Pressure

What is pressure?
Why does a stiletto dent a floor an elephant wouldn’t, why is a dam thickest at the bottom, and why does dal cook faster under a whistle? Squash sand, cut a tomato, lie on nails, fill a dam, pump out Magdeburg’s hemispheres and dive the Mariana Trench.

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

In 60 seconds

1. Force on an area. Pressure is force divided by area, P = F ÷ A, measured in pascals: one newton on one square metre. Press a plate into sand and halve its area: the pressure doubles and it sinks deeper. A brick on its end presses three times harder than lying flat, and a stiletto heel beats an elephant by about 75 times. The units board shows every pressure in Pa, kPa, bar, atm, psi and mmHg.
2. Sharp and wide. A sharp knife’s edge turns 10 N into megapascals and splits a tomato; a blunt one squashes it. Snowshoes, a bed of a thousand nails and fat low-pressure tractor tyres spread weight to keep pressure low. A tyre’s contact patch is about load ÷ pressure: palm-sized for a car at 2.2 bar, stamp-sized for a bike at 6 bar.
3. Deeper means harder. Under a liquid, P = ρ g h: every 10 m of water adds about 1 bar. A rooftop tank gives the ground floor about 1 bar, far short of an RO pump’s 5. Bhakra Dam’s 226 m of water press at 2.2 MPa at the base. Unequalised ears hurt a metre or two down, and blood pressure is highest at your ankles.
4. An ocean of air. Air presses at about 101 kPa at sea level. Pump air out of Magdeburg hemispheres and 20 kN clamps them. A straw lifts juice by lowering the pressure in your mouth. Pressure falls with height, to 65 kPa in Leh and a third on Everest, so water boils cooler; a pressure cooker’s 82 g whistle holds 1 bar extra and boils at 120 °C. A mercury barometer stands 760 mm tall.
5. Myths and extremes. Gauges show pressure above the air: a “flat” tyre still holds 1 atm. Suction never pulls, the outside air pushes, so a suction cup fails on Mars. The Mariana Trench presses at about 1,086 bar. And vessels of any shape filled to the same depth have the same pressure at the bottom: Stevin’s hydrostatic paradox.

Words worth knowing

Term	Meaning
Pressure	Force divided by the area it acts on, P = F ÷ A.
Pascal (Pa)	The SI unit of pressure: one newton per square metre. 1 bar = 100,000 Pa; 1 atm = 101,325 Pa.
Hydrostatic pressure	Pressure in a still liquid from the weight above: P = ρ g h. About 1 bar per 10 m of water.
Atmospheric pressure	The weight of the air above pressing on everything, about 101 kPa at sea level and less as you climb.
Gauge and absolute pressure	Gauges read the pressure above the surrounding air; absolute pressure adds the air’s 1 atm back.
Barometer	An instrument that measures air pressure, first a column of mercury about 760 mm tall (Torricelli, 1643).
Boiling point	The temperature where water’s vapour pressure matches the pressure around it: lower up mountains, higher in a pressure cooker.
Hydrostatic paradox	Pressure at the bottom of a liquid depends only on depth, not on the shape or size of the container.

A short history

From a pump that would not lift water past ten metres to a steel ball on the floor of the deepest sea: 400 years of learning that air and water push.

• 1586 · Stevin's hydrostatic paradox (Simon Stevin, Leiden, Netherlands)
• 1643 · Torricelli's mercury barometer (Evangelista Torricelli and Vincenzo Viviani, Florence, Italy)
• 1648 · A barometer climbs the Puy de Dôme (Florin Périer, for Blaise Pascal, Clermont-Ferrand, France)
• 1654 · Horses against the Magdeburg hemispheres (Otto von Guericke, Regensburg and Magdeburg, Germany)
• 1662 · Boyle's law: squeeze a gas, its pressure rises (Robert Boyle, Oxford, England)
• 1896 · Riva-Rocci's blood pressure cuff (Scipione Riva-Rocci, Turin, Italy)
• 1959 · Hawkins pressure cookers made in India (H. D. Vasudeva, Pressure Cookers and Appliances Ltd, Mumbai, India)
• 1960 · Trieste reaches the Challenger Deep (Jacques Piccard and Don Walsh, Mariana Trench, Pacific Ocean)

The full story, with 20 moments, charts, people and 36 sources: glassbox.how/e/pressureclear/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
pressureclear.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/pressureclear”.
• 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/pressure.js	Shared physics and parts: unit conversions, ρgh, the standard atmosphere, water's boiling point, chart boards, soft sand/snow beds, gas molecules, a person
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)