[{"data":1,"prerenderedAt":1135},["ShallowReactive",2],{"layer:gravity:discover":3},{"layer":4,"contentHash":1117,"dependencyHashes":1118,"approval":1129,"releaseId":1134},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":43,"sourceIds":1112,"reviewStatus":1113,"authoring":1114},1,"gravity","en","discover","Why does everything fall down?","Meet the pull that drops a pencil, bends the Moon’s path and holds the sky together","Start with a dropped pencil and end with galaxies. Discover what a force is, why heavy things do not fall faster, how air changes everything, the real difference between mass and weight, and the true reason astronauts float.",[13,14,15,16,17],"Say what a force is, and describe gravity as the attraction between any two masses.","Explain why a hammer and a feather land together on the Moon but not on Earth, using air resistance.","Tell mass and weight apart, and use weight = mass × g to find your weight on six different worlds.","Describe an orbit as falling sideways fast enough to keep missing the ground.","Explain correctly why astronauts float, and why \"there is no gravity in space\" is wrong.",35,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37,40],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 35 minutes",{"label":29,"value":30},"Prior knowledge","None — start here",{"label":32,"value":33},"Chapters","11",{"label":35,"value":36},"Labs","Drop race, weigh six worlds, Newton’s cannon, 2 games",{"label":38,"value":39},"Units used","kg, N, m\u002Fs, km\u002Fh, N\u002Fkg",{"label":41,"value":42},"Big misconception","\"There is no gravity in space\"",[44,48,54,60,63,68,73,93,98,103,106,111,132,137,142,145,149,153,200,203,221,226,229,233,237,242,279,292,297,300,304,307,357,361,387,391,396,399,415,419,423,479,484,487,526,530,559,573,588,601,606,609,647,651,656,678,683,686,690,719,732,737,740,743,747,775,781,784,789,792,797,801,865,901,949,1082,1099,1103],{"id":45,"type":46,"markdown":47},"intro-drop-something","prose","Pick up a pencil, hold it out at arm's length and let go.\n\nYou already know exactly what will happen. It falls. You have watched it happen ten thousand times: dropped rotis, dropped phones, a ball leaving your hand, rain arriving from a cloud, a coconut giving up its branch. Nothing has ever fallen *upwards* by accident.\n\nAnd yet the moment you ask **why**, the everyday answer runs out. \"Because it is heavy\" cannot be right, because a feather falls too. \"Because things go down\" is not an answer, it is the question written backwards.\n\nThe real answer is one of the biggest ideas humans have ever had, and it is startling: **the pencil is being pulled by the whole Earth** — and the pencil is pulling the Earth back. The same pull that drops the pencil holds the ocean in place, swings the Moon around us every month, and keeps the Earth circling the Sun. It is called **gravity**, and in this lesson you will meet it properly.",{"id":49,"type":50,"variant":51,"title":52,"markdown":53},"intro-how-to-read","callout","observation","How to use this lesson","Read the chapters in order the first time; each one sets up the next. Whenever you see a **prediction**, stop and decide your answer before reading on — being wrong for ten seconds is the fastest way to learn something.\n\nThe labs are safe to play with for as long as you like. Nothing in them can break.",{"id":55,"type":56,"title":57,"eyebrow":58,"navLabel":59},"ch1","chapter","Everything falls, and \"down\" is a direction with a secret","Chapter 01","1 Everything falls",{"id":61,"type":46,"markdown":62},"falls-list","Make a list, in your head, of everything you have seen fall today. Water from a tap. Crumbs from a plate. Dust drifting in a sunbeam. Your own foot, every time you took a step and let it drop.\n\nNow try to think of an exception — something that genuinely does not fall.\n\n- A balloon full of helium? It rises, but only because the air around it is *heavier* and pushes under it. Gravity is still pulling the balloon; the air simply wins. Take the same balloon somewhere with no air and it drops like a stone.\n- Smoke? Same story: hot air is lighter than cool air, so cool air slides underneath and shoves the smoke up.\n- A bird, a kite, an aeroplane? Each is being held up by something — wings pushing air downwards, a string, an engine. Stop the pushing and down they come.\n\nThere are no exceptions. **Everything with any mass at all is being pulled towards the Earth, all the time, everywhere, with no days off.**",{"id":64,"type":50,"variant":65,"title":66,"markdown":67},"down-is-centre","aha","\"Down\" points at the centre of the Earth","Here is the secret hiding inside the word *down*.\n\nA child in Chennai drops a ball and it goes down. A child in Argentina, on the opposite side of the planet, drops a ball and it goes down too — but their \"down\" points the **opposite way** in space from yours.\n\nBoth are right. \"Down\" is not a fixed direction in the universe. **Down means towards the centre of the Earth**, and since people stand all over the round Earth, everyone's down points a different way. Nobody in Argentina is upside down, and nobody there is in any danger of falling off.",{"id":69,"type":50,"variant":70,"title":71,"markdown":72},"misconception-fall-off","misconception","\"People on the other side would fall off\"","This one bothered clever people for centuries, and it bothers most nine-year-olds for about a week.\n\nThe Earth pulls you towards its **centre**, not towards some imaginary floor of the universe. Wherever you stand on the ball, the pull is downwards *under your feet*. There is no edge to slide off, and no bottom of the sky to fall into. A ship sailing round the world never has to climb over a hump or hang upside down — it is simply following the surface, with the centre always beneath it.",{"id":74,"type":75,"tone":76,"items":77},"spec-fall-facts","spec","blue",[78,82,86,90],{"label":79,"big":80,"value":81},"Direction of the pull","inwards","Always towards the centre of the Earth, from every point on the surface.",{"label":83,"big":84,"value":85},"Who feels it","everything","Every object with mass: dust, water, air, you, mountains and the Moon.",{"label":87,"big":88,"value":89},"Can it be switched off","no","Gravity has no off switch and no shield. You can only balance it with another force.",{"label":91,"big":88,"value":92},"Does it need touching","It reaches across empty space. The Moon is 384,400 km away and still feels it.",{"id":94,"type":50,"variant":95,"title":96,"markdown":97},"tryit-drop-test","try_it","Three drops in three minutes","You need: a coin, a sheet of paper, and a book.\n\n1. Hold the coin and the sheet of paper at the same height. Drop both. The coin wins easily.\n2. Now crumple the same sheet of paper into a tight ball and race it against the coin again. This time it is almost a dead heat.\n3. Put the flat sheet of paper **on top of** the book and drop the book. The paper arrives at the same instant as the book.\n\nNothing about the paper changed except its shape and what was in front of it. Remember this; we come back to it in Chapter 4.",{"id":99,"type":56,"title":100,"eyebrow":101,"navLabel":102},"ch2","A force is a push or a pull","Chapter 02","2 What a force is",{"id":104,"type":46,"markdown":105},"what-is-force","Before gravity, one small idea: **force**.\n\nA force is simply a **push or a pull**. That is the whole definition. You use forces all day without naming them:\n\n- Pushing a door open, pulling a drawer out.\n- A cricket bat pushing a ball, which is why the ball turns round and flies the other way.\n- The friction of your shoe pushing backwards on the floor so you can walk forwards.\n- A magnet pulling a pin across a table without touching it.\n\nForces do only three kinds of thing: they can make a still object **start moving**, make a moving object **speed up, slow down or change direction**, and they can **change an object's shape** (squash a ball of atta, stretch a rubber band).\n\nMost forces need contact — something has to touch something. A few do not. A magnet pulls iron through a gap. And gravity, the strangest of them all, pulls across empty space, across the whole Solar System, with nothing in between.",{"id":107,"type":50,"variant":108,"title":109,"markdown":110},"def-force","definition","Force, in one line","A **force** is a push or a pull on an object. It is measured in **newtons (N)**, named after Isaac Newton. One newton is roughly the pull of the Earth on a small apple — about 100 grams of it.",{"id":112,"type":113,"title":114,"items":115},"steps-what-forces-do","steps","The three things a force can do",[116,120,124,128],{"title":117,"tag":118,"text":119},"Start or stop motion","from still to moving","A still football does nothing until a foot pushes it. A rolling ball stops when friction pushes back on it.",{"title":121,"tag":122,"text":123},"Change speed","faster or slower","Keep pushing a trolley and it keeps speeding up. Pull backwards and it slows.",{"title":125,"tag":126,"text":127},"Change direction","turn the motion","A batsman does not stop the ball; the bat pushes it onto a new path. Gravity does this to the Moon.",{"title":129,"tag":130,"text":131},"Change shape","squash or stretch","Press dough, stretch a rubber band, dent a ball. Some shapes spring back, some do not.",{"id":133,"type":50,"variant":134,"title":135,"markdown":136},"nuance-balanced","nuance","Sitting still does not mean \"no force\"","You are sitting on a chair right now, going nowhere. That does **not** mean gravity has stopped pulling you.\n\nGravity is pulling you down with your full weight. The chair is pushing you up with exactly the same force. Two equal forces in opposite directions **balance**, and balanced forces leave motion unchanged — so you stay put.\n\nTake the chair away and the balance is gone instantly. The pull is still there; now there is nothing to cancel it.",{"id":138,"type":56,"title":139,"eyebrow":140,"navLabel":141},"ch3","Gravity: every mass pulls every other mass","Chapter 03","3 What gravity is",{"id":143,"type":46,"markdown":144},"gravity-explained","Here is gravity, as plainly as it can be put:\n\n**Every object that has mass pulls on every other object that has mass.**\n\nNot \"heavy things on Earth\". Not \"planets\". *Every* object. Your pencil pulls on your notebook. You pull on the person sitting next to you. A grain of rice pulls on the Sun.\n\nIf that sounds absurd, it is because of the second half of the rule: **the pull is unbelievably weak unless at least one of the objects is enormous.** Two friends of 50 kg each, standing a metre apart, pull on each other with a force of about **0.00000017 newtons** — far too small to feel, and far too small to move either of them against the friction of the floor.\n\nThe Earth, though, has a mass of about **6,000,000,000,000,000,000,000 tonnes**. That is why you notice *its* pull and nothing else's. Gravity is the weakest of nature's forces by a huge margin — but it is the only one that keeps adding up as objects get bigger, and it never cancels out. Pile up enough mass and it ends up running the universe.",{"id":146,"type":50,"variant":108,"title":147,"markdown":148},"def-gravity","Gravity","**Gravity** is the attraction between any two objects that have mass. It gets **stronger** when the masses are bigger, and **weaker** when the objects are further apart. It always pulls — never pushes — and it reaches across empty space.",{"id":150,"type":50,"variant":65,"title":151,"markdown":152},"aha-mutual","The apple pulls the Earth too","This is the part almost everyone finds hard to believe. When the Earth pulls an apple down with a force of 1 newton, the apple pulls the **Earth up** with a force of exactly 1 newton. The two pulls are always an equal pair.\n\nSo why does the apple move and the Earth stay still? Because the same force moves a small mass a lot and a huge mass almost not at all. The Earth does accelerate towards the apple — by a distance far smaller than the width of an atom. It is real, and it is utterly unnoticeable.",{"id":154,"type":155,"title":156,"note":157,"scale":158,"rungs":159},"ladder-masses","ladder","How much mass is doing the pulling?","A logarithmic scale: each step is ten times more massive than the one below. This is why only the big ones matter.","log",[160,164,168,172,176,180,184,188,192,196],{"label":161,"value":162,"display":163},"A grain of rice",0.00002,"0.02 g",{"label":165,"value":166,"display":167},"An apple",0.1,"100 g",{"label":169,"value":170,"display":171},"A 10-year-old child",32,"32 kg",{"label":173,"value":174,"display":175},"An Indian elephant",4000,"4 tonnes",{"label":177,"value":178,"display":179},"A loaded goods train",5000000,"5,000 tonnes",{"label":181,"value":182,"display":183},"Mount Everest (rough)",810000000000000,"about 810 trillion kg",{"label":185,"value":186,"display":187},"The Moon",7.342e+22,"7.3 x 10²² kg",{"label":189,"value":190,"display":191},"The Earth",5.9722e+24,"6.0 x 10²⁴ kg",{"label":193,"value":194,"display":195},"Jupiter",1.8982e+27,"1.9 x 10²⁷ kg",{"label":197,"value":198,"display":199},"The Sun",1.9885e+30,"2.0 x 10³⁰ kg",{"id":201,"type":46,"markdown":202},"two-rules","Gravity follows two rules that you can hold in your head without any arithmetic at all:\n\n**Rule 1 — more mass, more pull.** Jupiter is 318 times more massive than Earth, and it pulls correspondingly harder. The Moon is much less massive than Earth, so its pull at its surface is only about one-sixth of ours.\n\n**Rule 2 — more distance, less pull.** Move further away and the pull fades. It never quite reaches zero — gravity has infinite reach — but it drops off fast. Standing on top of Mount Everest, nearly 9 km above sea level, you are pulled slightly less than at the beach: about **0.3 % less**, which is far too small to feel but easy to measure.\n\nThat is it. Everything else in this topic — falling apples, orbiting satellites, ocean tides, the shape of galaxies — comes out of those two rules.",{"id":204,"type":205,"prompt":206,"options":207,"explanation":220},"predict-moon-pull","prediction","The Moon has much less mass than the Earth, and its surface gravity is about one-sixth as strong. A 40 kg child stands on the Moon. What happens to the child's **mass**?",[208,211,214,217],{"id":209,"label":210},"a","It becomes about one-sixth: roughly 6.7 kg",{"id":212,"label":213},"b","It stays exactly 40 kg",{"id":215,"label":216},"c","It becomes six times bigger: 240 kg",{"id":218,"label":219},"d","It becomes zero, because the Moon is small","**It stays exactly 40 kg.** Mass is the amount of *stuff* in you — the atoms in your bones, blood and breakfast. Flying to the Moon does not remove any of them.\n\nWhat changes is **weight**, which is how hard gravity pulls on that stuff. The child's weight drops from 392 newtons on Earth to about 65 newtons on the Moon. Same child, same mass, much smaller pull. Chapter 6 makes this precise, because it is the single most-muddled idea in the whole topic.",{"id":222,"type":56,"title":223,"eyebrow":224,"navLabel":225},"ch4","The great race: does heavy really fall faster?","Chapter 04","4 The great race",{"id":227,"type":46,"markdown":228},"aristotle-intuition","For about two thousand years, almost everybody in the world believed something that feels obviously true: **heavier things fall faster**.\n\nIt is easy to see why. Drop a stone and a leaf together and the stone lands first, every time. The Greek thinker Aristotle wrote it down as a rule, and for centuries people repeated it without checking.\n\nThen, in the late 1500s, an Italian named **Galileo Galilei** did something unusual. He checked.\n\nAnd the answer turned out to be: **no**. Drop a heavy ball and a light ball together and, apart from the effect of the air, they hit the ground **at the same moment**. A cannonball and a musket ball, released together, land together.",{"id":230,"type":50,"variant":134,"title":231,"markdown":232},"pisa-story","The Leaning Tower story, honestly told","You have probably heard that Galileo climbed the Leaning Tower of Pisa and dropped two balls of different weights in front of a crowd.\n\nIt is a wonderful story. It may not have happened. The only account comes from **Vincenzo Viviani**, Galileo's assistant, writing in 1654 — twelve years after Galileo died and more than sixty years after the supposed event. Galileo never described it himself in his own books, which is odd for a man who loved a good demonstration.\n\nWhat Galileo definitely did do was better: he rolled balls down gentle ramps, hundreds of times, timing them carefully, so that the motion was slow enough to measure. That is where the real evidence came from. You will try his ramp idea yourself in the **Investigate** layer.\n\n(If someone *had* dropped a ball from the tower's top gallery, about 55 m up, it would have taken roughly **3.3 seconds** to land.)",{"id":234,"type":50,"variant":70,"title":235,"markdown":236},"misconception-heavy-faster","\"Heavier things fall faster\"","This is the big one, and it is worth saying slowly.\n\n**In air**, heavier things often *do* land first — but not because they are heavy. It is because air pushes back on falling objects, and a light, wide object (a feather, a sheet of paper, a leaf) is affected by that push far more than a dense, compact one.\n\n**Without air**, a hammer and a feather fall at exactly the same rate and land together.\n\nSo the rule is not \"heavy falls faster\". The rule is \"**air slows some shapes far more than others**\".",{"id":238,"type":50,"variant":239,"title":240,"markdown":241},"example-apollo15","example","The hammer and the feather on the Moon","On **2 August 1971**, astronaut **David Scott** of Apollo 15 stood in front of a television camera on the Moon, held out a geology hammer in one hand and a falcon feather in the other, and let go.\n\nThe Moon has no air. The hammer (about 1.32 kg) and the feather (about 0.03 kg) fell side by side and touched the dust together. The hammer had around **44 times** the mass of the feather and it made no difference at all.\n\nFrom about 1.6 m up, on the Moon's gentler gravity, the fall took roughly **1.4 seconds**. On Earth, the same drop would take about **0.57 seconds** — the Moon's version is about two and a half times slower, and you can see it in the footage.",{"id":243,"type":244,"component":245,"componentVersion":5,"config":246,"objective":272,"textAlternative":273,"help":274},"lab-drop-earth-moon","interactive","gravity-drop",{"worlds":247,"objects":250,"modes":269,"dropHeightM":271},[248,249],"earth","moon",[251,256,261,265],{"id":252,"label":253,"massKg":254,"draggy":255},"hammer","Geology hammer (1.32 kg)",1.32,false,{"id":257,"label":258,"massKg":259,"draggy":260},"feather","Falcon feather (0.03 kg)",0.03,true,{"id":262,"label":263,"massKg":264,"draggy":255},"cricket-ball","Cricket ball (0.16 kg)",0.16,{"id":266,"label":267,"massKg":268,"draggy":260},"paper-flat","Flat sheet of paper",0.005,[270],"drop",20,"Race a hammer, a feather, a cricket ball and a sheet of paper on Earth (with air) and on the Moon (no air).","This lab drops four objects from 20 m and shows a stopwatch for each.\n\n**On Earth, with air:** the hammer and the cricket ball land together after about **2.0 seconds**. The feather and the flat sheet of paper drift down far more slowly, taking many seconds, wobbling as they go. The two dense objects are unaffected by the air; the two wide, light ones are held up by it.\n\n**On the Moon, with no air:** all four are released together and all four land together after about **5.0 seconds** — slower than on Earth, because the Moon pulls with 1.62 m\u002Fs² instead of 9.8, but perfectly level with each other. The feather falls as straight and as fast as the hammer.\n\nThe lesson: on the Moon, mass makes no difference to falling. On Earth, mass makes no difference either — **shape and air** do.",{"simplerExplanation":275,"hints":276},"Take the air away and everything falls at the same rate, whatever it weighs.",[277,278],"Watch the feather on Earth, then on the Moon. Only the air changed.","Compare the hammer on Earth with the hammer on the Moon: same object, different pull.",{"id":280,"type":205,"prompt":281,"options":282,"explanation":291},"predict-paper-crumple","You drop a flat sheet of paper and a coin together: the coin lands first. Now you crumple **the same sheet** into a tight ball and drop it against the coin again. What happens?",[283,285,287,289],{"id":209,"label":284},"Exactly the same as before: the coin wins easily",{"id":212,"label":286},"They land at nearly the same time",{"id":215,"label":288},"The paper now wins, because crumpling made it heavier",{"id":218,"label":290},"The paper floats, because crumpling traps air","**They land at nearly the same time.** Crumpling did not add a single atom — the paper has exactly the same mass as before. What changed is the **area facing the air**. A flat sheet has to shove aside a wide slab of air on the way down; a tight ball slices through.\n\nThis is the cleanest proof you can do at home that \"heavy falls faster\" is wrong. If weight were the cause, the flat sheet and the crumpled ball would behave identically.",{"id":293,"type":56,"title":294,"eyebrow":295,"navLabel":296},"ch5","Air pushes back","Chapter 05","5 Air pushes back",{"id":298,"type":46,"markdown":299},"air-resistance","Air feels like nothing. It is not nothing.\n\nA classroom-sized room holds roughly 60 kg of air — about the mass of an adult. When you move through it, you have to shove it out of the way, and it shoves back. That backwards push on a falling object is **air resistance**, or **drag**.\n\nDrag depends on three things:\n\n- **How fast you are going.** Stick your hand out of a moving car window at 20 km\u002Fh and you feel a gentle push. At 80 km\u002Fh it nearly rips your arm back. Drag grows *much* faster than speed does.\n- **How big the front of you is.** A flat sheet of paper meets a lot of air. The same paper crumpled meets very little.\n- **What shape you are.** A smooth, pointed shape slips through; a flat, blunt one piles the air up in front of it. This is why raindrops, bullets, fish, aeroplanes and the nose of a Vande Bharat train all end up looking oddly similar.",{"id":301,"type":50,"variant":108,"title":302,"markdown":303},"def-air-resistance","Air resistance","**Air resistance (drag)** is the backwards push that air gives to anything moving through it. It acts **opposite to the motion**, it grows rapidly as speed rises, and it is bigger for wide, flat, rough shapes than for small, smooth, pointed ones.\n\nIt is not a kind of gravity. It is a separate force that happens to be fighting gravity on the way down.",{"id":305,"type":46,"markdown":306},"terminal-velocity","Now put the two forces together and something neat happens.\n\nThe instant you jump off a diving board, you are barely moving, so drag is almost zero. Gravity has it all its own way and you speed up fast.\n\nAs you get faster, drag grows. Gravity has not changed — but now part of it is being cancelled. You still speed up, but less eagerly.\n\nEventually drag grows until it exactly equals your weight. The two forces balance, and balanced forces mean **no more change in speed**. From that moment you fall at a steady speed, all the way down. That steady speed is called **terminal velocity** — terminal meaning \"final\", nothing to do with a bus stand.\n\nA skydiver falling belly-down reaches about **55 metres per second**, which is close to **200 km\u002Fh**. A falling raindrop, only 2 mm across, tops out at about **6.5 m\u002Fs** — roughly **23 km\u002Fh**, slower than a cyclist. Which is extremely fortunate, because if raindrops kept accelerating all the way from the cloud they would arrive at over **870 km\u002Fh**, and standing outside in the monsoon would be fatal.",{"id":308,"type":309,"caption":310,"columns":311,"rows":316},"table-terminal","table","Terminal speeds in air: the speed at which drag finally balances weight (approximate values)",[312,313,314,315],"Falling thing","Steady speed","In km\u002Fh","Why",[317,322,327,332,337,342,347,352],[318,319,320,321],"Skydiver, arms and legs spread","about 55 m\u002Fs","about 200 km\u002Fh","Heavy, but spread wide to catch air",[323,324,325,326],"Skydiver, head-down and streamlined","about 90 m\u002Fs","about 320 km\u002Fh","Same person, much smaller front — less drag",[328,329,330,331],"Skydiver under an open parachute","about 5.5 m\u002Fs","about 20 km\u002Fh","Huge area, enormous drag, gentle landing",[333,334,335,336],"Raindrop, 2 mm across","about 6.5 m\u002Fs","about 23 km\u002Fh","Small and light; drag balances weight very early",[338,339,340,341],"Drizzle drop, 0.5 mm","about 2 m\u002Fs","about 7 km\u002Fh","Smaller still, so even slower",[343,344,345,346],"Mist droplet","about 3 cm\u002Fs","about 0.1 km\u002Fh","So slow it can hang in the air for hours",[348,349,350,351],"Hailstone, 2 cm across","about 20 m\u002Fs","about 70 km\u002Fh","Dense ice, compact shape — this is why hail hurts",[353,354,355,356],"A flat sheet of A4 paper","about 1 m\u002Fs","about 4 km\u002Fh","Almost no weight, enormous area",{"id":358,"type":50,"variant":65,"title":359,"markdown":360},"aha-parachute","Why a parachute works","A parachute does **not** make you lighter. Your weight is exactly the same with the canopy open as with it packed.\n\nWhat it does is make you enormously **wider**. Suddenly you are shoving aside a huge column of air, so drag becomes gigantic at a very low speed. Your terminal velocity crashes from around 200 km\u002Fh down to about 20 km\u002Fh — roughly the speed of jumping off a two-metre wall, which your legs can handle.\n\nSame gravity. Same weight. Different shape.",{"id":362,"type":113,"title":363,"items":364},"steps-skydive","One skydive, told as a balance of two forces",[365,369,373,376,380,383],{"title":366,"tag":367,"text":368},"Jump","0 seconds","Speed is almost zero, so drag is almost zero. Gravity acts alone and you accelerate hard.",{"title":370,"tag":371,"text":372},"Speeding up","first seconds","Drag grows quickly as you get faster. You are still speeding up, but less and less each second.",{"title":374,"tag":320,"text":375},"Terminal velocity","Drag now equals your weight. Forces balanced, so the speed stops changing. You fall steadily.",{"title":377,"tag":378,"text":379},"Pull the cord","the big change","The canopy opens. Area jumps, drag jumps far above your weight, and you slow down sharply.",{"title":381,"tag":330,"text":382},"New steady speed","At this much lower speed the big canopy makes drag equal your weight again. Balanced once more.",{"title":384,"tag":385,"text":386},"Land","knees bent","You touch down at walking-off-a-wall speed instead of car-crash speed.",{"id":388,"type":50,"variant":70,"title":389,"markdown":390},"misconception-no-air-no-gravity","\"No air means no gravity\"","Air and gravity are completely separate things, and it is easy to mix them up because both are invisible.\n\nThe Moon has **no air** and plenty of **gravity** — enough to hold astronauts, their footprints and their dust firmly on the ground.\n\nA sealed vacuum jar on a laboratory bench has no air inside it either, and a feather dropped inside drops like a rock. Removing the air does not remove the pull; it removes the *interference*.",{"id":392,"type":56,"title":393,"eyebrow":394,"navLabel":395},"ch6","Mass and weight are not the same thing","Chapter 06","6 Mass vs weight",{"id":397,"type":46,"markdown":398},"mass-vs-weight","In everyday Hindi, Tamil, Bengali or English we use \"weight\" for both ideas and nobody minds. In science they are two different quantities, measured in two different units, and confusing them causes more trouble in this topic than anything else.\n\n**Mass** is how much matter there is in something — how many atoms, roughly. It is measured in **kilograms (kg)**. It does not change when you move. A 40 kg child is 40 kg in Kolkata, 40 kg on the Moon, 40 kg drifting in deep space. To change your mass you must eat, grow or lose something real.\n\n**Weight** is the *force* with which gravity pulls on that mass. It is measured in **newtons (N)**, like every other force. It changes the moment the gravity around you changes.\n\nThe rule connecting them is beautifully short:\n\n**weight = mass × g**\n\nwhere *g* is the strength of gravity where you are standing. On Earth, g is about **9.8 newtons for every kilogram**. So a 40 kg child weighs 40 × 9.8 = **392 newtons**.",{"id":400,"type":401,"items":402},"formulas-weight","formulas",[403,406,409,412],{"expression":404,"caption":405},"weight = mass × g","Weight in newtons, mass in kilograms, g in newtons per kilogram.",{"expression":407,"caption":408},"g on Earth ≈ 9.8 N\u002Fkg","Every kilogram of you is pulled with about 9.8 newtons.",{"expression":410,"caption":411},"40 kg × 9.8 = 392 N","A 40 kg child weighs 392 newtons on Earth.",{"expression":413,"caption":414},"40 kg × 1.62 = 64.8 N","The same child on the Moon: same mass, far less weight.",{"id":416,"type":50,"variant":108,"title":417,"markdown":418},"def-mass-weight","Mass and weight, side by side","**Mass** — the amount of matter in an object. Unit: **kilogram (kg)**. The same everywhere in the universe. Measured with a balance, which compares one mass against another.\n\n**Weight** — the pull of gravity on that mass. Unit: **newton (N)**. Changes from world to world. Measured with a spring scale, which stretches according to the force.",{"id":420,"type":50,"variant":134,"title":421,"markdown":422},"nuance-bathroom-scale","So what does a bathroom scale measure?","Strictly, a bathroom scale measures **force** — how hard you press on its spring. It then quietly divides by 9.8 and prints the answer in kilograms, because on Earth that is a perfectly useful shortcut and nobody wants to see 392 on their bathroom floor.\n\nTake that same scale to the Moon and it would read about **6.6 kg** for a 40 kg child — a wrong number, because the scale is still dividing by Earth's 9.8. The child has not lost a gram. The scale has simply been lied to.\n\nA **balance** with pans, of the sort you see at a vegetable market, would be honest anywhere: it compares your mass against known masses, and gravity cancels out on both sides.",{"id":424,"type":244,"component":425,"componentVersion":5,"config":426,"objective":477,"textAlternative":478},"lab-sort-mass-weight","sort-game",{"prompt":427,"bins":428,"items":435,"seconds":476},"Does this change when you travel to the Moon, or stay the same?",[429,432],{"id":430,"label":431},"same","Stays the same",{"id":433,"label":434},"changes","Changes",[436,440,444,448,452,456,460,464,468,472],{"id":437,"label":438,"bin":430,"why":439},"i1","The number of atoms in your body","Travelling does not add or remove matter.",{"id":441,"label":442,"bin":430,"why":443},"i2","Your mass in kilograms","Mass is the amount of matter, so it is the same everywhere.",{"id":445,"label":446,"bin":433,"why":447},"i3","Your weight in newtons","Weight = mass x g, and g on the Moon is only 1.62 instead of 9.8.",{"id":449,"label":450,"bin":433,"why":451},"i4","How hard you press on the ground","That is your weight, so it changes with g.",{"id":453,"label":454,"bin":433,"why":455},"i5","How high you can jump","The same push from your legs lifts you about six times higher on the Moon.",{"id":457,"label":458,"bin":430,"why":459},"i6","The reading on a pan balance","A pan balance compares two masses, and gravity affects both pans equally.",{"id":461,"label":462,"bin":433,"why":463},"i7","The reading on a spring bathroom scale","A spring scale measures force, so it reads less where gravity is weaker.",{"id":465,"label":466,"bin":430,"why":467},"i8","How hard it is to get a heavy trolley moving","Resisting a push depends on mass, not on weight. A trolley is just as stubborn on the Moon.",{"id":469,"label":470,"bin":433,"why":471},"i9","The time a dropped hammer takes to land","Weaker gravity means slower falling: about 2.5 times longer on the Moon.",{"id":473,"label":474,"bin":430,"why":475},"i10","The amount of food you need to eat","Your body is unchanged; it still needs the same energy and nutrients.",0,"Sort ten things into \"stays the same\" and \"changes\" for a trip to the Moon, and check the reason for each.","A card-sorting game with two bins: **stays the same** and **changes**.\n\nSame on the Moon: atoms in your body, mass in kg, a pan balance reading, a trolley's stubbornness, and food needed.\n\nChanges on the Moon: weight in newtons (392 N becomes 64.8 N), how hard you press on the ground, jump height, a spring scale's reading (about 6.6 kg instead of 40 kg), and hammer-drop time.\n\nThe pattern: \"same\" is about **how much stuff** there is; \"changes\" is about **how hard gravity pulls** on it.",{"id":480,"type":56,"title":481,"eyebrow":482,"navLabel":483},"ch7","Your weight on six different worlds","Chapter 07","7 Other worlds",{"id":485,"type":46,"markdown":486},"other-worlds-intro","Every world pulls with its own strength. The number we call **g** — the pull on each kilogram — depends on how much mass that world has and how big it is.\n\nHere is what a **40 kg child** would weigh standing on each of six places in the Solar System. The mass is 40 kg in every single row. Only the pull changes.",{"id":488,"type":309,"caption":489,"columns":490,"rows":495},"table-worlds","Gravity and weight across the Solar System, for a child whose mass is 40 kg everywhere",[491,492,493,494],"World","g (N\u002Fkg)","Weight of a 40 kg child","What it would feel like",[496,501,506,511,516,521],[497,498,499,500],"**Pluto**","0.62","24.8 N","About 1\u002F16 of Earth. A gentle hop would carry you over a house.",[502,503,504,505],"**The Moon**","1.62","64.8 N","About 1\u002F6 of Earth. Apollo astronauts found bunny-hopping easier than walking.",[507,508,509,510],"**Mars**","3.72","148.8 N","Just over a third of Earth. You could carry your own body weight easily.",[512,513,514,515],"**Earth**","9.8","392 N","Home. Exactly what your bones and muscles were built for.",[517,518,519,520],"**Jupiter**","24.79","991.6 N","Two and a half times Earth. Standing up would feel like carrying two more children.",[522,523,524,525],"**The Sun**","274","10,960 N","Twenty-eight times Earth. You could not stand, breathe or survive for an instant.",{"id":527,"type":50,"variant":134,"title":528,"markdown":529},"nuance-jupiter-surface","Jupiter and the Sun have no surface to stand on","The table quietly cheats twice.\n\n**Jupiter** is a ball of gas that gets thicker and hotter as you go down, with no solid ground anywhere. The figure 24.79 N\u002Fkg is measured at the top of its clouds, where a spacecraft could fly past.\n\n**The Sun** is not solid either — it is a ball of incandescent gas and plasma at about 5,500 °C on the outside. The figure 274 N\u002Fkg applies at its visible edge.\n\nSo read those two rows as \"how hard gravity would pull if you could be there\", not as an invitation.",{"id":531,"type":244,"component":245,"componentVersion":5,"config":532,"objective":554,"textAlternative":555,"help":556},"lab-weigh-worlds",{"worlds":533,"objects":538,"modes":552,"dropHeightM":271},[534,249,535,248,536,537],"pluto","mars","jupiter","sun",[539,543,547,549],{"id":540,"label":541,"massKg":542,"draggy":255},"child","You (40 kg)",40,{"id":544,"label":545,"massKg":546,"draggy":255},"rice","Bag of rice (5 kg)",5,{"id":548,"label":263,"massKg":264,"draggy":255},"ball",{"id":550,"label":551,"massKg":5,"draggy":255},"water","Bottle of water (1 kg)",[553,270],"weigh","Pick a world and see what four familiar objects weigh there, then drop them and time the fall.","Choose a world from Pluto, the Moon, Mars, Earth, Jupiter and the Sun. The lab then shows the weight of four objects there, and lets you drop them 20 m.\n\n**A 40 kg child weighs:** 24.8 N on Pluto, 64.8 N on the Moon, 148.8 N on Mars, 392 N on Earth, 991.6 N on Jupiter and 10,960 N at the Sun's surface.\n\n**A 5 kg bag of rice weighs:** 3.1 N, 8.1 N, 18.6 N, 49 N, 123.9 N and 1,370 N on the same six worlds.\n\n**A 1 kg bottle of water** weighs exactly g newtons on each world: 0.62, 1.62, 3.72, 9.8, 24.79 and 274 N. That is what \"newtons per kilogram\" means.\n\n**Dropping 20 m takes:** 8.03 s on Pluto, 4.97 s on the Moon, 3.28 s on Mars, 2.02 s on Earth, 1.27 s on Jupiter and only 0.38 s at the Sun. On every world, all four objects land together, because none of these worlds' air is being simulated.",{"simplerExplanation":557,"anotherExample":558},"Same objects, same masses, six different pulls. Weight follows the world; mass never does.","On Mars a 30 kg suitcase would weigh 111.6 N instead of 294 N — you could lift it with one hand.",{"id":560,"type":561,"title":562,"problem":563,"steps":564,"help":571},"we-weight-mars","worked_example","What would you weigh on Mars?","A student has a mass of **35 kg**. Mars pulls with **g = 3.72 N\u002Fkg**. Work out the student's weight on Earth and on Mars, and say how many times smaller the Mars figure is.",[565,566,567,568,569,570],"Use weight = mass × g every time.","On Earth: 35 × 9.8 = **343 N**.","On Mars: 35 × 3.72 = **130.2 N**.","How many times smaller? 343 ÷ 130.2 ≈ **2.63 times**.","Check it the other way: the two g values themselves are in the ratio 9.8 ÷ 3.72 ≈ 2.63. They must match, because the mass of 35 kg is the same on both lines and cancels out.","And the mass? Still 35 kg on both worlds. Only the pull moved.",{"simplerExplanation":572},"Multiply the mass in kilograms by the world’s g to get the weight in newtons.",{"id":574,"type":575,"itemId":576,"prompt":577,"check":578,"hints":582,"feedback":585},"practice-weight-moon","practice","gravity.discover-weight-moon","A cricket bag has a mass of **10 kg**. What would it **weigh**, in newtons, on the Moon, where g = 1.62 N\u002Fkg?",{"kind":579,"answer":580,"tolerance":166,"unit":581},"number",16.2,"N",[583,584],"Use weight = mass × g.","10 × 1.62. The mass stays 10 kg on the Moon.",{"correct":586,"incorrect":587},"Correct: 10 × 1.62 = **16.2 N**. On Earth the same bag would weigh 98 N, so it feels about six times lighter — but it still has all 10 kg of stuff in it.","Multiply the mass by the Moon’s g: 10 × 1.62 = 16.2 N. Do not change the 10 kg — mass never changes when you travel.",{"id":589,"type":205,"prompt":590,"options":591,"explanation":600},"predict-jump-moon","On Earth you can jump about 40 cm straight up. Wearing a spacesuit on the Moon, where gravity pulls with about one-sixth the strength, roughly how high could the same jump take you?",[592,594,596,598],{"id":209,"label":593},"About the same: 40 cm",{"id":212,"label":595},"About 6 cm — the suit is heavy",{"id":215,"label":597},"Around 2 m, if the suit were not in the way",{"id":218,"label":599},"You would float away and never come down","**Around 2 m** — roughly six times higher — if the bulky suit did not get in the way. Your legs give the same push, but gravity takes six times longer to stop you and pull you back.\n\nApollo astronauts really did find it easier to hop than to walk. But **(d) is wrong**: you cannot jump away from the Moon. To escape it entirely you would need to leave the surface at about 2.4 km\u002Fs, which is roughly a thousand times faster than a human jump.",{"id":602,"type":56,"title":603,"eyebrow":604,"navLabel":605},"ch8","Falling gets faster and faster","Chapter 08","8 Faster and faster",{"id":607,"type":46,"markdown":608},"falling-speeds-up","Watch a ball dropped from a first-floor balcony and then from a fifth-floor one. The second fall does not just last longer — the ball arrives **much** harder.\n\nThat is because a falling object does not pick a speed and keep it. Gravity keeps pulling the whole way down, so the speed keeps **growing**. On Earth, with no air in the way, every second of falling adds another **9.8 metres per second** to the speed. That is what \"g = 9.8\" really means: not a speed, but a gain in speed, every single second.\n\nSo:\n\n- After **1 second** you are falling at 9.8 m\u002Fs (about 35 km\u002Fh) and you have dropped **4.9 m** — about one storey of a building.\n- After **2 seconds** you are at 19.6 m\u002Fs (about 71 km\u002Fh) and you have dropped **19.6 m** — four times as far, not twice.\n- After **3 seconds** you are at 29.4 m\u002Fs (about 106 km\u002Fh) and you have dropped **44.1 m** — nine times the first second's distance.\n\nLook at those distances again: 4.9, 19.6, 44.1. They go up as 1, 4, 9 — the square numbers. Doubling the time does not double the fall; it **quadruples** it. This is why a fall from ten metres is so much worse than a fall from five.",{"id":610,"type":309,"caption":611,"columns":612,"rows":618},"table-fall","Falling from rest with no air resistance, on Earth (g = 9.8 m\u002Fs²)",[613,614,615,616,617],"Time falling","Speed now","Speed in km\u002Fh","Distance fallen so far","Fallen in that second alone",[619,624,630,636,642],[620,621,622,623,623],"1 s","9.8 m\u002Fs","35.3 km\u002Fh","4.9 m",[625,626,627,628,629],"2 s","19.6 m\u002Fs","70.6 km\u002Fh","19.6 m","14.7 m",[631,632,633,634,635],"3 s","29.4 m\u002Fs","105.8 km\u002Fh","44.1 m","24.5 m",[637,638,639,640,641],"4 s","39.2 m\u002Fs","141.1 km\u002Fh","78.4 m","34.3 m",[643,644,645,646,634],"5 s","49.0 m\u002Fs","176.4 km\u002Fh","122.5 m",{"id":648,"type":50,"variant":65,"title":649,"markdown":650},"aha-odd-numbers","Galileo’s beautiful pattern: 1, 3, 5, 7","Look at the last column of the table: **4.9, 14.7, 24.5, 34.3**.\n\nDivide each by 4.9 and you get **1, 3, 5, 7** — the odd numbers, in order.\n\nIn the first second a falling body covers one unit. In the next second, three units. Then five. Then seven. Galileo spotted this on his ramps four hundred years ago, before stopwatches existed, and it was the clue that unlocked everything. A simple counting pattern, hiding inside something as ordinary as a dropped stone.",{"id":652,"type":50,"variant":653,"title":654,"markdown":655},"careful-heights","careful","Please do not test this from a height","Everything in this chapter can be explored safely with drops of a metre or two onto a soft floor, or in the labs on this page.\n\nNever drop anything from a balcony, a window, a roof or a staircase, even something soft, and never lean out to watch. A falling object from four floors up is arriving at highway speed, and there may be somebody below.",{"id":657,"type":575,"itemId":658,"prompt":659,"check":660,"hints":672,"feedback":675},"practice-fall-distance","gravity.discover-fall-2s","A stone falls for **1 second** and covers 4.9 m. How far will it have fallen after **2 seconds**?",{"kind":661,"options":662,"correct":671},"choice",[663,665,667,669],{"id":209,"label":664},"9.8 m — twice as far",{"id":212,"label":666},"19.6 m — four times as far",{"id":215,"label":668},"4.9 m — the same each second",{"id":218,"label":670},"14.7 m — three times as far",[212],[673,674],"The stone is going faster in the second second than it was in the first.","Distance grows with the *square* of the time: 1, 4, 9, 16.",{"correct":676,"incorrect":677},"Right: **19.6 m**. Doubling the time quadruples the distance, because the stone spends the second second travelling much faster than the first.","It is not simply double. In the first second the stone falls 4.9 m; in the *second* second it is moving faster and falls 14.7 m more. Total: 4.9 + 14.7 = 19.6 m.",{"id":679,"type":56,"title":680,"eyebrow":681,"navLabel":682},"ch9","So why does the Moon not fall down?","Chapter 09","9 The falling Moon",{"id":684,"type":46,"markdown":685},"moon-question","If gravity pulls everything, and the Moon has mass, and the Earth has mass — why has the Moon not crashed into us?\n\nThe answer, first worked out by **Isaac Newton** in the 1600s, is one of the most satisfying sentences in all of science:\n\n**It is falling. It has always been falling. It just keeps missing.**\n\nHere is Newton's own thought experiment, and it is worth picturing carefully.\n\nImagine a cannon on top of an impossibly high mountain, firing horizontally — straight out sideways, not up.\n\n- Fire it **gently** and the ball curves down and lands not far away.\n- Fire it **harder** and it goes further before landing. It is still falling at exactly the same rate; it simply travels further sideways while it falls.\n- Fire it **much harder** and it goes so far sideways that the ground has **curved away beneath it**. The Earth is a ball, after all. The ball falls, the ground drops away, and the gap never closes.\n- Fire it at just the right speed and the ball falls around the Earth forever, never getting closer, never getting further away. That is an **orbit**.\n\nThe magic speed, fired from just above the ground, is about **7.9 kilometres every second** — roughly Delhi to Agra in under half a minute.",{"id":687,"type":50,"variant":65,"title":688,"markdown":689},"aha-orbit-is-falling","An orbit is a permanent, beautifully aimed miss","Nothing in orbit is \"beyond gravity\". Everything in orbit is falling, every second, exactly like a dropped stone.\n\nThe only difference is sideways speed. A stone dropped from your hand has none, so it falls straight down and hits the floor. A satellite has so much sideways speed that by the time it has fallen, the round Earth has curved out from underneath it by the same amount.\n\nThe Moon, right now, is falling towards the Earth at about **1.35 millimetres per second** — while sweeping sideways at about **1 km every second**. Those two numbers together bend its path into a circle. Take away the sideways motion and the Moon would drop on us. Take away gravity and it would sail off in a straight line.",{"id":691,"type":244,"component":692,"componentVersion":5,"config":693,"objective":712,"textAlternative":713,"help":714},"lab-cannon-basic","orbit-lab",{"speedKmS":694,"presets":697,"showMoon":255},{"min":5,"max":695,"initial":696},12,3,[698,700,703,706,709],{"label":699,"speedKmS":696},"Too slow: falls back",{"label":701,"speedKmS":702},"Still falls, but far away",6,{"label":704,"speedKmS":705},"Orbit! About 7.9 km\u002Fs",7.9,{"label":707,"speedKmS":708},"Stretched, oval orbit",9.5,{"label":710,"speedKmS":711},"Escape: 11.2 km\u002Fs",11.2,"Fire Newton's cannon horizontally from a mountain top and find the speed at which the ball stops landing.","A cannon on a very tall mountain fires horizontally, with a speed slider from 1 to 12 km\u002Fs and a drawn path.\n\n**At 3 km\u002Fs** the ball arcs over and lands perhaps a thousand kilometres away.\n\n**At 6 km\u002Fs** it travels much further round the curve of the Earth, but still comes down.\n\n**At about 7.9 km\u002Fs** the path closes into a circle. The ball falls forever and never lands: it is in **orbit**.\n\n**At 9.5 km\u002Fs** the orbit becomes a long oval — the ball swings far out, slows, and comes sweeping back past the mountain.\n\n**At about 11.2 km\u002Fs** the path stops closing at all. The ball leaves Earth for good. This is **escape velocity**.\n\nIn every one of these cases gravity is pulling just as hard. The only thing you changed was how fast the ball was thrown sideways.",{"simplerExplanation":715,"hints":716},"Throw sideways fast enough and the ground curves away as fast as you fall.",[717,718],"Start slow and increase in small steps. Watch where the landing point goes.","Find the slowest speed that never lands. That is the orbit speed.",{"id":720,"type":205,"prompt":721,"options":722,"explanation":731},"predict-turn-off-gravity","Suppose gravity could be switched off, this instant, everywhere. What would the Moon do?",[723,725,727,729],{"id":209,"label":724},"Fly off in a straight line, at a tangent to its orbit",{"id":212,"label":726},"Drop straight down onto the Earth",{"id":215,"label":728},"Stay exactly where it is, forever",{"id":218,"label":730},"Slow down and gently stop","**It would fly off in a straight line**, carrying on in whatever direction it happened to be travelling at that moment, at about 1 km\u002Fs, forever.\n\nMoving things travel in straight lines unless a force bends them. Gravity is the force doing the bending, every second, all the way round. Remove it and there is nothing left to curve the path — so the Moon simply keeps going the way it was already going, and never comes back.\n\n(**b** is the opposite mistake: without gravity there is nothing to pull it down. **c** and **d** both forget that the Moon is already moving fast.)",{"id":733,"type":56,"title":734,"eyebrow":735,"navLabel":736},"ch10","Why astronauts float","Chapter 10","10 Why they float",{"id":738,"type":46,"markdown":739},"weightless-intro","Everyone has seen the videos: astronauts on the International Space Station tumbling gently past the camera, water forming wobbling silver spheres, a pen drifting away from a hand.\n\nThe usual explanation — the one in a hundred cartoons and a few careless books — is that there is **no gravity** up there.\n\nThat explanation is wrong, and it is worth being precise about why, because the true reason is far more interesting.\n\nThe ISS orbits about **400 kilometres** above the ground. That is high, but on the scale of the Earth it is nothing: the Earth's radius is 6,371 km, so the station is only about **6 % further** from the centre than you are. Gravity there is about **8.7 N\u002Fkg** — nearly **89 %** of its strength on the ground.\n\nAn astronaut who weighs 600 N standing in Bengaluru still weighs about 530 N aboard the station. Gravity has barely noticed they left.",{"id":741,"type":50,"variant":70,"title":42,"markdown":742},"misconception-zero-gravity","This is the most important misunderstanding in this whole topic, so here it is in bold:\n\n**Astronauts float because they are falling, not because gravity has stopped.**\n\nThe space station is falling towards the Earth continuously — and so is everything inside it, at exactly the same rate. The astronaut falls, the floor falls, the camera falls, the drinking water falls, all together. Since nothing catches up with anything else, nobody presses on anything, and nothing feels heavy.\n\nGravity has not gone. It is doing its job perfectly. It is simply pulling on the person and their surroundings **equally**, so there is nothing left to feel.\n\nScientists therefore prefer the word **microgravity** to \"zero gravity\" — and even that is a little generous.",{"id":744,"type":50,"variant":95,"title":745,"markdown":746},"tryit-lift-drop","Feel weightlessness for half a second","You do not need a rocket.\n\n**In a lift:** stand still in a lift and pay attention to your feet as it starts going down. For a moment you feel lighter — because the floor has begun dropping away from you and is pressing on you less. When the lift stops at the bottom you feel briefly heavier.\n\n**On a swing:** at the very top of the forward arc, just before you come back, there is a small moment where the seat stops pushing on you and your stomach lifts. That is the same effect.\n\n**Jump:** jump straight up from the floor. For the fraction of a second you are in the air, you are in free fall and you are, genuinely, weightless. An astronaut's experience is exactly this, stretched out for six months, because the floor is falling too and never arrives.",{"id":748,"type":113,"title":749,"items":750},"steps-iss-facts","The International Space Station, by the numbers",[751,755,759,763,767,771],{"title":752,"tag":753,"text":754},"How high","about 400 km","Roughly Chennai to Bengaluru, but straight up. Well inside Earth’s gravity.",{"title":756,"tag":757,"text":758},"Gravity there","about 8.7 N\u002Fkg","Around 89 % of the pull you feel standing on the ground.",{"title":760,"tag":761,"text":762},"How fast","about 7.7 km\u002Fs","Close to 27,600 km\u002Fh. Fast enough to keep missing the Earth.",{"title":764,"tag":765,"text":766},"One lap","about 92 minutes","A whole orbit in about an hour and a half.",{"title":768,"tag":769,"text":770},"Sunrises per day","about 16","The crew sees a sunrise roughly every 92 minutes.",{"title":772,"tag":773,"text":774},"Why they float","free fall","Station and crew fall together, so nobody presses on anything.",{"id":776,"type":777,"conceptId":778,"relation":779,"explanation":780},"connect-body-systems","connection","body-systems","related_to","Bones, muscles and blood pressure are all built for a lifetime of pulling against gravity, which is why months in free fall weaken astronauts.",{"id":782,"type":46,"markdown":783},"body-in-space","Falling for months on end turns out to be hard on a human body, precisely because our bodies were designed by evolution for a world that pulls.\n\n- **Bones** thin. Weight-bearing bones such as the hip and spine lose roughly **1 % of their mineral each month** in orbit, because bone rebuilds itself in response to being loaded, and nothing is loading it. Over six months that is close to 6 %.\n- **Muscles** shrink, especially the big ones in the legs and back that normally spend all day holding you upright against gravity.\n- **Fluids shift.** On Earth, gravity keeps a good deal of your blood and water in your legs. In free fall it spreads out, so astronauts arrive in orbit with puffy faces and thin \"bird legs\" for the first days.\n- **Spines stretch.** Without the constant squeeze of gravity, the discs between the vertebrae expand and astronauts grow a few centimetres taller — then shrink back after landing.\n\nThe remedy is exercise: crews on the ISS work out about **two hours a day**, strapped down to a treadmill or pulling against a resistance machine, deliberately re-creating the loads that gravity used to provide for free.",{"id":785,"type":56,"title":786,"eyebrow":787,"navLabel":788},"ch11","Gravity holds the whole sky together","Chapter 11","11 The bigger picture",{"id":790,"type":46,"markdown":791},"gravity-everywhere","Step back far enough and you can see that gravity is not really about dropped pencils at all. It is the force that builds and runs the universe.\n\n- **The tides.** Twice a day the sea rises and falls along the coasts of Gujarat, Odisha and Kerala. That is the Moon's gravity, pulling the near side of the ocean a little harder than it pulls the far side.\n- **The Moon's month.** The Moon takes about 27.3 days to circle us, held on its path by Earth's pull. That journey is what gives us the changing phases.\n- **The year.** The Earth is in orbit around the Sun, held by a pull 330,000 times more massive than our own planet's. Every planet, comet and asteroid is doing the same.\n- **The atmosphere.** The air you are breathing is held down by gravity. The Moon, with a weaker pull, could not keep an atmosphere and lost it long ago.\n- **Stars and galaxies.** Gravity pulled clouds of gas together until they squeezed hot enough to ignite as stars. It then gathered hundreds of billions of those stars into the slow, turning wheel of our galaxy, the Milky Way.\n\nEvery one of those is the same rule that dropped your pencil: masses pull on masses.",{"id":793,"type":777,"conceptId":794,"relation":795,"explanation":796},"connect-tides","tides","helps_understand","Tides are gravity made visible: the Moon pulls the near ocean harder than the far ocean, and the sea rises and falls twice a day.",{"id":798,"type":777,"conceptId":799,"relation":795,"explanation":800},"connect-moon-phases","phases-of-the-moon","Gravity is what keeps the Moon on the monthly orbit that produces the cycle of phases.",{"id":802,"type":803,"title":804,"prompt":805,"options":806},"explorer-gravity-jobs","explorer","Five jobs gravity does every single day","Pick one to see what gravity is holding together, and what would happen without it.",[807,819,830,842,853],{"id":794,"label":808,"chain":809,"badge":815,"note":818},"The tides",[810,811,812,813,814],"Moon pulls the ocean","Near water pulled hardest","Sea bulges","Earth spins beneath","Two tides a day",{"text":816,"tone":817},"Moon’s gravity","yes","The Moon pulls the ocean nearest to it slightly harder than the solid planet, and the planet slightly harder than the far ocean, raising a bulge of water on both sides. As Earth spins beneath those bulges, each coast gets a high tide about twice a day — a rhythm fishing communities from Gujarat to the Sundarbans have planned around for thousands of years.",{"id":249,"label":820,"chain":821,"badge":827,"note":829},"The Moon’s orbit",[822,823,824,825,826],"Moon moves sideways","Earth pulls it inward","Path bends","Circle closes","27.3 days per lap",{"text":828,"tone":817},"Earth’s gravity","The Moon travels sideways at about 1 km\u002Fs while Earth's gravity bends its path inward, balancing into a nearly circular orbit 384,400 km across, taking 27.3 days. Seeing different amounts of its sunlit half as it goes round gives us the cycle of phases.",{"id":831,"label":832,"chain":833,"badge":839,"note":841},"year","The year",[834,835,836,837,838],"Sun’s huge mass","Earth moves sideways","Orbit bends round","365.25 days","Seasons follow",{"text":840,"tone":817},"Sun’s gravity","The Sun holds 99.8 % of the Solar System's mass and keeps eight planets on curved paths around it. Earth takes about 365.25 days per lap, hence a leap day every four years. Seasons come from Earth's axial tilt, not distance from the Sun.",{"id":843,"label":844,"chain":845,"badge":851,"note":852},"air","The air",[846,847,848,849,850],"Gas molecules move fast","Gravity pulls them down","Atmosphere stays","Densest at the ground","Thin on Everest",{"text":828,"tone":817},"Air molecules fly about at hundreds of metres per second and would escape into space without gravity holding them in a shell that thins with height — why Everest climbers need bottled oxygen. The Moon's weaker gravity could not keep an atmosphere at all, hence its black daytime sky.",{"id":854,"label":855,"chain":856,"badge":862,"note":864},"stars","Stars and galaxies",[857,858,859,860,861],"Cloud of gas","Gravity pulls it in","Centre gets hot","Star ignites","Galaxies gather",{"text":863,"tone":817},"Gravity everywhere","A star begins as a cold gas cloud; gravity pulls it together until the centre is hot enough for fusion to ignite it. Gravity then gathers stars into galaxies — our Milky Way holds a few hundred billion, all orbiting a common centre.",{"id":866,"type":244,"component":867,"componentVersion":5,"config":868,"objective":899,"textAlternative":900},"lab-match-gravity-words","match-pairs",{"prompt":869,"mode":870,"pairs":871},"Match each gravity word to what it means.","connect",[872,875,877,880,883,886,888,890,893,896],{"a":873,"b":874},"Force","A push or a pull, measured in newtons",{"a":147,"b":876},"The attraction between any two masses",{"a":878,"b":879},"Mass","How much matter is in something, in kilograms",{"a":881,"b":882},"Weight","The pull of gravity on a mass, in newtons",{"a":884,"b":885},"g on Earth","About 9.8 newtons for every kilogram",{"a":302,"b":887},"The backwards push of air on a moving object",{"a":374,"b":889},"The steady speed when drag balances weight",{"a":891,"b":892},"Free fall","Falling with only gravity acting on you",{"a":894,"b":895},"Orbit","Falling around a world and always missing it",{"a":897,"b":898},"Microgravity","Floating because everything is falling together","Match ten gravity words to their plain-language meanings.","A matching game with ten pairs.\n\nForce goes with \"a push or a pull, measured in newtons\". Gravity goes with \"the attraction between any two masses\". Mass goes with \"how much matter is in something, in kilograms\", while weight goes with \"the pull of gravity on a mass, in newtons\".\n\ng on Earth goes with \"about 9.8 newtons for every kilogram\". Air resistance goes with \"the backwards push of air on a moving object\", and terminal velocity with \"the steady speed when drag balances weight\".\n\nFree fall goes with \"falling with only gravity acting on you\". Orbit goes with \"falling around a world and always missing it\", and microgravity with \"floating because everything is falling together\".",{"id":902,"type":903,"title":904,"terms":905},"glossary-discover","glossary","Gravity vocabulary",[906,909,912,915,918,922,926,930,933,936,939,942,945],{"term":873,"meaning":907,"example":908},"A push or a pull on an object. Measured in newtons (N).","The chair pushes up on you; gravity pulls down.",{"term":147,"meaning":910,"example":911},"The attraction between any two objects that have mass.","The Earth and an apple pull on each other.",{"term":878,"meaning":913,"example":914},"The amount of matter in an object, measured in kilograms (kg). It never changes when you travel.","A 40 kg child is 40 kg on the Moon too.",{"term":881,"meaning":916,"example":917},"The force of gravity on an object, measured in newtons (N). It changes from world to world.","40 kg weighs 392 N on Earth, 64.8 N on the Moon.",{"term":919,"meaning":920,"example":921},"Newton (N)","The unit of force, named after Isaac Newton. One newton is roughly the pull on a small apple.","Your weight is your mass in kg times 9.8.",{"term":923,"meaning":924,"example":925},"g","The strength of gravity at a place: how many newtons pull on each kilogram. On Earth, about 9.8 N\u002Fkg.","On Mars g is 3.72; on Jupiter, 24.79.",{"term":927,"meaning":928,"example":929},"Balanced forces","Two equal forces acting in opposite directions, leaving the motion unchanged.","Sitting on a chair: gravity down, chair up.",{"term":302,"meaning":931,"example":932},"The backwards push that air gives to anything moving through it. Also called drag.","A flat sheet of paper feels a lot of it.",{"term":891,"meaning":934,"example":935},"Falling with only gravity acting, and nothing else in the way.","On the Moon, every drop is a free fall.",{"term":374,"meaning":937,"example":938},"The steady speed reached when air resistance grows to equal weight, so the falling stops speeding up.","A skydiver: about 200 km\u002Fh; a raindrop: about 23 km\u002Fh.",{"term":894,"meaning":940,"example":941},"The curved path of one object round another, caused by falling sideways fast enough to keep missing.","The Moon orbits the Earth in 27.3 days.",{"term":897,"meaning":943,"example":944},"The floating feeling when everything around you is falling at the same rate as you are.","Astronauts aboard the ISS.",{"term":946,"meaning":947,"example":948},"Atmosphere","The layer of air that gravity holds around a planet.","The Moon is too small to keep one.",{"id":950,"type":951,"title":952,"questions":953},"quiz-discover","quiz","Check your gravity sense",[954,967,980,993,1006,1019,1032,1045,1058,1069],{"itemId":955,"prompt":956,"options":957,"correct":209,"why":966},"gravity.discover-q-mass-moon","A 40 kg child travels to the Moon. What is the child’s mass there?",[958,960,962,964],{"id":209,"label":959},"40 kg",{"id":212,"label":961},"About 6.7 kg",{"id":215,"label":963},"64.8 kg",{"id":218,"label":965},"Zero","Mass never changes with travel. Only *weight* changes: 392 N on Earth to 64.8 N on the Moon.",{"itemId":968,"prompt":969,"options":970,"correct":212,"why":979},"gravity.discover-q-hammer-feather","On the Moon, a hammer and a feather are dropped from the same height at the same moment. What happens?",[971,973,975,977],{"id":209,"label":972},"The hammer lands well before the feather",{"id":212,"label":974},"They land together",{"id":215,"label":976},"The feather floats and never lands",{"id":218,"label":978},"The feather lands first, being lighter","Apollo 15 did this in 1971. With no air, mass makes no difference to falling — they touched down together.",{"itemId":981,"prompt":982,"options":983,"correct":215,"why":992},"gravity.discover-q-crumple","Why does a crumpled ball of paper fall faster than the same sheet left flat?",[984,986,988,990],{"id":209,"label":985},"Crumpling makes it heavier",{"id":212,"label":987},"Crumpling makes gravity stronger on it",{"id":215,"label":989},"A smaller area means much less air resistance",{"id":218,"label":991},"The flat sheet is held up by static electricity","Same mass, not one atom added. Only the area facing the air changed, so drag changed.",{"itemId":994,"prompt":995,"options":996,"correct":212,"why":1005},"gravity.discover-q-down","What does \"down\" actually mean?",[997,999,1001,1003],{"id":209,"label":998},"Towards the bottom of the universe",{"id":212,"label":1000},"Towards the centre of the Earth",{"id":215,"label":1002},"Towards the South Pole",{"id":218,"label":1004},"Away from the Sun","Down points at Earth’s centre, so everyone’s down points a different way — nobody falls off the other side.",{"itemId":1007,"prompt":1008,"options":1009,"correct":215,"why":1018},"gravity.discover-q-astronauts","Why do astronauts on the space station float?",[1010,1012,1014,1016],{"id":209,"label":1011},"There is no gravity that far from Earth",{"id":212,"label":1013},"They are too far away to be pulled",{"id":215,"label":1015},"They and the station are falling together",{"id":218,"label":1017},"The station spins to cancel gravity","Gravity at 400 km is still about 89 % of ground strength. They float because station and crew fall together, so nobody presses on anything.",{"itemId":1020,"prompt":1021,"options":1022,"correct":215,"why":1031},"gravity.discover-q-orbit","What would the Moon do if gravity suddenly vanished?",[1023,1025,1027,1029],{"id":209,"label":1024},"Fall straight down onto Earth",{"id":212,"label":1026},"Stay exactly where it is",{"id":215,"label":1028},"Fly off in a straight line",{"id":218,"label":1030},"Slow down and stop","A moving object keeps going straight unless a force bends it. Gravity is what curves the Moon’s path.",{"itemId":1033,"prompt":1034,"options":1035,"correct":212,"why":1044},"gravity.discover-q-weight-mars","A 20 kg suitcase is taken to Mars, where g = 3.72 N\u002Fkg. What does it weigh there?",[1036,1038,1040,1042],{"id":209,"label":1037},"20 N",{"id":212,"label":1039},"74.4 N",{"id":215,"label":1041},"196 N",{"id":218,"label":1043},"3.72 N","Weight = mass × g = 20 × 3.72 = 74.4 N (on Earth: 20 × 9.8 = 196 N).",{"itemId":1046,"prompt":1047,"options":1048,"correct":215,"why":1057},"gravity.discover-q-parachute","A parachute helps a skydiver land safely because it...",[1049,1051,1053,1055],{"id":209,"label":1050},"makes the skydiver lighter",{"id":212,"label":1052},"blocks gravity above the skydiver",{"id":215,"label":1054},"creates a huge area, so drag balances weight at a low speed",{"id":218,"label":1056},"pushes upwards with a rocket effect","Weight is unchanged; the huge area makes drag balance it at about 20 km\u002Fh instead of 200 km\u002Fh.",{"itemId":1059,"prompt":1060,"options":1061,"correct":215,"why":1068},"gravity.discover-q-fall-3s","With no air, how far does a stone fall in 3 seconds on Earth?",[1062,1064,1065,1066],{"id":209,"label":1063},"29.4 m",{"id":212,"label":629},{"id":215,"label":634},{"id":218,"label":1067},"9.8 m","Distance = ½ × 9.8 × 3² = 44.1 m. (29.4 m\u002Fs is its *speed*, a different thing.)",{"itemId":1070,"prompt":1071,"options":1072,"correct":212,"why":1081},"gravity.discover-q-two-friends","Do two children standing side by side pull on each other gravitationally?",[1073,1075,1077,1079],{"id":209,"label":1074},"No, only planets have gravity",{"id":212,"label":1076},"Yes, but far too weakly to notice",{"id":215,"label":1078},"Only if they are touching",{"id":218,"label":1080},"Only if one of them is heavier","Every mass pulls every mass. Two 50 kg people a metre apart pull with about 0.00000017 N.",{"id":1083,"type":1084,"title":1085,"points":1086},"cheat-sheet-discover","summary","Cheat sheet: gravity in twelve lines",[1087,1088,1089,1090,1091,1092,1093,1094,1095,1096,1097,1098],"**Gravity** is the attraction between any two objects with mass. Bigger masses pull harder; greater distance weakens the pull.","**A force** is a push or a pull, measured in **newtons (N)**. Gravity is a force that works across empty space.","**Down** means towards the centre of the Earth, which is why nobody falls off the far side.","**Everything falls at the same rate** when air is out of the way — hammer and feather together, as Apollo 15 showed on the Moon in 1971.","**Air resistance** is the backwards push of air. It affects wide, light shapes far more than dense, compact ones. That is why a flat sheet loses to a crumpled one.","**Terminal velocity** is the steady speed where drag equals weight: about 200 km\u002Fh for a skydiver, about 20 km\u002Fh under a parachute, about 23 km\u002Fh for a raindrop.","**Mass** (kg) is how much stuff there is and never changes. **Weight** (N) is how hard gravity pulls, and changes with the world.","**weight = mass × g.** On Earth g ≈ 9.8 N\u002Fkg, so a 40 kg child weighs 392 N.","**g elsewhere:** Moon 1.62, Mars 3.72, Jupiter 24.79, the Sun 274, Pluto 0.62 N\u002Fkg. The same 40 kg child weighs 64.8, 148.8, 991.6, 10,960 and 24.8 N.","**Falling speeds up** by 9.8 m\u002Fs every second. After 1, 2 and 3 seconds you have fallen 4.9 m, 19.6 m and 44.1 m — the square numbers 1, 4, 9.","**An orbit is a permanent miss:** go sideways fast enough (about 7.9 km\u002Fs near the ground) and the Earth curves away as fast as you fall.","**Astronauts float because they are falling**, not because gravity has stopped. At 400 km up, gravity is still about 89 % as strong as on the ground.",{"id":1100,"type":1101,"prompt":1102},"reflect-discover","reflection","Imagine waking up on a world where gravity is exactly half as strong as Earth's.\n\nWrite down three things that would be easier, two that would be harder or stranger, and one thing that would be **exactly the same as it is here**. For the last one, say why gravity has nothing to do with it.",{"id":1104,"type":1105,"sourceIds":1106},"sources-discover","sources",[1107,1108,1109,1110,1111],"gravity-nasa-planetary-factsheet","gravity-physicsclassroom-free-fall","gravity-nasa-iss","gravity-hyperphysics-gravity","gravity-britannica-gravity",[1107,1108,1109,1110,1111],"needs_review",{"generatedBy":1115,"notes":1116},"claude-code","Draft. Every number (weights on six worlds, fall distances and times, terminal speeds, ISS figures, orbit and escape speeds) computed and asserted in Python. Pisa story explicitly hedged as probably apocryphal. Pending owner review.","476784b35e85b0417de6f066c5bea012dbf1913c5175524840f09b1a7f9a9474",{"component:gravity-drop@1":1119,"component:sort-game@1":1120,"logic:practice":1121,"component:orbit-lab@1":1122,"component:match-pairs@1":1123,"source:gravity-britannica-gravity":1124,"source:gravity-hyperphysics-gravity":1125,"source:gravity-nasa-iss":1126,"source:gravity-nasa-planetary-factsheet":1127,"source:gravity-physicsclassroom-free-fall":1128},"60b1b2628472895c3a36f45610b158c8d87f74e411a582d1c8293f4f80ecf53b","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","e59aa1a3427977ca02681e15776a720cd37d878bd774ea8c5531e2116616b667","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","8a68b69eee24a181ce32e96e29a3f9d05221ee983d659a93152038805c82ae06","ad0b66634225092960d8f417463e2d74017822199cc2886992381487d6b8cbd9","bfc3220795160e3b0ddf890b11835e5452f06f2c29b03d7541b29110035b675a","9c57a129761cacac5b2946c90eff33acba841871c7bbd0b69bdd1ec642680db9","8c311b8ddd919ef66d07a631e86b5823f4c525e1b8bf33ae94ffd5f066a7d201",{"state":1130,"reviewer":1131,"selfReview":260,"reviewedAt":1132,"method":1133},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597257]