We Live at the Bottom of an Ocean of Air! A Plastic Bottle Experiment Reveals the Truth About Air Pressure (A High School Physics Refresher for Adults)
Hi, I’m Ken Kuwako, your friendly Science Trainer. Every day is an experiment!
【This article is also available as a radio episode!】
Quick question: do you have a pencil nearby? If so, let’s try a little experiment. Pinch the pencil between your two index fingers, one on each end, and press them together. How does it feel? Which finger hurts more, the right or the left?
If you think about balanced forces and Newton’s third law, both fingers should be feeling exactly the same amount of force. And yet the finger on the sharp tip hurts way more. Same force, so why does it feel so different? The answer lies in a wonderfully magical idea that governs everything from our daily lives to the workings of the universe: pressure.
The Pressure Formula: How Force and Area Work Together
The answer is simple: the force is concentrated on a tiny area. To compare the effect of forces fairly, we need to look at them over the same amount of area, and that is exactly why the concept of pressure was born. Pressure is the amount of force acting on every 1m^2.
P = F/S
(Pressure = Force ÷ Area over which the force acts)
Pressure is measured in Pa (pascals). Let’s try it with our pencil. Say you’re pressing with a force of 0.1N (about the weight of a 10g object) and compare the areas.
Left finger (flat end: 0.5cm^2):
P_left = 20,000 Pa
Right finger (sharp tip: 0.01cm2):
P_right = 100,000 Pa
That means your right finger is under 50 times more pressure than your left! No wonder it hurts!
We’re Deep-Sea Fish at the Bottom of an Ocean of Air!?
Now let’s talk about pressure on a much grander scale. Right now, we’re all living at the bottom of an atmosphere that wraps around the Earth and stretches over 150km high. Just like fish living at the bottom of the sea, we’re basically deep-atmosphere creatures. Above our heads, 150km worth of nitrogen, oxygen and other air molecules are piled up. Each one is light, but when you stack up that many, the total weight is staggering. The pressure created by this weight is atmospheric pressure. The average pressure at ground level is about 1,000hPa (hectopascals).
“Hecto” means 100 times, so that’s equal to 100,000Pa. In other words, a force of 100,000N (about 10 tons!) is pressing down on every 1m^2. Hard to picture 100,000N, right?
Think of it in milk cartons (1kg ≈ 10N). It’s like lying down with 10,000 milk cartons stacked on top of you. Just imagining it makes you feel squashed, but we stay in balance by pushing back from the inside with the same force.
Atmospheric pressure is stronger than you might think. You can even lift a whole desk using just a plastic sheet!
An Experiment to Catch the Invisible Power of Air
Don’t believe you’re being pushed that hard? Then give this experiment a try. Pour a little hot water into an empty plastic bottle, shake it, and let the inside fill with steam. Pour out the water, quickly screw the cap on tight, and just leave it. After about a minute… the bottle suddenly starts crunching and crumpling with a loud crackling sound.
What happened? The steam inside the bottle cooled and turned back into water, leaving something close to a vacuum inside. The bottle couldn’t withstand the air pressure outside (100,000Pa) and got crushed. Air pressure has the strength of an invisible monster.
Why Potato Chip Bags Puff Up on Mountaintops
Air pressure doesn’t just push down from above. It pushes on us from every direction. Fluids like air and water squeeze into any gap they can find and push against any wall in their way. Have you ever hiked up a mountain and noticed that the snack bag in your backpack has swollen up like a balloon?
That happens because the air sealed in the bag at ground level (1,000hPa) was carried up to a mountaintop where the pressure is lower (say, 970hPa). As a result, the air inside pushes outward harder than the air outside pushes in.
When I visited the Nobeyama Radio Observatory of the National Astronomical Observatory of Japan, I brought a bag of potato chips along. Let’s compare how the bag looked in Tokyo and at Nobeyama…
As you can see, the bag at Nobeyama is clearly more puffed up.
This is how the bag looked when I took it to Tsuboniwa in the Kita-Yatsugatake mountains, at an elevation of 2,237m.
Now for the opposite case, where the pressure goes up instead of down. There was a fun exhibit at the Ibaraki Nature Museum that I’d like to share: a cup noodle container that shrinks as the pressure increases.
It has already shrunk at -200m!
The Secret Behind Buoyancy: Water Pressure Teamwork
Finally, let’s uncover the truth about buoyancy, the force you feel in the bath or at the pool. Many people think of it as a mysterious force that makes your body feel lighter in water, but it too comes from a difference in pressure. The deeper you go, the more water is stacked above you, so the water pressure goes up. By calculation, diving just 10m down adds 1,000hPa, the same as the pressure from 150km of air. That shows just how dense water is.
Now picture an object underwater. The water pressure pushing on its bottom surface, which sits deeper, is always greater than the pressure pushing on its top surface. The pushes from the left and right cancel each other out, but the difference between top and bottom is left over as an upward force. That is the true identity of buoyancy. It’s not a magic force at all. It’s the combined push of water pressure, with the surrounding water working together to lift the object up.
Air and water seem so ordinary, but hidden inside them is a surprisingly dynamic world of physics, woven from area and force. The next time you pick up a pencil or open a bag of potato chips, remember the invisible power of pressure.
Contact and Requests
Make the wonders and fun of science more familiar! This site collects fun science experiments you can do at home, along with tips for making them work. Try searching around!
・The content of “Kagaku no Neta-cho” (Science Idea Notebook) has been published as a book. Details are here.
・Learn more about the site’s author, Ken Kuwako, here.
・For all kinds of requests (writing, lectures, science workshops, TV supervision and appearances, etc.), please click here.
・Article updates are posted on X!
The Science Neta Channel features experiment videos!
１０月のイチオシ実験！
https://youtube.com/shorts/jLVKYRVI1uY?feature=share
テレビ番組監修・イベント等のお知らせ
- 12月26日（土） ナリカサイエンスアカデミー（教員向け実験講習会）開催
書籍のお知らせ
- 『高校入試 分解問題集 理科』（学研）…難しい問題も小さな問題に分解することで、問題を解くことができます。そんな分解の技術が身につくように深く関わりを持って作りました。
『大人のための高校物理復習帳』（講談社）…一般向けに日常の物理について公式を元に紐解きました。特設サイトでは実験を多数紹介しています。※増刷がかかり６刷となりました（2026/02/01）
- 『きめる!共通テスト 物理基礎 改訂版』（学研）… 高校物理の参考書です。イラストを多くしてイメージが持てるように描きました。授業についていけない、物理が苦手、そんな生徒におすすめです。特設サイトはこちら。
各種SNS（更新情報をお届け！）
【日本語】X(Twitter)／instagram／Facebook 【英語】BlueSky／Threads
Explore
- 楽しい実験…お子さんと一緒に夢中になれるイチオシの科学実験を多数紹介しています。また、高校物理の理解を深めるための動画教材も用意しました。
- 理科の教材… 理科教師をバックアップ！授業の質を高め、準備を効率化するための選りすぐりの教材を紹介しています。
- Youtube…科学実験等の動画を配信しています。
- 科学ラジオ …科学トピックをほぼ毎日配信中！AI技術を駆使して作成した「耳で楽しむ科学」をお届けします。
- 講演 …全国各地で実験講習会・サイエンスショー等を行っています。
- About …「科学のネタ帳」のコンセプトや、運営者である桑子研のプロフィール・想いをまとめています。
- お問い合わせ …実験教室のご依頼、執筆・講演の相談、科学監修等はこちらのフォームからお寄せください。
- Experiment! What Happens When You Take a Bag of Potato Chips Up a Mountain? Almost Bursting at 2,237 m?! (The Mystery of Air Pressure)
- We Live at the Bottom of an Ocean of Air! A Plastic Bottle Experiment Reveals the Truth About Air Pressure (A High School Physics Refresher for Adults)
- Seeing the World Through an Insect’s Eyes? The Hidden Secrets of Dandelions and Azaleas
- Did the Chopstick Snap? Did It Move Backward? The Strange Images in a Glass of Water
- Making the Invisible Visible: A Laser and Fog Machine Light Show!
- The 1,300-View Wall! What Middle Schoolers Learned About “Reaching People” Through Short Videos (Science Communication Course)
- Is the Image on Your Retina Upside Down? Discover the Magic of Light with a Squid Eye Experiment
- We Built a Volcano—and It Was Explosively Fun! The Ultimate Experiment Recipe Using Plaster and Baking Soda!
- Data So Clean It’ll Wow You! Cracking the Link Between Potential Energy and Work with Just a Rail and a Marble
