Gravity

Pick something up — a pencil, your shoe, an apple — and let go. It drops straight to the floor. It never drifts up to the ceiling. It never floats out sideways through the window. Down, every single time.

That never-resting downward tug is gravity: the whole Earth quietly pulling on everything near it, all the time, always towards the ground. You can't see it, you can't switch it off, and it never gets tired. Watch the apple.

Gravity grabs everything

Gravity doesn't only pull things that are touching the ground. It reaches up and drags the raindrops out of the clouds. It hauls a thrown ball back down. It holds you in your seat, keeps the sea lying in its bed, and pins the whole atmosphere around the planet so the air doesn't drift off into space.

Anything made of stuff gets pulled — you, your dog, a mountain, a grain of sand, a feather. And gravity reaches a very long way: the Earth's pull stretches all the way out to the Moon and holds it looping around us, month after month, without a rope in sight.

The great surprise: heavy and light fall together

Here is something that fooled clever people for two thousand years. Drop a heavy hammer and a light feather at the same moment — which lands first?

"The hammer, obviously." And on an ordinary day, it does. But only because the feather catches on the air and dawdles down, wafting from side to side. Take the air away, and the hammer and the feather fall exactly together and hit the ground at the very same instant. Gravity speeds up everything at the same rate, no matter how heavy it is.

It sounds too strange to be true, so in 1971 an astronaut proved it — live on television. On the airless surface of the Moon, Commander David Scott of Apollo 15 held out a heavy geology hammer in one hand and a light falcon feather in the other, and let go of both at once. With no air to slow the feather, they fell side by side and landed together in the grey dust. "How about that," he said. Galileo — who first worked the idea out, roughly 400 years ago — would have loved it.

Weight: how hard gravity is pulling

Gravity's pull is so important that we measure it. The strength of gravity's pull on a thing is called its weight. The more stuff packed into something — we call the amount of stuff its mass — the harder gravity heaves on it, so the more it weighs.

A bowling ball is crammed with stuff, so gravity hauls on it hard: heavy. A beach ball is nearly all air, so gravity barely bothers: light. In the box below, drag the mass up and watch the downward weight arrow stretch. (Ignore the world switch for a moment — we'll use it next.)

Somewhere the pull is weaker

Gravity isn't equally strong everywhere. The Moon is a much smaller ball than the Earth, so its gravity is much weaker — about six times weaker. Now flip the switch in the box above from Earth to the Moon. The object doesn't change — it's the same rock with the same stuff inside — but its weight arrow shrinks right down, because the Moon simply can't pull as hard.

That is why the Apollo astronauts bounced across the Moon in those slow, floating leaps: their weight was tiny, but their muscles were still Earth-strong. A child who could just about lift one friend on Earth could lift six friends on the Moon. Same child, same friends — only the strength of gravity has changed.

Mass is how much stuff is in something. It doesn't change — you have the same mass on Earth, on the Moon, or floating in space.
Weight is how hard gravity pulls on that stuff. It does change: weaker gravity means less weight, even though the stuff is the same.

These three catch almost everybody out:

Gravity is not air pushing down. There is no air in space, yet gravity is still there — it's exactly what keeps the Moon and every satellite swinging round the Earth.
Astronauts up in the Space Station are not "beyond gravity." Gravity is nearly as strong up there as down here. They float because they are falling round and round the Earth along with their spaceship — you'll see how when you meet orbits.
"Down" is not a fixed direction. Down means towards the centre of the Earth. To a child standing on the opposite side of the world, "down" points the exact opposite way to yours — and neither of you falls off, because the Earth is pulling you both inwards.

Not quite — but nearly. Isaac Newton told friends that, sheltering from the plague in his mother's garden, he watched an apple drop and fell to wondering: the same pull that tugs an apple to the ground… might it reach all the way up to the Moon? That single thought grew into the idea of gravity we still use today to steer rockets to other planets.

And here's the twist that melts most people's brains: while the Earth pulls the apple down, the apple pulls the Earth up by exactly as much. We never notice because the Earth has so stupendously much more stuff that it barely twitches — far too little to ever measure. Gravity always pulls both ways.