Physical science: energy, forces and motion

Why this matters on the test

About 40 percent of the GED® science test is physical science. Many questions give you a formula and a few numbers. You need to know what each letter means, put the numbers in, and check that the units make sense.

Each formula below has one worked example. Write the formula first, then put in the numbers. That one habit prevents most mistakes.

Speed, velocity and acceleration

Speed = distance ÷ time. A bus goes 150 miles in 3 hours: 150 ÷ 3 = 50 miles per hour.

Velocity is speed with a direction, such as 50 miles per hour north.

Acceleration is how fast velocity changes: change in velocity ÷ time. A car goes from 0 to 20 meters per second in 5 seconds: 20 ÷ 5 = 4 meters per second each second (4 m/s²). Slowing down is also acceleration, in the opposite direction.

Newton's three laws

First law (inertia): an object keeps doing what it is doing, staying still or moving in a straight line at the same speed, unless a force acts on it. That is why a seat belt matters. When a car stops suddenly, your body keeps moving forward.

Second law: force = mass × acceleration (F = m × a). Force is measured in newtons (N). Worked: a 1,000 kg car speeding up at 2 m/s² needs 1,000 × 2 = 2,000 N. The same push on a heavier object gives less acceleration.

Third law: for every action force there is an equal and opposite reaction force. A swimmer pushes water backward, and the water pushes the swimmer forward.

Mass is how much matter an object has, in kilograms. Weight is the pull of gravity on that mass: weight = mass × g. On Earth g is about 9.8 m/s², so a 60 kg person weighs about 60 × 9.8 = 588 N. On the Moon the same person has the same mass but weighs about one sixth as much.

Momentum and collisions

Momentum = mass × velocity (p = m × v). A 2 kg cart rolling at 3 m/s has 2 × 3 = 6 kg·m/s of momentum.

In a collision, the total momentum before equals the total momentum after, as long as no outside force acts. A heavy truck and a small car at the same speed do not have the same momentum. The truck has much more, so it is much harder to stop.

Work, power and simple machines

Work = force × distance (W = F × d), measured in joules (J). Work only counts when the object moves in the direction of the force. Worked: pushing a box with 200 N for 3 m is 200 × 3 = 600 J. Pushing on a wall that does not move is zero work.

Power = work ÷ time, measured in watts (W). If that 600 J of work takes 4 seconds, the power is 600 ÷ 4 = 150 W.

Simple machines are the lever, pulley, inclined plane (ramp), wedge, screw, and wheel and axle. They let you use less force by moving a longer distance. A ramp lets you push a heavy cart with less force, but you push it farther. The work is never less than without the machine.

Kinds of energy, and energy changing form

Kinetic energy is the energy of motion: KE = ½ × m × v². A 2 kg ball moving at 3 m/s has ½ × 2 × 3² = ½ × 2 × 9 = 9 J. Speed is squared, so doubling the speed makes the kinetic energy 4 times as large.

Potential energy is stored energy. Gravitational potential energy is PE = m × g × h. A 5 kg box on a shelf 2 m high has 5 × 9.8 × 2 = 98 J. Chemical energy is stored in food, fuel and batteries.

Energy is never created or destroyed. It only changes form. A flashlight turns chemical energy in the battery into electrical energy, then into light, and some becomes heat. On the test, some energy "lost" usually means it became heat.

Heat, reactions and waves

Heat always flows from a warmer object to a cooler one. Conduction moves heat by direct touch, like a hot pan handle. Convection moves heat as warm liquid or gas rises and cool sinks, like warm air rising from a heater. Radiation moves heat as waves through space, like sunlight warming your face.

An exothermic reaction gives off heat, like burning wood. An endothermic reaction takes in heat, like an instant cold pack.

Waves carry energy. Wavelength is the distance from one crest to the next. Frequency is how many waves pass each second, in hertz (Hz). Amplitude is the wave's height. Wave speed = frequency × wavelength. Worked: 5 Hz × 2 m = 10 m/s.

Light is part of the electromagnetic spectrum. From longest to shortest wavelength: radio, microwaves, infrared, visible light, ultraviolet (UV), X-rays, gamma rays. Shorter wavelengths carry more energy. That is why UV light can burn skin and X-rays need shielding.

Energy sources

Renewable sources are replaced by nature in a human lifetime: sunlight (solar), wind, moving water (hydroelectric) and heat from inside Earth (geothermal).

Nonrenewable sources run out: coal, oil and natural gas (fossil fuels), and the uranium used in nuclear power plants. Burning fossil fuels releases carbon dioxide and other pollution. Questions may ask you to compare sources by cost, pollution or how steady the supply is. For example, solar power only works while the sun shines unless the energy is stored.

Where this comes from

GED Testing Service's Assessment Guide for Educators: Science lists physical-science topics P.a (energy: heat, kinds of energy, sources and waves) and P.b (work, motion and forces: speed, acceleration, momentum, Newton's laws, gravity, mass and weight, work, simple machines and power). It says about 40 percent of the test is physical science. We checked the guide, NASA's page on Newton's laws and OpenStax Physics on October 28, 2026.

The explanations, worked numbers and questions are original FinishHigh practice, not GED® test questions. GED® is a registered trademark of the American Council on Education. FinishHigh is independent and not affiliated with or endorsed by it. This lesson teaches a few skills. It is not a full course or an official readiness result.

Next, try the questions below. For balancing equations and solutions, open the lesson "Understand matter, chemical reactions and solutions."

Trusted sources and further study

Sources guide the topics and factual information. The lesson wording, examples, and questions are original FinishHigh material.

• Science: GED Testing Service assessment targets for educators ↗GED Testing Service · checked 2026-10-20
• NASA: Newton's laws of motion ↗NASA Glenn Research Center · checked 2026-10-28
• OpenStax Physics: Newton's second law ↗OpenStax, Rice University · checked 2026-10-28

Another small step

Compare a science experiment fairly ↗

Read graph axes and units before the numbers ↗