What is gravity?
Two tonnes of lead pull on each other with less than the weight of a grain of sand, yet the same pull holds the Moon in orbit. Weigh the Earth with Cavendish's twisting rod, fire Newton's cannon into orbit, watch a satellite hang over India, stretch the tides, and fix GPS clocks with Einstein.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Every mass pulls every mass. F = G m₁ m₂ ÷ r². G is tiny, 6.674 × 10⁻¹¹, so two 1-tonne balls 1 m apart pull with 0.000 067 N. Double the distance and the pull drops to a quarter. Cavendish measured it with a twisting rod in 1798 and so weighed the Earth.
- Weight, falling and floating. Near a planet everything falls with the same g: 9.81 m/s² on Earth, 1.62 on the Moon. Your weight is m × g. At the ISS gravity is still 89% as strong; the crew float because they are falling round the Earth, like Newton's cannonball fired at 7.8 km/s.
- Orbits and tides. Orbital speed is √(GM ÷ r): 7.66 km/s and 92 minutes for the ISS, 12 hours for GPS, and one day at 35,786 km, where INSAT and GSAT hang still. The Moon's uneven pull stretches the oceans into two bulges; lined up with the Sun it makes spring tides.
- Gravity at home. A tank 10 m up gives about 1 bar at the tap, too little for an RO membrane. Standing, your blood pressure is 27 mmHg lower at your head than your heart. A bike freewheels to about 40 km/h on a 5% slope, and a 99.4 cm pendulum ticks once a second.
- Beyond Newton, and two myths. Einstein explained gravity as curved spacetime. GPS clocks gain 38 µs a day and must be corrected. Black holes merging shook LIGO's arms by a thousandth of a proton in 2015. There is gravity in space, and heavy things don't fall faster: a hammer and a feather land together on the Moon.
| Term | Meaning |
|---|---|
| Gravity | The attraction between any two masses. It always pulls and never cancels out. |
| Gravitational constant (G) | 6.674 × 10⁻¹¹ N m²/kg²: the pull between two 1 kg masses 1 m apart. |
| Inverse-square law | A pull that falls with the square of distance: twice as far, a quarter as strong. |
| Weight | The pull of gravity on a mass, W = m × g, in newtons. Mass stays the same everywhere. |
| Free fall | Moving under gravity alone. Everything falls together, so you feel weightless. |
| Orbit | A path where an object keeps falling but moves sideways fast enough to keep missing the ground. |
| Tides | Two ocean bulges raised by the difference in the Moon's (and Sun's) pull across the Earth. |
| Curved spacetime | Einstein's picture of gravity: mass bends space and time, and things follow the curves. |
From falling stones and a famous apple to weighing the Earth, curved spacetime and a lander on the Moon's south pole.
- 628 · The Earth attracts (Aryabhata and Brahmagupta, Kusumapura and Bhillamala, India)
- 1609 · Planets move on ellipses (Johannes Kepler, Prague and Linz)
- 1666 · The apple and the Moon (Isaac Newton, Woolsthorpe Manor, Lincolnshire, England)
- 1687 · Principia: the law of universal gravitation (Isaac Newton, London, England)
- 1798 · Weighing the Earth with a twisting rod (Henry Cavendish, Clapham, London, England)
- 1846 · A planet found with a pencil (Urbain Le Verrier and Johann Galle, Paris, France, and Berlin, Prussia)
- 1915 · Gravity is curved spacetime (Albert Einstein, Berlin, Germany)
- 1919 · Starlight bends round the Sun (Arthur Eddington, Frank Dyson and team, Príncipe and Sobral, Brazil)
The full story, with 20 moments, charts, people and 28 sources: glassbox.how/e/gravityclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
gravityclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/gravityclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
- The Glassbox name and logo aren't covered by either licence. See the terms.
Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/gravity.js |
Shared parts and physics: G and Earth constants, g on four worlds, orbit speed and period, number formatting, chart boards, arrows that point anywhere, painted planets, a satellite, people, bead trails, stars |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |