[1] What Is Gravity?
Type: Space agency, written for children
Written by NASA for young readers. Simplified on purpose, but accurate.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 6–7 (quiz); Ages 6–7 (text)
Sources for the gravity lessons
Every fact in the three gravity lessons (text, experiments, quizzes and worksheets) is listed here with where it comes from, so you can judge for yourself how much to trust it. 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 history is less certain: the Delft tower drop (1586) and the doubts about Galileo's Pisa drop rest on the two medium-rated sources [10] and [11]. They agree with standard histories of science, but if you teach this part in depth, check a library history-of-science book. The apple story is known from William Stukeley's written account [9]; it is Newton's own story, told about 60 years later.
All sources
[2] Planetary Fact Sheet (Metric)
Type: Space agency data table
NASA's reference table of planet data. The surface gravity values (m/s²) used here are taken from it, rounded.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text); Ages 6–7 (quiz); Ages 6–7 (text)
[3] The Apollo 15 Hammer-Feather Drop
Type: Space agency, quoting the mission's science report
Describes Commander David Scott's demonstration on the Moon (Apollo 15, 1971) and quotes the Apollo 15 Preliminary Science Report. The page links NASA's original video.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 6–7 (experiment); Ages 6–7 (quiz); Ages 6–7 (text)
https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html
[4] What Is Microgravity?
Type: Space agency explainer
Explains that gravity at about 250 miles (400 km) up is 88.8% of its surface strength, and that astronauts float because they are in free fall.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text)
[5] College Physics 2e, 6.5 Newton's Universal Law of Gravitation
Type: Peer-reviewed open textbook
Free university-level textbook. Gives F = GmM/r², G = 6.674×10⁻¹¹ N·m²/kg², why mass cancels out in free fall, and the Moon's role in tides. It calls the apple story 'part of worldwide folklore', which may be based in fact.
Used in: Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (experiment); Ages 15–16 (quiz); Ages 15–16 (text)
https://openstax.org/books/college-physics-2e/pages/6-5-newtons-universal-law-of-gravitation
[6] University Physics Volume 1, 3.5 Free Fall
Type: Peer-reviewed open textbook
Gives the free-fall equation y = y₀ + v₀t − ½gt² and states that without air resistance objects fall with the same acceleration, independent of mass.
Used in: Ages 10–11 (experiment); Ages 10–11 (quiz); Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text); Ages 6–7 (experiment); Ages 6–7 (quiz)
https://openstax.org/books/university-physics-volume-1/pages/3-5-free-fall
[7] University Physics Volume 1, 15.4 Pendulums
Type: Peer-reviewed open textbook
Gives T = 2π√(L/g) for a simple pendulum and the small-angle condition (below about 15°).
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz)
https://openstax.org/books/university-physics-volume-1/pages/15-4-pendulums
[8] June 1798: Cavendish weighs the world
Type: Professional physics society, history column
Describes Cavendish's torsion balance with large lead spheres and his result: Earth's density about 5.48 times that of water.
Used in: Ages 15–16 (text)
[9] Story of Newton's encounter with apple goes online
Type: Science news site reporting on a primary document
Reports the Royal Society putting William Stukeley's manuscript online. In it Stukeley records Newton telling the apple story over tea in 1726, about an event in the mid-1660s. The manuscript itself is the primary source; this article is a reliable report of it.
Used in: Ages 15–16 (text)
[10] Galileo Galilei's and Simon Stevin's experiments
Type: Essay by a computer scientist and historian of science
States that Stevin published the Delft drop in the appendix to De Beghinselen der Weeghconst (1586), and that Galileo's Pisa drop is only documented by Viviani's 1654 biography. A single-author essay, not peer reviewed. The claims agree with standard histories of science, but a teacher may want to check a library history-of-science book as well.
Used in: Ages 10–11 (text); Ages 15–16 (quiz); Ages 15–16 (text)
https://johnplaice.substack.com/p/galileo-galileis-and-simon-stevins
[11] The Delft Tower Experiment
Type: Popular history website
Gives the setting of Stevin and Jan Cornets de Groot's 1586 drop from the Nieuwe Kerk tower in Delft, with two lead balls of different weight. Popular, not scholarly. Used only together with the source above.
Used in: Ages 10–11 (text); Ages 15–16 (text)
https://www.amusingplanet.com/2024/10/the-delft-tower-experiment.html