT, but it feeds job line scores. The Air Force E (Electronics) composite is GS + AR + MK + EI, and the Navy and Marine Corps EL composites use the same four subtests. Electronics, nuclear and many technical jobs care about this score.

What You Will Learn in This Lesson

Physics on the ASVAB comes down to a short list of ideas: how things move, why they move (forces), how energy changes form, how waves carry sound and light, and how charges and magnets push and pull. This part starts with the toolkit every one of those questions uses: units.

Warm-up. 🎯 Answer from what you already know. Don't worry if you miss some: each idea is taught later in this lesson.

The Units Physics Uses

Science uses the metric system (SI). Every quantity has a unit, and the ASVAB often tests whether you can match a quantity to its unit.

Base units

Units named after scientists

Many derived units are named after people. Learn the pairs:

Key Fact: If a question asks "which unit measures energy?", the answer is the joule, not the watt. Watts measure power: how fast energy is used (1 watt = 1 joule per second).

Metric Prefixes

A prefix in front of a unit multiplies it by a power of 10. These seven go in order, each step 10 times the next:

Mnemonic: King Henry Died By Drinking Chocolate Milk.

Three more prefixes appear in science and electronics:

Converting with the ladder

Moving down the ladder (toward milli-) the number gets bigger: move the decimal point right one place per step. Moving up (toward kilo-) the number gets smaller: move it left.

Example 1. Convert 3.5 kilometers to meters. Kilo- to base is 3 steps down, so move the decimal 3 places right: 3,500 m.

Example 2. Convert 250 centimeters to meters. Centi- to base is 2 steps up, so move the decimal 2 places left: 2.5 m.

Example 3. Convert 4,000 milligrams to grams. Milli- to base is 3 steps up: 4 g.

Key Fact: A big unit needs a small number and a small unit needs a big number. 2 km and 2,000 m are the same distance.

Match each quantity to its unit. 🔽

Use the prefix ladder. ✍️

1) How many meters are in 7 kilometers?

2) How many grams are in 600 centigrams?

3) How many milliliters are in 2.4 liters?

Units and prefixes. 🎯

Key Takeaways — Part 1

• GS is the first CAT-ASVAB subtest: 15 questions, 12 minutes, about 48 seconds each
• Base units: meter, kilogram, second, kelvin, ampere
• Named units: newton = force, joule = energy and work, watt = power, pascal = pressure, hertz = frequency, volt = voltage, ohm = resistance
• King Henry Died By Drinking Chocolate Milk: kilo-, hecto-, deka-, base, deci-, centi-, milli-, each step a factor of 10
• Mega- = 1,000,000; micro- = 1/1,000,000; nano- = 1/1,000,000,000
• Going to a smaller unit makes the number bigger

Up Next: Part 2 — speed, acceleration, mass and weight, and Newton's three laws.

⚡ Physics: Motion, Energy and Waves

Part 2 of 7 — Motion, Forces and Newton's Laws

Describing Motion

The distance triangle. Speed, distance and time are linked by $d = s \times t$. Cover the one you want:

• distance = speed × time
• speed = distance ÷ time
• time = distance ÷ speed

Example 1. A convoy drives at 50 mph for 3 hours. Distance = 50 × 3 = 150 miles.

Example 2. A truck covers 180 miles at 60 mph. Time = 180 ÷ 60 = 3 hours.

Example 3. A car speeds up from 0 to 20 m/s in 5 seconds. Acceleration = 20 ÷ 5 = 4 m/s²: its speed grows by 4 m/s every second.

Key Fact: Acceleration means any change in velocity: speeding up, slowing down, or changing direction. A car going around a curve at a steady 30 mph is accelerating, because its direction changes.

Reading a distance–time graph

On a graph of distance against time, the steepness (slope) is the speed. A steeper line means faster, and a flat line means the object is stopped.

Read the graph. 📈 Figure 1 shows a runner during a timed drill.

Gravity, Free Fall, Mass and Weight

Near Earth's surface, gravity makes a falling object speed up by about 9.8 m/s² (about 32 ft/s²). This number is called $g$.

Free fall rule: without air resistance, all objects fall at the same rate, no matter how heavy. A hammer and a feather dropped on the Moon (no air) land together. On Earth the feather falls slower only because air pushes back on it.

Example. A 10 kg pack weighs about $10 \times 9.8 = 98$ N on Earth. Taken to the Moon, it still has a mass of 10 kg, but it weighs only about one-sixth as much.

Key Fact: Mass never changes with location. Weight depends on how strong gravity is where you are.

Newton's Three Laws

Mnemonic: Inertia, Force = ma, Action–reaction: "I Found Answers."

Using F = ma. A net force of 30 N on a 6 kg box: $a = \frac{F}{m} = \frac{30}{6} = 5$ m/s².

Friction

Friction is a force that opposes sliding. It acts in the direction opposite to the motion (or the attempted motion), and it turns motion energy into heat (rub your hands together). Rough surfaces give more friction; lubricants such as oil reduce it.

Momentum

Momentum is mass in motion: $p = mv$. A slow truck can have more momentum than a fast bicycle because its mass is so much larger. In a collision with no outside forces, the total momentum before equals the total momentum after.

Example. A 2,000 kg truck at 5 m/s has momentum $2{,}000 \times 5 = 10{,}000$ kg·m/s. A 100 kg cyclist at 10 m/s has only $1{,}000$ kg·m/s.

Key Fact: Inertia depends only on mass. The more mass, the harder it is to start, stop or turn the object.

Forces and motion. 🎯

Work the numbers. ✍️ No calculator.

1) A helicopter flies 240 km in 2 hours. What is its average speed, in km/h?

2) A cart's speed increases from 0 to 12 m/s in 4 seconds. What is its acceleration, in m/s²?

3) A net force of 40 N acts on an 8 kg crate. What is its acceleration, in m/s²?

Name the idea. 🔽

Key Takeaways — Part 2

• Speed = distance ÷ time; velocity is speed with a direction; acceleration is change in velocity ÷ time (m/s²)
• On a distance–time graph, slope = speed and a flat line = stopped
• Gravity: $g \approx 9.8$ m/s² (about 32 ft/s²); without air, everything falls at the same rate
• Mass (kg) stays the same everywhere; weight = $mg$ changes with gravity (Moon about 1/6)
• Newton: 1st inertia, 2nd $F = ma$, 3rd equal and opposite forces
• Friction opposes motion; momentum $p = mv$

Up Next: Part 3 — work, energy, power, and how heat moves.

⚡ Physics: Motion, Energy and Waves

Part 3 of 7 — Work, Energy, Power and Heat

Work

In physics, work is done only when a force moves something in the direction of the force:

$W = F \times d$

Force in newtons times distance in meters gives work in joules (J).

Example 1. You push a crate with 50 N of force for 4 m. Work = 50 × 4 = 200 J.

Example 2. You hold a heavy box perfectly still for a minute. Your arms get tired, but the box does not move, so the work done on the box is zero.

Two kinds of mechanical energy

Example 3. A 2 kg ball moving at 3 m/s has $KE = \frac{1}{2} \times 2 \times 3^2 = 9$ J.

Example 4. A 5 kg box on a shelf 2 m high has $PE = 5 \times 10 \times 2 = 100$ J (using $g \approx 10$ m/s² to keep the numbers friendly).

Key Fact: Double the speed and the kinetic energy becomes four times as large, because speed is squared. That is why a car at 60 mph needs about four times the braking distance it needs at 30 mph.

Conservation of Energy and Power

Energy cannot be created or destroyed, only changed from one form to another. The total stays the same.

A swinging pendulum trades energy back and forth: most PE and least KE at the highest point, most KE (fastest) at the lowest point. Friction slowly turns some of the energy into heat, so a real pendulum gradually stops.

Power

Power is how fast work is done or energy is used:

$P = \frac{W}{t}$

Joules per second are watts (W). One horsepower is about 746 watts.

Example 5. A motor lifts a load, doing 600 J of work in 3 seconds. Power = 600 ÷ 3 = 200 W.

Key Fact: Two people climb the same stairs. They do the same work if they weigh the same, but the one who climbs faster uses more power.

Work, energy and power. 🎯

Calculate. ✍️

1) A worker pushes a cart with a force of 25 N for 8 m. How much work is done, in joules?

2) A 3 kg rock rests on a ledge 4 m high. Using $g = 10$ m/s², what is its potential energy, in joules?

3) A pump does 1,200 J of work in 4 seconds. What is its power, in watts?

Heat and Temperature

Temperature measures how fast the particles in a substance move on average. Heat is energy that flows from a hotter object to a colder one, never the other way on its own. A bathtub of warm water holds more heat energy than a cup of boiling water, even though the cup is at a higher temperature.

Temperature scales

Conversions:

• $F = \frac{9}{5}C + 32$
• $C = \frac{5}{9}(F - 32)$
• $K = C + 273$

Example 6. Convert 25 °C to Fahrenheit: $\frac{9}{5} \times 25 + 32 = 45 + 32 = 77$ °F.

Example 7. Convert 50 °F to Celsius: $\frac{5}{9} \times (50 - 32) = \frac{5}{9} \times 18 = 10$ °C.

Key Fact: A change of 1 °C is a bigger step than a change of 1 °F (it equals 1.8 °F). A change of 1 °C equals a change of 1 K.

Three ways heat moves

Memory hook: Conduction = Contact, conVection = moVing fluid, Radiation = Rays.

Conductors, insulators and other heat facts

• Conductors pass heat easily: metals such as copper, aluminum and silver.
• Insulators slow heat flow: wood, plastic, rubber, foam, still air (why a puffy jacket keeps you warm).
• Thermal expansion: most materials expand when heated and contract when cooled. Bridges have expansion joints; a tight metal jar lid loosens under hot water.
• Specific heat is how much energy it takes to warm a substance. Water's specific heat is very high, so water heats and cools slowly. That is why coastal towns have milder weather than inland towns and why water is used as engine coolant.

Name the heat transfer. 🔽

Convert temperatures. ✍️

1) Convert 20 °C to Fahrenheit.

2) Convert 122 °F to Celsius.

3) Convert 27 °C to kelvins.

Key Takeaways — Part 3

• Work $W = Fd$ in joules; no movement means no work
• $KE = \frac{1}{2}mv^2$ (motion), $PE = mgh$ (height); doubling speed quadruples KE
• Energy is conserved: it only changes form
• Power $P = \frac{W}{t}$ in watts; 1 horsepower ≈ 746 W
• Heat flows hot → cold; water freezes at 0 °C = 32 °F and boils at 100 °C = 212 °F; −40 is the same on both scales
• $F = \frac{9}{5}C + 32$, $K = C + 273$
• Conduction = contact, convection = moving fluid, radiation = waves
• Metals conduct, foam and still air insulate, water has a high specific heat

Up Next: Part 4 — waves, sound and light.

⚡ Physics: Motion, Energy and Waves

Part 4 of 7 — Waves, Sound and Light

Anatomy of a Wave

A wave carries energy from place to place without carrying matter along with it. A cork on a pond bobs up and down as waves pass, but it does not travel across the pond.

The wave equation

$v = f\lambda$

Wave speed = frequency × wavelength.

Example 1. A wave has a frequency of 5 Hz and a wavelength of 2 m. Speed = 5 × 2 = 10 m/s.

Example 2. A wave travels at 12 m/s with a wavelength of 3 m. Frequency = 12 ÷ 3 = 4 Hz, and its period is 1/4 second.

Key Fact: For waves moving at the same speed, higher frequency means shorter wavelength. Frequency and wavelength trade off.

Two kinds of waves

Read the wave. 🌊 Use Figure 1.

Sound

Sound is a longitudinal wave of compressions and spread-out regions in a material. It needs a medium (air, water, a solid), so sound cannot travel through a vacuum such as outer space.

Sound is slowest in gases and fastest in solids, the opposite of what many people guess.

• Humans hear roughly 20 Hz to 20,000 Hz. Above that is ultrasound (used for medical imaging and sonar); below it is infrasound.
• Sonar times an echo to find distance. If a ping returns in 2 seconds, the sound went out and back, so the object is 1 second's travel away.
• Doppler effect: a source moving toward you sounds higher in pitch; moving away, lower. An ambulance siren drops in pitch as it passes.

Example 3. You see lightning and hear the thunder 3 seconds later. Sound covers about 343 m each second, so the strike is about 3 × 343 ≈ 1,000 m (1 km) away. (Rule of thumb: about 5 seconds per mile.)

Light and the Electromagnetic Spectrum

Light is an electromagnetic (EM) wave. EM waves are transverse and need no medium: they cross empty space. In a vacuum all of them travel at the speed of light, about 300,000 km/s (186,000 miles per second), nearly a million times faster than sound in air.

The spectrum, from long wavelength to short

Mnemonic: Raging Martians Invaded Venus Using X-ray Guns.

ROY G BIV = red, orange, yellow, green, blue, indigo, violet.

What light does

Lenses. A convex lens (thicker in the middle) bends light together to a focus: magnifying glasses, and glasses for farsightedness. A concave lens (thinner in the middle) spreads light apart: glasses for nearsightedness.

Color. White light is a mix of all colors. An object looks red because it reflects red and absorbs the other colors. A white object reflects all colors; a black object absorbs them all.

Sound and light. 🎯

Waves vocabulary. 🔽

Wave math. ✍️

1) A water wave has a frequency of 2 Hz and a wavelength of 6 m. What is its speed, in m/s?

2) A wave moves at 20 m/s and has a wavelength of 5 m. What is its frequency, in Hz?

3) A sonar ping returns to a ship 4 seconds after it was sent. Sound travels about 1,500 m/s in seawater. How deep is the sea floor, in meters?

Key Takeaways — Part 4

• Waves carry energy, not matter; wavelength = crest to crest, amplitude = rest line to crest
• $v = f\lambda$; frequency in hertz; period = 1 ÷ frequency
• Transverse: light, rope waves. Longitudinal: sound
• Sound needs a medium: fastest in solids, slowest in gases, none in a vacuum; about 343 m/s in air
• Pitch = frequency, loudness = amplitude (decibels); Doppler: approaching sounds higher
• Light: about 300,000 km/s, needs no medium; spectrum radio, microwave, infrared, visible, UV, X-ray, gamma (long → short wavelength, low → high energy)
• ROY G BIV; reflection bounces, refraction bends; convex lens converges, concave lens spreads

Up Next: Part 5 — charges, magnets and electromagnets.

⚡ Physics: Motion, Energy and Waves

Part 5 of 7 — Electricity and Magnetism Basics

Electric Charge

All matter is made of atoms with positive protons and negative electrons. Normally the charges balance. Rub two materials together (a balloon on your hair, socks on carpet) and electrons move from one to the other: one object becomes negative (extra electrons), the other positive (missing electrons). That built-up charge is static electricity.

Key Fact: Only electrons move when objects become charged. Protons stay locked in the nucleus.

Conductors and Insulators

That is why wires are copper on the inside and covered in plastic or rubber on the outside.

Three words you need

They are linked by Ohm's law, $V = IR$. Working with circuits (series, parallel, power in a circuit) belongs to the Electronics Information subtest; for GS, know the three words, their units, and that a battery's direct current (DC) flows one way while wall outlets supply alternating current (AC), which reverses direction many times a second.

Magnets

Every magnet has a north pole and a south pole. Cut a magnet in half and you get two smaller magnets, each with its own north and south.

Magnetic materials. Only a few metals are strongly attracted to magnets: iron, nickel and cobalt (and steel, which is mostly iron). Aluminum, copper, gold, wood and plastic are not. Memory hook: "I Need Coffee": Iron, Nickel, Cobalt.

Earth is a giant magnet. A compass needle is a small magnet free to turn. Its north-seeking end points toward Earth's geographic North Pole, which means a magnetic south pole sits near there (opposites attract).

Electricity and magnetism are linked

An electromagnet gets stronger with more current, more turns of wire, or an iron core. Junkyard cranes use electromagnets so they can drop the load by switching off the current.

Key Fact: Motor and generator are opposites. A motor uses electricity to make motion; a generator uses motion to make electricity.

Read the field. 🧲 Use Figure 1.

Charges and magnets. 🎯

Units and rules. 🔽

Key Takeaways — Part 5

• Like charges repel, opposite charges attract; only electrons move when objects get charged; lightning is a static discharge
• Conductors: metals (silver best, copper common). Insulators: rubber, plastic, glass, dry wood
• Current = flow (amperes), voltage = push (volts), resistance = opposition (ohms); $V = IR$; DC flows one way, AC reverses
• Like poles repel, opposite poles attract; field lines run N → S outside the magnet and crowd together where the field is strong
• Magnetic metals: iron, nickel, cobalt ("I Need Coffee")
• A compass's north-seeking end points to geographic north; Earth acts like a giant magnet
• Electromagnet = current makes magnetism; motor = electricity → motion; generator = motion → electricity

Up Next: Part 6 — how to work the physics questions fast and dodge the common traps.

⚡ Physics: Motion, Energy and Waves

Part 6 of 7 — Test Strategy and Common Traps

Your Clock on GS

Most physics questions are recall questions you can answer in 15 to 20 seconds if you know the fact. That banks time for the ones that need a quick calculation.

Time budget for a physics question

1. Read the last line first. What is being asked for: a unit, a direction, a number?
2. Name the idea. "This is Newton's third law" or "this is $v = f\lambda$." Most GS physics needs only one formula.
3. Estimate before you compute. Friendly numbers are the rule (no calculator is allowed). If your answer is ugly, recheck.
4. Eliminate. Cross out choices with the wrong unit or the wrong direction first.

Running short on time

On the CAT, unfinished questions at the time limit are scored as if you had guessed at random, and the official guidance warns that a string of wrong answers near the end of a subtest is penalized heavily. Since GS is mostly recall, you rarely run short; if you do, keep reading and answering each question for real rather than clicking randomly.

The Physics Traps the Test Loves

Key Fact: When two choices are exact opposites (attract vs repel, higher vs lower, motor vs generator), the question is testing a rule. Say the rule to yourself before you pick.

Spot the trap. 🪤 Each of these is built around one of the traps above.

Fix the misconception. 🔽 Choose the word that makes each statement TRUE.

Quick numbers, no calculator. ✍️ Aim for under a minute total.

1) A diver's sonar ping returns after 6 seconds. Sound travels 1,500 m/s in the water. How far away is the object, in meters?

2) Convert 15 °C to Fahrenheit.

Key Takeaways — Part 6

• GS: 15 questions in 12 minutes, about 48 seconds each, and it comes first on the CAT
• No going back: confirm your answer before you click; treat every question as scored
• Read the last line, name the idea, estimate, eliminate wrong units and wrong directions
• Know the traps: equal fall rates, mass vs weight, amplitude is half of crest-to-trough, sound fastest in solids, watt = power, multiply by 9/5 before adding 32, halve echo times, turning is acceleration

Up Next: Part 7 — a mixed mastery set at ASVAB difficulty.

⚡ Physics: Motion, Energy and Waves

Part 7 of 7 — Mixed Mastery Set

How This Set Works

These questions mix every part of the lesson, in the style and difficulty range of the GS subtest: mostly quick recall, a few one- or two-step calculations, and a couple of harder "which idea applies?" questions. Give yourself about 48 seconds per question, the CAT pace.

Tip: After each question, read the explanation even when you are right. Explanations show the fastest route.

Set 1 — Motion and units. 🎯

Set 2 — Energy and heat. 🎯

Set 3 — Waves, electricity and magnetism. 🎯

Finish strong. ✍️

1) A radio wave has a frequency of 100,000,000 Hz and travels at 300,000,000 m/s. What is its wavelength, in meters?

2) A 4 kg cart moves at 5 m/s. What is its kinetic energy, in joules?

Key Takeaways — Part 7

• Most GS physics questions test one idea: name it, then answer
• Formulas worth knowing cold: $s = \frac{d}{t}$, $F = ma$, weight $= mg$, $p = mv$, $W = Fd$, $KE = \frac{1}{2}mv^2$, $PE = mgh$, $P = \frac{W}{t}$, $v = f\lambda$, $F = \frac{9}{5}C + 32$
• Units worth knowing cold: newton, joule, watt, hertz, volt, ampere, ohm
• Lists worth knowing cold: King Henry's prefixes, the EM spectrum, ROY G BIV, iron/nickel/cobalt

Next step: Take the exit quiz. Then review the flashcards for this topic, especially the ULTRA-HIGH ones, before your next practice test.