What is inertia?
Why do you lurch when the bus brakes, and how does a coin drop neatly into a glass when you flick the card away? Push carts, yank a tablecloth, spin a skater, hang a bicycle wheel from a string, weigh an astronaut without weight, and test the most precise coincidence in physics.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Hard to budge. Inertia is how hard it is to change something's motion, and mass, in kilograms, measures it: the same force gives half the acceleration to twice the mass, a = F ÷ m. Mass is not weight: weight is m g, in newtons, and changes on the Moon. Flick a card or yank a tablecloth fast and inertia keeps the coin and plates where they were.
- Riding. When a bus brakes, you keep going until something pushes you back with F = m a: about 180 N for a 60 kg passenger at 3 m/s². Unaided, you tip beyond about 1.3 m/s². Seat belts, a rider's arms and lorry straps all supply that force; friction alone can hold a load only up to μ g.
- Spinning. For turning, inertia is the moment of inertia I = Σ m r². A skater pulling her arms in cuts I nearly four times, so she spins nearly four times faster, keeping L = I ω. A spinning wheel hung from a string precesses instead of falling, and a flywheel smooths an engine's power strokes.
- Measuring mass. A spring scale measures weight and reads a sixth on the Moon; a balance compares masses and reads true, but fails in orbit. Skylab's crews wobbled on a spring chair (T = 2π √(m ÷ k)); the ISS's SLAMMD pulls with a known force and computes m = F ÷ a.
- Myths and limits. Things don't naturally slow down: friction stops them, and on ice a heavy sledge slides as far as a light one. Inertial and gravitational mass agree to 1 part in 10¹⁵ (MICROSCOPE, 2022), the equivalence principle behind general relativity. 'Relativistic mass' is an old idea; mass today means rest mass.
| Term | Meaning |
|---|---|
| Inertia | The tendency of an object to keep still, or keep moving in a straight line at steady speed, until a force acts. |
| Mass | The amount of inertia an object has, measured in kilograms: m = F ÷ a. |
| Weight | The pull of gravity on a mass, W = m g, in newtons. It changes from place to place; mass does not. |
| Kilogram | The SI unit of mass, defined since 2019 by fixing the Planck constant at 6.626 070 15 × 10⁻³⁴ J·s. |
| Moment of inertia | Resistance to changes in spin, I = Σ m r², in kg·m². Mass far from the axis counts most. |
| Angular momentum | L = I ω. It stays constant unless a torque acts, which is why skaters spin faster with arms in. |
| Inertial balance | A mass on springs whose wobble period, T = 2π √(m ÷ k), measures mass without gravity. |
| Equivalence principle | Inertial and gravitational mass are the same, so everything falls alike. Tested to about 1 part in 10¹⁵. |
From Aristotle's pushing air to a kilogram set by the Planck constant: 2,400 years of learning why things keep moving.
- 350 BCE · Motion needs a mover, says Aristotle (Aristotle, Athens, Greece)
- 1350 · Buridan's impetus (Jean Buridan, Paris, France)
- 1604 · Galileo's rolling balls (Galileo Galilei, Padua, Italy)
- 1618 · Kepler names 'inertia' (Johannes Kepler, Linz, Austria)
- 1638 · Two New Sciences: level motion lasts (Galileo Galilei, Leiden, Netherlands)
- 1687 · Newton's first law, and mass (Isaac Newton, London, England)
- 1889 · The kilogram gets a body (1st General Conference on Weights and Measures, Sèvres, near Paris, France)
- 1889 · Eötvös weighs inertia against gravity (Loránd Eötvös, with Dezső Pekár and Jenő Fekete, Budapest, Hungary)
The full story, with 20 moments, charts, people and 24 sources: glassbox.how/e/inertiaclear/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 |
|---|---|
inertiaclear.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/inertiaclear”. - 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/inertia.js |
Shared parts for this box: chart boards, arrows, carts, bus, lorry, motorcycle, skater, bicycle wheel, glass and people |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
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) |