11.3 Physical Science Forces, Energy, and Motion
Key Takeaways
- Physical Science is roughly 40% of the GED Science test, and prompts usually supply any needed formula, so the skill tested is choosing the right relationship and the right units.
- Speed is how fast distance changes with time, velocity adds direction, and acceleration is a change in velocity over time.
- Newton's laws connect force, mass, and acceleration (F = m x a) and explain collisions, seat belts, falling objects, and pushes or pulls.
- Work, power, and simple machines involve a tradeoff: a machine can reduce force by increasing distance, but it never creates energy.
- Energy transformations and wave data are tested through diagrams, tables, or practical scenarios rather than abstract memorization.
Reading Motion and Force Like a GED Scientist
Physical science often looks mathematical, but the GED is not asking you to memorize a formula sheet. Prompts usually provide the formula or enough information to reason from the relationship. Your job is to decide what the quantities mean, use consistent units, and check whether the result makes sense. With about 40 questions in 90 minutes, you have just over two minutes each, so do not over-calculate.
Motion From Tables and Graphs
Speed is distance divided by time. Velocity is speed with a direction. Acceleration is a change in velocity over time. On a distance-time graph, a steeper slope means a faster object and a flat line means the distance from the start is not changing.
| Time (s) | Cart A Distance (m) | Cart B Distance (m) |
|---|---|---|
| 0 | 0 | 0 |
| 2 | 4 | 3 |
| 4 | 8 | 6 |
| 6 | 12 | 9 |
Cart A travels 12 m in 6 s, an average speed of 2 m/s. Cart B travels 9 m in 6 s, an average speed of 1.5 m/s. Because each cart's distance increases by the same amount every 2 seconds, both move at a constant speed (zero acceleration) over this interval.
Force, Mass, and Acceleration
Newton's three laws explain common GED scenarios:
- First law (inertia): an object at rest stays at rest, and an object in motion keeps moving, unless a net force acts. This is why seat belts matter in a sudden stop.
- Second law: F = m x a. A larger net force gives more acceleration; a larger mass resists acceleration.
- Third law: forces occur in equal and opposite pairs; when one object pushes another, the second pushes back equally.
Use this experimental data for carts pushed with different forces.
| Cart Mass (kg) | Net Force (N) | Acceleration (m/s^2) |
|---|---|---|
| 2 | 4 | 2 |
| 2 | 8 | 4 |
| 4 | 8 | 2 |
Hold mass at 2 kg and double the force from 4 N to 8 N: acceleration doubles. Hold force at 8 N and double the mass from 2 kg to 4 kg: acceleration is cut in half. That is F = m x a in action.
Work, Power, and Simple Machines
Work is done when a force moves an object through a distance (work = force x distance). Power is how quickly work is done (work divided by time). Simple machines, ramps, pulleys, levers, and wheels, can make a job feel easier by reducing the force needed, but the tradeoff is moving the object over a longer distance. A ramp does not remove the energy requirement; it spreads the effort out. On the GED, if a ramp lets a worker use less force to lift a box, expect the distance along the ramp to be longer than the straight vertical lift.
Energy and Waves
Energy changes form: chemical energy in food becomes motion and heat; electrical energy in a lamp becomes light and heat; gravitational potential energy in a raised object becomes kinetic energy as it falls. By the law of conservation of energy, total energy in a closed system stays constant, though useful energy spreads out as heat.
Waves transfer energy. GED items may use wavelength (distance between peaks), frequency (waves per second), or amplitude (height, linked to loudness or brightness). For electromagnetic radiation, higher frequency means higher energy: ultraviolet light carries more energy than visible light and can damage skin, while low-frequency radio waves carry little energy and are used for communication. ### The Electromagnetic Spectrum
GED items sometimes order types of light by energy. From lowest to highest frequency and energy:
| Wave Type | Relative Energy | Everyday Use or Effect |
|---|---|---|
| Radio | Lowest | Broadcasting, communication |
| Microwave | Low | Heating food, radar |
| Infrared | Moderate | Heat lamps, remote controls |
| Visible light | Moderate-high | What the eye detects |
| Ultraviolet | High | Sunburn, sterilization |
| X-ray / Gamma | Highest | Medical imaging, very damaging |
If a question asks which wave carries the most energy, choose the higher-frequency, shorter-wavelength option. This is why sunscreen blocks ultraviolet rays but no one worries about radio waves from a phone.
Worked Example: Reading an Acceleration Result
A 10 kg cart is pushed with a net force of 30 N. Using F = m x a, solve for acceleration: a = F / m = 30 N / 10 kg = 3 m/s squared. Now suppose friction adds an opposing 10 N force. The net force is only 30 - 10 = 20 N, so a = 20 / 10 = 2 m/s squared. The lesson: always use the net force (the sum of all forces), not just the applied push.
Common GED Physical-Science Traps
- Forgetting units, an answer of "3" is incomplete; it must be 3 m/s or 3 m/s squared.
- Using total force instead of net force when friction or an opposing force is present.
- Believing a simple machine multiplies energy rather than trading force for distance.
- Confusing speed (no direction) with velocity (with direction) on a graph question.
For any physical-science data set, read the headings and units first, compare one variable at a time, and choose the conclusion that matches the measured pattern.
A toy car travels 18 meters in 6 seconds. Using speed = distance / time, what is the car's average speed?
A worker uses a 4-meter ramp instead of lifting a crate straight up 1 meter onto a truck bed. Compared with the straight lift, the ramp lets the worker apply less force. What is the tradeoff?