Force

What is a force?
A newton is about the weight of an apple. Drag force arrows across a table and watch them add up, weigh rice on Jupiter, ride a lift standing on a scale, feel the 7,500 N a seat belt gives you in a crash, and meet the four forces behind every push in the universe.

▶ Play with it  ·  Read the 60-second explainer  ·  Watch the 40-second video

In 60 seconds

1. A push or a pull, with a direction. A force has a size and a direction, so we draw it as an arrow. It is measured in newtons: 1 N speeds 1 kg up by 1 m/s every second, and is about the weight of a 100 g apple. A spring scale measures it.
2. Forces add like arrows. Put the arrows tip to tail: 30 N and 40 N at right angles make 50 N, not 70. Equal and opposite forces cancel. Only the net force that is left changes motion: a = F ÷ m. A free-body diagram shows every force on one object.
3. Weight, lifts and springs. Weight is a force, W = m × g: 5 kg of rice weighs 49 N on Earth and 124 N on Jupiter. A bathroom scale reads the floor's push, so it reads more in a lift speeding up. Ropes carry tension; springs push back with k × x.
4. Forces in machines. A bike chain pulls with about twice your foot's push. A braking car relies on the road's grip, about μ × weight. A seat belt gives about 7,500 N in a 50 km/h crash, and a spinning drum pushes each sock inwards with hundreds of newtons.
5. Pairs, balance and myths. Forces come in pairs acting on different things: rocket and gas, foot and ground, fan and air. Balanced forces hold a bridge up. Every push is one of four fundamental forces, and gravity is by far the weakest. You don't need a force to keep moving, centrifugal force isn't a push, and heavy things don't fall faster.

Words worth knowing

Term	Meaning
Force	A push or a pull on an object, with a size and a direction.
Newton (N)	The SI unit of force: it gives 1 kg an acceleration of 1 m/s². About the weight of a 100 g apple.
Net force	All the forces on an object added together as arrows. Only the net force changes motion.
Weight	The pull of gravity on a mass: W = m × g, about 9.81 N for each kilogram on Earth.
Normal force	The push of a surface on something resting on it, at right angles to the surface.
Tension	The pull carried along a rope, cable or chain.
Friction	The force between surfaces that resists sliding, up to about μ times the force pressing them together.
Fundamental forces	Strong, electromagnetic, weak and gravity: the four forces behind every push and pull.

A short history

From Aristotle's pushers to springs, twisted threads and quartz crystals: how people learned to see, weigh and name the pushes and pulls that move the world.

• 350 BCE · Every motion needs a mover (Aristotle, Athens, Greece)
• 1586 · The wreath of spheres (Simon Stevin, Leiden, Dutch Republic)
• 1678 · As the stretch, so the force (Robert Hooke, London, England)
• 1687 · Force gets a definition (Isaac Newton, London, England)
• 1798 · Weighing the Earth (Henry Cavendish, using John Michell's design, Clapham, London, England)
• 1938 · The strain gauge (Edward Simmons and Arthur Ruge, Caltech and MIT)
• 1948 · A unit called the newton (9th General Conference on Weights and Measures, Paris, France)
• 1983 · Catching the carriers of the weak force (UA1 and UA2 teams, with Carlo Rubbia and Simon van der Meer, CERN, near Geneva)

The full story, with 20 moments, charts, people and 40 sources: glassbox.how/e/forceclear/history. The data lives in history.json.

Video and slides

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

Privacy

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
forceclear.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.

Licences

• 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/forceclear”.
• 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.

Run it

It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.

python3 -m http.server 8000

Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.

Then open http://localhost:8000.

How it's built

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/force.js	Shared parts and physics: force arrows that point anywhere, dragging arrow tips, a spring scale, chart boards, people, a car, a crash dummy, a desk fan, g on four worlds
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)