--- title: "Air Around Us" book: "Primary & Middle School Physics" subject: physics language: en chapter: 10 exercises: 10 source: https://one-course.com/books/physics/1/en/chapter/10-air-around-us --- # Chapter 10 — Air Around Us Wave your hand fast in front of your face: something soft brushes your cheek. You cannot see it, grab it, or put it in a drawer — yet it fills every room, every bottle, every playground on Earth. This chapter is about [air](#def-g2-air-around-us-air): the invisible stuff we live at the bottom of. ## 10.1 Is air really something? **Definition 10.1 (Air).** *Air* is the invisible stuff all around us. It has no color, no smell of its own, and [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) passes straight through it — which is why we forget it is there. But it is real stuff, as the experiments of this chapter will prove, and it is precious: every breath you take is a gulp of it. **Example 10.2 (Try it: catch some air).** Open a plastic bag wide, sweep it through the room, and twist it shut. The bag stands puffed up like a cushion — squeeze it: it pushes back! Something is clearly trapped inside. Open a corner and squeeze again: you feel the invisible prisoner stream out over your fingers. An “empty” bag full of nothing could not push back. **Example 10.3 (Where air shows itself).** [Air](#def-g2-air-around-us-air) is invisible, but its tricks are not: it wobbles as shimmering above a hot road, it lifts kites, it rustles the leaves, and it makes bubbles wherever it escapes underwater. Blow through a straw into your glass: every silver bubble is a little parcel of [air](#def-g2-air-around-us-air) on its way up. ## 10.2 Air takes up room **Method 10.4 (The dry-towel dive).** A magic trick that is pure physics — do it over a basin. 1. Stuff a dry paper towel snugly into the bottom of a glass so it cannot fall out; 2. turn the glass exactly upside down; 3. push it straight down into a basin of water, deeper than the whole glass, without tilting; 4. pull it straight back out and touch the towel. The towel is perfectly dry! The glass was never empty: it was full of [air](#def-g2-air-around-us-air), and the trapped [air](#def-g2-air-around-us-air) kept its room, so the water could not come in. ![The dry-towel dive: the upside-down glass is full of air, and two things cannot be in the same place at once.](https://one-course.com/images/onecourse/chapters/physics-1/g2-air-around-us/fig-3ebc4e8b7ab6.svg) *The dry-towel dive: the upside-down glass is full of [air](#def-g2-air-around-us-air), and two things cannot be in the same place at once.* **Example 10.5 (The glugging bottle).** Push an “empty” bottle mouth-down into the basin and tip it slightly: glug, glug, glug — big silver bubbles race to the surface, and water takes their place inside. The bottle was full of [air](#def-g2-air-around-us-air); the water can only move in as fast as the [air](#def-g2-air-around-us-air) moves out. That glug at the kitchen [sink](https://one-course.com/books/physics/1/en/chapter/5-floating-and-sinking#def-g1-floating-and-sinking-words) is the sound of [air](#def-g2-air-around-us-air) and water swapping rooms. ![The glugging bottle: as water pours out, big bubbles of air fight their way in — the “empty” space must be filled.](https://one-course.com/images/onecourse/chapters/physics-1/g2-air-around-us/img-3917a33ef88c.jpg) *The glugging bottle: as water pours out, big bubbles of [air](#def-g2-air-around-us-air) fight their way in — the “empty” space must be filled.* **Remark 10.6 (No such thing as an empty bottle).** From now on, be suspicious of the word “empty”. The empty bottle, the empty glass, the empty box: all full of [air](#def-g2-air-around-us-air), right up to the brim. Truly empty — with nothing at all inside, not even [air](#def-g2-air-around-us-air) — is very hard to arrange, and one day you will meet the strange name scientists give it. ## 10.3 Air pushes **Definition 10.7 (Wind).** *Wind* is [air](#def-g2-air-around-us-air) on the move. Still [air](#def-g2-air-around-us-air) brushes nobody; moving [air](#def-g2-air-around-us-air) pushes whatever stands in its way — gently on a breezy day, hard enough in a storm to bend trees. **Example 10.8 (Air at work).** Moving [air](#def-g2-air-around-us-air) does real jobs: it fills the sails of boats, spins [wind](#def-g2-air-around-us-wind) toys, dries the laundry, and carries kites. You make [wind](#def-g2-air-around-us-wind) yourself every time you blow out a birthday candle — your breath is a small private gust, strong enough to push the flame right off its wick. **Example 10.9 (Try it: the balloon jet).** Blow up a balloon and hold its neck shut. Inside, squeezed [air](#def-g2-air-around-us-air) is waiting. Let go: the [air](#def-g2-air-around-us-air) rushes out backward, and the balloon shoots forward across the room, wobbling wildly. The escaping [air](#def-g2-air-around-us-air) pushes the balloon — a tiny cousin of the way squeezed-out gas pushes real rockets. ![The balloon jet: air escaping one way pushes the balloon the other way.](https://one-course.com/images/onecourse/chapters/physics-1/g2-air-around-us/fig-d4ecce9d9ea7.svg) *The balloon jet: [air](#def-g2-air-around-us-air) escaping one way pushes the balloon the other way.* **Remark 10.10 (Air is precious).** The most important push of all happens quietly in your chest: with every breath, [air](#def-g2-air-around-us-air) flows in, your body keeps a little of what it needs, and the rest flows out. People, cats, birds, even fish (who sip the [air](#def-g2-air-around-us-air) hidden in water) all need their share. Invisible does not mean unimportant — with [air](#def-g2-air-around-us-air) it means the opposite. ## 10.4 Exercises **Exercise 10.1 ★.** Name three clues, seen this week, that [air](#def-g2-air-around-us-air) is real — without using any experiment from a book. **Solution of Exercise 10.1.** For example: the [wind](#def-g2-air-around-us-wind) bending trees or rustling leaves; a kite or flag flying; breath-clouds on a cold morning; bubbles when blowing through a straw; a door slamming in a draught. **Exercise 10.2 ★.** Why does the twisted-shut bag of [Example 10.2](#ex-g2-air-around-us-catch) push back when you squeeze it? What is inside? **Solution of Exercise 10.2.** [Air](#def-g2-air-around-us-air) is trapped inside. Squeezing tries to push the [air](#def-g2-air-around-us-air) into less room, and the [air](#def-g2-air-around-us-air) pushes back — something truly empty could not. **Exercise 10.3 ★.** In the dry-towel dive, why does the water not reach the towel? Say it with the chapter’s words: what keeps its room? **Solution of Exercise 10.3.** The glass is full of [air](#def-g2-air-around-us-air), and the trapped [air](#def-g2-air-around-us-air) keeps its room: two things cannot be in the same place at once, so the water cannot rise to the towel. **Exercise 10.4 ★.** What must you *not* do with the glass during the dry-towel dive, and what would happen to the towel if you did? **Solution of Exercise 10.4.** Do not tilt it. Tilted, the trapped [air](#def-g2-air-around-us-air) escapes upward as big bubbles, water takes its room — and the towel gets soaked. **Exercise 10.5 ★.** What is the glug-glug of a bottle filling underwater? What goes out, what comes in, and why must they take turns? **Solution of Exercise 10.5.** Each glug is a parcel of [air](#def-g2-air-around-us-air) escaping as a bubble while a gulp of water takes its place. They take turns because the bottle’s neck is narrow and [air](#def-g2-air-around-us-air) and water cannot pass through it at the same time — and neither can share the same room inside. **Exercise 10.6 ★.** “Pass me the empty bottle.” What is really inside an empty bottle? (See [Remark 10.6](#rem-g2-air-around-us-noempty).) **Solution of Exercise 10.6.** [Air](#def-g2-air-around-us-air) — right up to the brim. “Empty” only means “no drink left”; the bottle is full of invisible stuff. **Exercise 10.7 ★.** What is [wind](#def-g2-air-around-us-wind), in the words of [Definition 10.7](#def-g2-air-around-us-wind)? Give two jobs that moving [air](#def-g2-air-around-us-air) does. **Solution of Exercise 10.7.** [Wind](#def-g2-air-around-us-wind) is [air](#def-g2-air-around-us-air) on the move. Jobs: filling sails, flying kites, drying laundry, spinning [wind](#def-g2-air-around-us-wind) toys — any two. **Exercise 10.8 ★.** When the balloon jet flies, which way does the [air](#def-g2-air-around-us-air) rush out, and which way does the balloon go? **Solution of Exercise 10.8.** The [air](#def-g2-air-around-us-air) rushes out backward through the neck, and the balloon shoots forward, the opposite way. **Exercise 10.9 ★★.** A funnel sits snugly in the neck of a bottle, sealed tight all around with sticky putty. Ana pours water into the funnel — and after a small gulp, the water just sits in the funnel, refusing to go down into the bottle. Explain what blocks it, and propose the one-second fix. **Solution of Exercise 10.9.** The bottle is already full of [air](#def-g2-air-around-us-air), and the sealed funnel leaves the [air](#def-g2-air-around-us-air) no way out — so the water cannot get in. The fix: loosen the seal (or lift the funnel a moment) so the [air](#def-g2-air-around-us-air) can escape past it; then the water glugs down happily. **Exercise 10.10 ★★.** Underwater in the bath, tip a glass full of [air](#def-g2-air-around-us-air) so its mouth faces a glass full of water, also underwater. Can you pour the [air](#def-g2-air-around-us-air) *upward* from one glass into the other? Describe what you would see, and why the bubbles go up while poured water goes down. **Solution of Exercise 10.10.** Yes! Tip the air-filled glass so its mouth is under the other glass’s mouth: the [air](#def-g2-air-around-us-air) leaves as big silver bubbles that rise *up* into the water-filled glass and gather at its top, pushing the water out downward. [Air](#def-g2-air-around-us-air) climbs and water falls because [air](#def-g2-air-around-us-air) always [floats](https://one-course.com/books/physics/1/en/chapter/5-floating-and-sinking#def-g1-floating-and-sinking-words) up through water — pouring, upside down.