Why does everything fall down?

Pick up a pencil, hold it out at arm's length and let go.

You 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.

And 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.

The 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.

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.

Now try to think of an exception — something that genuinely does not fall.

• 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.
• Smoke? Same story: hot air is lighter than cool air, so cool air slides underneath and shoves the smoke up.
• 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.

There are no exceptions. Everything with any mass at all is being pulled towards the Earth, all the time, everywhere, with no days off.

Before gravity, one small idea: force.

A force is simply a push or a pull. That is the whole definition. You use forces all day without naming them:

• Pushing a door open, pulling a drawer out.
• A cricket bat pushing a ball, which is why the ball turns round and flies the other way.
• The friction of your shoe pushing backwards on the floor so you can walk forwards.
• A magnet pulling a pin across a table without touching it.

Forces 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).

Most 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.

Here is gravity, as plainly as it can be put:

Every object that has mass pulls on every other object that has mass.

Not "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.

If 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.

The 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.

Gravity follows two rules that you can hold in your head without any arithmetic at all:

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.

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.

That is it. Everything else in this topic — falling apples, orbiting satellites, ocean tides, the shape of galaxies — comes out of those two rules.

For about two thousand years, almost everybody in the world believed something that feels obviously true: heavier things fall faster.

It 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.

Then, in the late 1500s, an Italian named Galileo Galilei did something unusual. He checked.

And 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.

Air feels like nothing. It is not nothing.

A 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.

Drag depends on three things:

• How fast you are going. Stick your hand out of a moving car window at 20 km/h and you feel a gentle push. At 80 km/h it nearly rips your arm back. Drag grows much faster than speed does.
• How big the front of you is. A flat sheet of paper meets a lot of air. The same paper crumpled meets very little.
• 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.

Now put the two forces together and something neat happens.

The 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.

As you get faster, drag grows. Gravity has not changed — but now part of it is being cancelled. You still speed up, but less eagerly.

Eventually 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.

A skydiver falling belly-down reaches about 55 metres per second, which is close to 200 km/h. A falling raindrop, only 2 mm across, tops out at about 6.5 m/s — roughly 23 km/h, 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/h, and standing outside in the monsoon would be fatal.

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.

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.

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.

The rule connecting them is beautifully short:

weight = mass × g

where 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.

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.

Here 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.

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.

That 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.

So:

• After 1 second you are falling at 9.8 m/s (about 35 km/h) and you have dropped 4.9 m — about one storey of a building.
• After 2 seconds you are at 19.6 m/s (about 71 km/h) and you have dropped 19.6 m — four times as far, not twice.
• After 3 seconds you are at 29.4 m/s (about 106 km/h) and you have dropped 44.1 m — nine times the first second's distance.

Look 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.

If gravity pulls everything, and the Moon has mass, and the Earth has mass — why has the Moon not crashed into us?

The answer, first worked out by Isaac Newton in the 1600s, is one of the most satisfying sentences in all of science:

It is falling. It has always been falling. It just keeps missing.

Here is Newton's own thought experiment, and it is worth picturing carefully.

Imagine a cannon on top of an impossibly high mountain, firing horizontally — straight out sideways, not up.

• Fire it gently and the ball curves down and lands not far away.
• 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.
• 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.
• 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.

The magic speed, fired from just above the ground, is about 7.9 kilometres every second — roughly Delhi to Agra in under half a minute.

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.

The usual explanation — the one in a hundred cartoons and a few careless books — is that there is no gravity up there.

That explanation is wrong, and it is worth being precise about why, because the true reason is far more interesting.

The 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/kg — nearly 89 % of its strength on the ground.

An astronaut who weighs 600 N standing in Bengaluru still weighs about 530 N aboard the station. Gravity has barely noticed they left.

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.

• 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 %.
• Muscles shrink, especially the big ones in the legs and back that normally spend all day holding you upright against gravity.
• 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.
• 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.

The 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.

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.

• 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.
• 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.
• 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.
• 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.
• 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.

Every one of those is the same rule that dropped your pencil: masses pull on masses.