Every fact in the three energy lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the lessons simplify: the youngest lessons use the U.S. Energy Information Administration's two groups, 'stored' (potential) and 'working' (kinetic) energy, and its list of forms; physicists group energy in several slightly different ways, so other books may sort sound, light or electricity differently. The efficiency values (OpenStax) are described by the source as representative values; real machines vary. The figures for light bulbs (U.S. Department of Energy and OpenStax) are for typical bulbs and differ between models. In the bouncing-ball experiment we say the 'lost' energy becomes mainly heat and sound; measuring exactly where it goes is beyond a classroom experiment.
All sources
[1] What is energy?
U.S. Energy Information Administration (EIA), Energy Explained · Reliability: High
Type: Government statistics agency, education page
Scientists define energy as the ability to do work; energy can be converted from one form to another; forms include heat, light, motion, electrical, chemical and gravitational; energy is grouped into potential (stored) and kinetic (working) energy; the food you eat contains chemical energy; stored chemical energy in coal or natural gas can be converted into electrical energy, which can be converted into light and heat; we use energy for walking, cycling, moving cars and cooking.
U.S. Energy Information Administration (EIA), Energy Explained · Reliability: High
Type: Government statistics agency, education page
Energy is either potential or kinetic; potential energy is stored energy and the energy of position. Chemical energy is stored in the bonds of atoms and molecules (batteries, biomass, petroleum, natural gas, coal); stretched rubber bands and compressed springs store mechanical energy; gravitational energy is stored in an object's height: the higher and heavier the object, the more is stored; a cyclist going downhill converts gravitational energy into motion energy; in hydropower, gravity forces water through a turbine to produce electricity. Radiant energy includes visible light; sunshine provides the fuel and warmth that make life on Earth possible; thermal energy (heat) comes from moving atoms and molecules; the faster an object moves, the more motion energy it has, and wind is an example; sound is produced when a force makes an object vibrate; lightning is an example of electrical energy; burning wood converts chemical energy into thermal energy.
[3] Physics, 9.2 Mechanical Energy and Conservation of Energy
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Mechanical energy can be potential or kinetic; kinetic energy KE = ½mv²; gravitational potential energy PE = mgh; work and energy are measured in joules; in a closed system total energy is conserved; energy–work transformations are never perfect because of friction, which causes the loss of some useful energy (heat). Worked example: a 10 kg rock falling from a 20 m cliff has, after falling 10 m, lost 980 J of potential energy and gained 980 J of kinetic energy.
[4] College Physics 2e, 7.6 Conservation of Energy
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Total energy is constant in any process: it may change form or be transferred, but the total stays the same; all forms of energy can be converted into one another; the chemical energy in food is converted into thermal energy through metabolism; batteries turn chemical energy into electrical energy; solar cells turn sunlight into electrical energy; efficiency = useful energy or work output ÷ total energy input. Representative efficiencies: electric motor 98%, residential gas heater 90%, coal power plant 42%, diesel engine 35%, gasoline engine 30%, compact fluorescent light 20%, human body cycling 20%, steam engine 17%, solar cell 10%.
Power is the rate at which work is done, P = W/t; 1 watt = 1 joule per second; electricity bills give energy in kilowatt-hours (power in kW × time in hours); a 60 W incandescent bulb converts only 5 W into light, with 55 W becoming thermal energy; a 0.200 kW computer used 6.00 h a day for 30.0 days uses 36.0 kWh.
U.S. Department of Energy, Energy Saver · Reliability: High
Type: Government energy agency, consumer guidance
Residential LEDs, especially ENERGY STAR rated ones, use at least 75% less energy and last up to 25 times longer than incandescent lighting; incandescent bulbs release 90% of their energy as heat.