Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one idea. Numbers come from NASA, ESA, the DOE and OpenStax; the pictures are not to scale.
Tap the buttons and slide the sliders.
Drag the two balls apart and change their mass. When is the pull strongest?
F ∝ m·M / r². Double the distance and watch the force drop to a quarter; the two arrows are always equal and opposite.
Pull the balls apart, then push them close. Make one ball heavy.
Arrow lengths show the pull, relative to the starting setup.
Pick air or no air, then drop them together. Which lands first?
With air, drag slows the feather. In a vacuum (the Moon, g ≈ 1.62 m/s²) both fall at the same rate: the mass cancels out.
Try it on Earth, then on the Moon. Tap Drop!
Set the speed and fire. Can you find the speed that never lands?
Below orbital speed the path hits the ground; at it, a circle; above it, a wider ellipse.
Slide the speed and fire! Can you make it go all the way around?
Not to scale: the mountain is impossibly tall so you can see it.
Drag the heavy ball and change its mass, then roll marbles past. Can you get one to orbit?
Mass curves spacetime; free objects follow the curvature. The sheet is an analogy, not a literal picture.
Only an analogy: real spacetime curves in four dimensions.
Switch gravity off, then on. What does the Moon do?
Without a force toward Earth, the Moon keeps going in a straight line at its current velocity.
Tap Gravity off. Where does the Moon go?
Gravity is an invisible pull. Everything pulls on everything else! Big, heavy things pull the hardest. Our Earth is very, very big, so it pulls you down. That's why you don't float away.
An apple falls off a tree and lands on the grass. The Moon is falling too! But it is also zooming sideways, so it keeps missing the Earth. It goes around and around.
On Earth, a feather floats down slowly, because the air holds it up. On the Moon there is no air. An astronaut dropped a hammer and a feather there, and they landed at the same time!
Throw a ball super fast from a very tall mountain, and the round Earth curves away under it. It falls and falls, and never lands. That's how the space station goes around the Earth.
Gravity is an invisible pull between things that have mass. Anything that has mass has gravity, objects with more mass pull harder, and the pull gets weaker with distance.1 Earth's gravity comes from all of its mass pulling together on the mass in your body. That pull is your weight, and it's why a dropped apple heads for the ground.1
The same pull reaches the Moon. The Moon doesn't crash into us because it's also moving sideways very fast. It falls toward Earth all the time, but Earth's round surface keeps curving away underneath it, so it keeps missing. That endless fall is an orbit.27
Isaac Newton published his law of universal gravitation in 1687.9 It says that every object in the universe pulls on every other object, along the line joining them.4 The strength of the pull depends on only two things: how much mass the objects have, and how far apart they are.4
F = G × m × M ÷ r²
Here F is the pull, m and M are the two masses, r is the distance between their centers, and G is the gravitational constant, a tiny number that's the same everywhere in the universe. Henry Cavendish first measured it accurately in 1798.4
Mass is the amount of matter in something. Weight is the pull of gravity on that mass. Your mass is the same on the Moon, but your weight is about one sixth, because the Moon's surface gravity is about one sixth of Earth's.411
Gravity always works both ways. You pull on Earth exactly as hard as Earth pulls on you. Earth has so much more mass than you that your pull makes no noticeable difference to it.15
Gravity follows an inverse square law: double the distance and the pull drops to a quarter; triple it and the pull is nine times weaker.4 The reason is geometry. The pull spreads out over the surface of an imaginary sphere around the mass, and that sphere's area grows with the square of its radius.5
| Distance from Earth's center | Strength of Earth's pull |
|---|---|
| 1× (standing on the ground) | 100% |
| 2× | 25% (¼) |
| 3× | 11% (1/9) |
| 10× | 1% (1/100) |
| About 60× (the Moon) | About 1/3,600 |
This was Newton's big clue. The Moon is about 60 times farther from Earth's center than we are, and its acceleration toward Earth is about 60 × 60 = 3,600 times smaller than a falling apple's.5 The same law explained both.
No, not when there's no air. Near Earth's surface, a falling object speeds up by about 9.8 meters per second, every second.6 A heavier object gets a bigger pull, but it also takes more pull to get it moving, and the two cancel out. So in a vacuum, everything falls at the same rate, just as Galileo had concluded hundreds of years earlier.32
At the end of the Apollo 15 mission in 1971, Commander David Scott tested it live on TV from the Moon. He dropped a 1.32 kg aluminium hammer and a 0.03 kg falcon feather from about 1.6 meters. With no air in the way, they hit the ground at the same time.3 On Earth, air resistance makes the feather drift down slowly.2
Try it at home: drop a book and a sheet of paper side by side. The paper floats down. Now lay the paper flat on top of the book and drop them together. The book pushes the air out of the way, and the paper falls just as fast.
Newton imagined a cannon on a very tall mountain.7 Fire a ball slowly, and gravity curves it down to the ground close by. Fire faster, and it lands farther away. Fire it at exactly the right speed, and Earth's surface curves away beneath the ball just as fast as the ball falls. It never lands. It's in orbit.72
Without gravity, a moving object would keep going in a straight line. Gravity bends that straight line into a curve around the planet.12 The International Space Station does exactly this, moving at about 17,500 miles per hour and going round Earth about every 90 minutes.210 The Moon is falling around Earth for the same reason.2
Yes, everywhere. Gravity gets weaker with distance but never switches off.2 The space station orbits between 200 and 250 miles up, where Earth's gravity is still about 90% as strong as on the ground. A person who weighs 100 pounds on the ground would weigh about 90 pounds at the top of a ladder that tall.2
So why do astronauts float? Because they're in free fall. The station, the crew and everything on board are all falling around Earth together, so the floor doesn't have to hold anyone up and everything floats. NASA calls this microgravity and says "zero gravity" is a misleading name.2 You can feel a few seconds of it at the top of a roller coaster hill, and NASA's training planes give 20 to 30 seconds at a time.2
Newton's law predicted gravity very well, but Newton himself admitted it only described gravity. It didn't explain what gravity is.8 In 1916 Albert Einstein offered a new picture, his general theory of relativity.5 In it, gravity isn't a pull at all. Mass and energy curve space and time, woven together into "spacetime", and objects follow those curves.8
The usual picture is a heavy ball on a stretched rubber sheet. The ball makes a dip, and a marble rolled past curves around it, either circling like a planet or falling in.9 (It's only an analogy, but a useful one.)
Einstein's theory explained things Newton's couldn't, such as the slow shift of Mercury's orbit.9 It predicted that light bends as it passes a heavy object, and astronomers saw starlight bend around the Sun during a solar eclipse in 1919.4 GPS satellites have to correct for relativity to give your position accurately.9 And in 2015, detectors picked up gravitational waves, ripples in spacetime, for the first time.9
Both theories are still used. Newton's law is simpler and good enough to put satellites in orbit and send spacecraft to other planets.8 Nobody yet has a working quantum theory of gravity. It's one of the biggest open questions in physics.8
Gravity is by far the weakest of nature's four basic forces. The others are the strong force, electromagnetism and the weak force.8 Earth has more than 13 septillion pounds of mass, yet you can still easily jump off it. But gravity has two superpowers: it builds up with mass, and it reaches across any distance.8 Over the history of the universe, it pulled scattered matter together into stars, planets and galaxies.81
Gravity shapes lots of other things we explain here: it is why the Moon goes round us and shows phases, how black holes form when a huge star collapses, and why a plane needs lift to stay up at all.
Everything that has mass pulls on everything else. The more mass, the stronger the pull, and the farther apart things are, the weaker it gets. Earth has a huge amount of mass, so it pulls you, apples and the Moon toward its center.
Yes. Gravity reaches everywhere in space and just gets weaker with distance. On the International Space Station, Earth's gravity is still about 90% as strong as on the ground. Astronauts float because they and the station are falling around Earth together.
It is falling, all the time. But it is also moving sideways so fast that Earth's curved surface keeps falling away beneath it, so it never gets closer. That endless falling-and-missing is an orbit.
Not without air. In a vacuum everything falls at the same rate. In 1971 Apollo 15 astronaut David Scott dropped a hammer and a feather on the Moon, and they hit the ground together. On Earth, air slows the feather.
The pull between two objects is proportional to both of their masses. Double one mass and the pull doubles. That's one reason Earth has about six times the Moon's surface gravity: it has far more mass (its bigger size works the other way, but mass wins).
Near Earth's surface, falling objects speed up by about 9.8 meters per second every second (ignoring air). The exact value comes from Earth's mass and size; 10 is just a handy round number for quick sums.
Newton described gravity as a force between masses but said he couldn't explain what it is. Einstein's general relativity says mass and energy curve spacetime, and objects follow those curves. Scientists still don't have a quantum theory of gravity.