Gravity is the invisible tug between everything that has stuff in it. Step onto another world and that tug changes — so you'd weigh something completely different on the Moon than on Jupiter. Let's drop a few things and find out.
Let it fallGravity is an invisible pull between any two objects that are made of stuff. The more stuff an object has, the harder it pulls — and the closer you are, the stronger the tug feels.
That one quiet rule is doing an enormous amount of work right now. It's why you're sitting in your chair instead of drifting off into the ceiling. It's why a dropped pencil heads straight for the floor. And it's why the Moon keeps looping around the Earth, year after year, instead of wandering off into space. Same pull, three very different jobs.
Here's the surprise this whole page is built around: gravity isn't the same everywhere. Carry yourself to the Moon and you'd feel feather-light. Stand on giant Jupiter and you'd feel crushingly heavy. You haven't changed one bit — but the pull has. Let's go feel the difference for ourselves.
Let's get the big word out of the way first. Mass is just a measure of how much stuff an object is made of — how many atoms are packed inside it. We measure it in kilograms (kg). A feather has a tiny mass; you have a medium one; the Earth has an absolutely colossal one.
Now the magic rule: anything with mass pulls on anything else with mass. Your pencil pulls on your rubber. You pull on your phone. You even pull on the entire Earth! The reason you never notice most of these tugs is that the pull only gets big when the masses are big. Two everyday objects pull on each other so faintly that you'd never feel it. But put one truly gigantic mass in the mix — like a planet — and suddenly the pull is impossible to ignore.
So gravity follows two simple patterns, and they're worth keeping in your pocket for the rest of the page:
Pile on more stuff and the tug grows. A planet pulls hard; a pebble barely pulls at all.
Move two objects nearer to each other and the pull strengthens. Drift far apart and it fades.
You pull the Earth exactly as hard as it pulls you — but it's so massive that you do all the moving.
That last one trips people up, so it's worth a beat. When you jump, the Earth really does get tugged up toward you — but the Earth is so unimaginably more massive than you that its movement is far too tiny to ever measure. You, on the other hand, get yanked back down in a heartbeat. The pull is shared equally; the response is wildly lopsided. We'll play with that exact idea further down.
Pick a world for the left side and a world for the right, then hit Drop on both. It's the very same object each time — only the gravity changes. Watch which one wins the race to the ground, and check the readouts to see how long the fall takes and what a scale would say you weigh there.
Same object, two gravities. The world with the stronger pull drags the object down faster — so it lands first. Try sliding the mass: notice the fall time never budges. Heavier objects don't fall any faster!
Did you spot the sneaky lesson hiding in there? You can crank your object's mass all the way up, and the time to fall never changes on a given world. A 1 kg ball and a 20 kg ball hit the ground together. The only thing that changes the fall is which world you're standing on — that is, how strong the gravity is. (We'll come back to why heavy things don't fall faster; it surprises almost everyone.)
And look how wildly the worlds differ. On the Moon, with about one-sixth of Earth's pull, the object drifts down in lazy slow-motion. On Mars (around a third of Earth's gravity) it's quicker but still gentle. Jupiter's pull is roughly two and a half times Earth's, so things slam down fast. And the Sun's gravity is so monstrous — about 28 times Earth's — that the same drop is over almost before it begins.
So why do you get glued to the ground while the Moon just circles overhead? It's the same gravity doing both — the difference is all about how much sideways speed something has.
Start with you. The Earth is staggeringly massive, and you're standing right on it — about as close as you can get. By those two rules from earlier (more mass, and closer), the Earth's pull on you is strong and constant. You have basically no sideways speed compared to the planet, so the pull simply holds you flat against the ground. That downward hold is what you feel as your weight.
Now the Moon. The Moon is being pulled toward the Earth too — gravity doesn't switch off just because something is far away. So why doesn't it crash into us? Because the Moon is also racing sideways, incredibly fast. Picture throwing a ball: the harder you throw it, the farther it travels before gravity curves it down to the ground. Now imagine throwing it so unbelievably fast that, as it falls, the ground curves away beneath it just as quickly. It would keep falling forever and never land. That's an orbit — and it's exactly what the Moon is doing. The Moon is perpetually falling toward Earth and perpetually missing, looping around us instead of dropping on us.
An orbit isn't gravity being switched off. It's gravity plus a huge sideways speed — falling and missing, over and over. The Earth orbits the Sun for the very same reason.
This is the single most useful idea on the whole page, so let's be really clear. Mass and weight sound like the same thing in everyday talk, but to a scientist they're two different ideas.
Mass is how much stuff you're made of, in kilograms. It never changes. The same number of atoms make up your body on Earth, on the Moon, or floating in deep space. Weight is something else: it's how hard gravity is pulling on that stuff. Weight is a force, and it changes the moment the local gravity changes. (Scientists measure force in units called newtons, but a bathroom scale is built to translate that pull back into friendly kilograms — so we'll just read off "what the scale says.")
A simple way to hold the two ideas together: weight = mass × gravity. Your mass is fixed, so it's the gravity part that does all the changing as you travel. Slide your mass below and visit a few worlds to feel it.
On Earth, the scale reads your full weight — this is the gravity you grew up with.
Watch what moved and what didn't. Your mass sat perfectly still — same stuff, every world. But the scale reading swung all over the place, because the strength of gravity changed under your feet. On the Moon you'd weigh about a sixth of your Earth weight and could bounce around in giant slow-motion leaps. On the Sun's surface, gravity would press down so hard you couldn't even stand. Same you, completely different pull — that's the heart of this whole topic.
Here are two objects floating in space. Make each one heavier with the sliders, and drag them around to move them closer or farther apart. The arrows are the gravity pull between them — watch how they grow when you add mass or close the gap, and shrink when you pull the objects apart.
The two arrows are always the same length — the objects tug each other equally, even when one is much heavier. Add mass or move them closer and both arrows grow together.
Two things should jump out. First, adding mass to either object makes the pull stronger. Second, moving them closer makes it stronger too — and the effect of distance is dramatic, growing fast as they near each other. That's exactly why a whole planet beneath your feet grips you so firmly, while the same planet's pull on a far-off spacecraft is gentle enough to let it coast for years.
And notice the arrows stay matched in length no matter how lopsided the masses get. That's the two-way-street rule again: gravity is always a mutual hug, never a one-sided shove. The tiny object pulls the giant just as hard as the giant pulls the tiny one.
You felt it in the first toy: changing your object's mass didn't change the fall time at all. That feels deeply wrong. Surely a bowling ball should beat a grape to the floor? Drop them on your desk and the bowling ball really does seem to win. So what's going on?
The culprit is air resistance — the air pushing back on things as they fall through it. Air shoves up much harder on a wide, light thing (a feather, a leaf, a sheet of paper) than on a small, dense thing (a coin, a marble). That extra push is what makes a feather drift down slowly. It has nothing to do with gravity playing favourites.
Take the air away, and the illusion vanishes completely. In a sealed chamber with the air pumped out — a vacuum — a feather and a hammer dropped together fall at exactly the same rate and hit the bottom at the same instant. Astronauts actually did this for real on the airless Moon: a hammer and a feather, released together, landed together. In the same gravity, everything falls at the same rate, whatever its mass.
Why does this happen? Here's the gentle version. Gravity pulls harder on a heavier object — but a heavier object is also harder to get moving in the first place (it has more stuff to shift). Those two effects cancel out perfectly, so every object ends up speeding up at the same rate. Stronger pull, but more stubbornness to overcome — and it's always a tie. That's the deep reason your drop-toy never cared how heavy the object was.
Your everyday experience is a brilliant teacher and an occasional liar. Two beliefs about gravity feel completely obvious and are completely false — and spotting them is half of really understanding the topic.
"Heavier things fall faster."
In the same gravity, everything falls at the same rate, whatever its mass. A feather only loses a race to a coin because air pushes back on it. Remove the air and they fall as one. Mass changes your weight, never your falling speed.
"There's no gravity in space."
Space is full of gravity — it's exactly what keeps the Moon circling Earth and Earth circling the Sun. Astronauts on the Space Station look weightless not because gravity is gone, but because they and their station are falling around the Earth together, in a never-ending orbit. Everything falls at the same rate, so nothing presses against anything — and that floating feeling is really endless falling.
Both traps come from trusting only what you see on the ground, where air and the Earth's giant pull hide the simpler truth. Once you imagine the air switched off, or picture an orbit as endless falling, the real rules click into place.
Three quick questions. Pick an answer and the reason pops up — no pressure, you can't get it "wrong" for long.
Mass is how much stuff you're made of, and that doesn't change when you travel. Only your weight — how hard gravity pulls — gets smaller on the Moon.
With no air to slow the feather, both objects speed up at the same rate and land at the same instant. Mass doesn't change how fast something falls.
Gravity absolutely reaches the Moon — that's what holds it in orbit. The Moon's huge sideways speed means it's forever falling toward Earth and forever missing, looping around instead of landing.
Anything with mass pulls on anything else. More mass and closer distance both mean a stronger tug.
Your mass stays the same everywhere, but your weight follows the local gravity — light on the Moon, crushing on the Sun.
In the same gravity, everything falls at the same rate. A feather and a hammer land together with no air.