What is friction?
Friction is the force that holds before it slips. Pull a block with a spring scale and watch the grip peak then drop, stop a car on dry roads and ice, float a crankshaft on oil, hold a boat with one hand and three turns of rope, and bust the myth that pressure makes ice slippery.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Grip, then slip. Friction resists sliding. Static friction matches your pull up to a limit, μs × N; past it the surface lets go and sliding friction, μk × N, is usually smaller. μ has no unit. Tilt a ramp until a block slides and tan θ = μs. Surfaces really touch only at the tips of tiny bumps.
- Grip on the road. Tyres drive, steer and stop through static friction: μ about 0.8 dry, 0.5 wet, 0.1 on ice. Braking distance is v² ÷ 2μg, so ice means eight times as far. ABS keeps wheels just short of locking. Worn tread can't clear water and the tyre aquaplanes.
- Friction we fight. Rolling needs about 1% of the weight for a car, a hundredth of dragging it. Ball bearings run at μ ≈ 0.0015. An oil film floats a crankshaft off the metal, as the Stribeck curve shows. Whatever friction remains becomes heat, enough to make brake discs glow.
- Friction we need. Each step pushes the ground back so the ground pushes you forward, needing μ ≈ 0.18. A pen's ball rolls only because paper grips it. Rope round a post grips exponentially, T₀ e^(μθ). Rosin makes a violin bow stick and slip hundreds of times a second.
- Myths and limits. Friction doesn't depend on contact area. Ultra-smooth clean metals stick and can even cold-weld. Pressure lowers ice's melting point by only about 1.5 °C under a skate: ice is slippery because of its wet surface layer and frictional heating. Twisted graphite is superlubric, and air drag grows with speed squared.
| Term | Meaning |
|---|---|
| Friction | The force between touching surfaces that resists them sliding over each other. |
| Normal force | How hard two surfaces are pressed together, at right angles to them. On a flat table it equals the weight. |
| Coefficient of friction (μ) | Friction divided by normal force. It has no unit and depends on the two surfaces. |
| Static and kinetic friction | Static friction stops sliding from starting, up to μs × N; kinetic friction acts once sliding, about μk × N. |
| Real contact area | The tiny fraction of a surface that actually touches, at the tips of its bumps. It grows with the load. |
| Rolling resistance | The small drag on a rolling wheel, Crr × weight: about 1% for a car tyre. |
| Lubrication | A film of oil or grease that keeps surfaces apart and cuts friction. |
| Stick–slip | Jerky motion where surfaces stick, build up force, slip and stick again: violins, squeaks and earthquakes. |
From Egyptian workers pouring water under a 58-tonne statue to graphite flakes that slide with almost no friction at all: how people learned to measure, tame and explain the force that grips.
- 1900 BCE · Water under the sledge (Workers of the nomarch Djehutihotep, Deir el-Bersha, Egypt)
- 1493 · Leonardo's sliding blocks (Leonardo da Vinci, Milan, Italy)
- 1699 · Amontons's laws (Guillaume Amontons, Paris, France)
- 1785 · Coulomb tests everything (Charles-Augustin de Coulomb, Rochefort and Paris, France)
- 1850 · A wet skin on ice (Michael Faraday, Royal Institution, London, England)
- 1950 · The real area of contact (Frank Philip Bowden and David Tabor, Cambridge, England)
- 1978 · Anti-lock brakes in showrooms (Bosch and Mercedes-Benz, Stuttgart, Germany)
- 2004 · Superlubricity seen (Martin Dienwiebel, Joost Frenken and colleagues, Leiden, Netherlands)
The full story, with 20 moments, charts, people and 24 sources: glassbox.how/e/frictionclear/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 |
|---|---|
frictionclear.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/frictionclear”. - 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/friction.js |
Shared parts and physics: friction coefficients, braking distance, chart boards, force arrows, a spring scale, rough surfaces for the close-ups, people, a car and a road |
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) |