What are speed, velocity and acceleration?
Speed tells you how fast. Velocity adds which way. Acceleration says how quickly that changes. Drive a car past distance posts with a stopwatch, race an EV against a bicycle, stop in time for a child near a school, spin a washing machine drum at 400 g, and find out why nothing catches light.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- How fast, which way, how quickly it changes. Speed is distance ÷ time, in m/s (1 m/s = 3.6 km/h). Velocity is speed with a direction. Acceleration is the change in velocity per second, in m/s². With steady acceleration, v = u + a t and s = u t + ½ a t², and the area under a velocity–time graph is the distance.
- Speeding up. 0–100 km/h is an acceleration test: about 10 s (2.8 m/s²) for a hatchback, about 3 s for a sports EV. Grip limits the start, then power does, because the push fades as P ÷ v while drag grows with v².
- Stopping and falling. Stopping distance is thinking distance (speed × reaction time) plus braking distance (v² ÷ 2a): about 18 m from 30 km/h and 36 m from 50 km/h. Dropped things gain 9.81 m/s every second, and in a lift the scales read m (g + a).
- Going round, and the needle. At steady speed in a circle the velocity still turns, so there is an acceleration v² ÷ r towards the centre: 3,700 m/s² at a fan blade tip, about 400 g in a spinning washer drum. A speedometer shows instantaneous speed; the trip computer, the average.
- Myths and limits. A lap of a 400 m track has zero displacement and zero average velocity. Heavy and light balls fall together; only air slows the feather. Velocities depend on your frame of reference, and near light speed they no longer simply add: nothing with mass reaches c.
| Term | Meaning |
|---|---|
| Speed | How far something travels per second: distance ÷ time, in metres per second. |
| Velocity | Speed in a stated direction. It changes when either the speed or the direction changes. |
| Acceleration | The rate of change of velocity: a = Δv ÷ Δt, in metres per second per second (m/s²). |
| Displacement | How far, and in which direction, you are from where you started. |
| Equations of motion | For steady acceleration: v = u + a t and s = u t + ½ a t². |
| Free fall | Motion under gravity alone. Near the Earth everything accelerates at g = 9.81 m/s². |
| Centripetal acceleration | The acceleration towards the centre needed to move in a circle: a = v² ÷ r. |
| Frame of reference | The point of view velocities are measured from, such as the platform or the moving train. |
From Aristotle's falling stones to satellites that clock your bike ride: 2,300 years of learning what 'how fast' means.
- 350 BCE · Heavier things fall faster, says Aristotle (Aristotle, Athens, Greece)
- 1335 · The Oxford Calculators measure motion (Thomas Bradwardine, William Heytesbury, Richard Swineshead, John Dumbleton, Merton College, Oxford, England)
- 1350 · Motion drawn as a graph (Nicole Oresme, Paris, France)
- 1604 · Distance grows with time squared (Galileo Galilei, Padua, Italy)
- 1638 · Uniform acceleration is defined (Galileo Galilei, Leiden (printed), from Arcetri, Italy)
- 1687 · Newton ties acceleration to force (Isaac Newton, London, England)
- 1902 · The eddy-current speedometer (Otto Schultze (after Josip Belušić's 1888 patent), Berlin, Germany)
- 1905 · Nothing catches light (Albert Einstein, Bern, Switzerland)
The full story, with 19 moments, charts, people and 28 sources: glassbox.how/e/motionclear/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 |
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|---|---|
motionclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
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| 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 |
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