Gravity, the sculptor
Gravity is the weakest force and still shapes everything big. See why an orbit is a fall that never lands, why large bodies are round, and how a nearly smooth universe turned into stars and galaxies.
THE RULE
Everything pulls on everything
Gravity is a pull between any two things that have mass. Two facts set its strength. More mass, more pull. And more distance, much less pull: move twice as far away and the pull drops to a quarter; three times as far, a ninth. It gets weak quickly, but it never reaches zero.
You pull on the Earth exactly as hard as it pulls on you. The Earth just has so much mass that it barely notices, while you notice a lot.
Check yourself
- A probe 10,000 km from a planet's centre feels a pull of 16 units.
- The same probe at 20,000 km from the centre feels a pull of 4 units.
What pull will it feel at 40,000 km?
- 2 units
- 1 unit
- 8 units
- 0 units
Show the answer
1 unit
Right. Each doubling of distance cuts the pull to a quarter: 16, then 4, then 1. It shrinks fast but does not vanish.
Newton's cannon: how to fall and keep missing
- Fire a cannonball sideways from a mountain
It curves down and lands some distance away. Gravity pulled it to the ground.
- Fire it faster
It lands farther away. While it falls, the ground below is also curving away, because the Earth is a ball.
- Find the magic speed
In one second, anything falls about 5 metres. The Earth's surface curves down by about 5 metres for every 8 km you travel. So at about 8 km per second, the ball drops 5 m and the ground drops 5 m with it.
- That is an orbit
The ball is falling the whole time and never gets any closer to the ground. It circles the planet. Nothing holds it up. It is simply falling around the Earth instead of into it.
Check yourself
Roya watches a video of astronauts floating inside the space station and asks why they don't fall. Which answer is right?
- They are falling, together with the station, around the Earth
- They are too far from Earth for its gravity to reach them
- There is no gravity once you leave the atmosphere
- The air pressure inside the station holds them up
Show the answer
They are falling, together with the station, around the Earth
Yes. The station and everyone in it are in continuous free fall at about 28,000 km/h, circling the Earth every 90 minutes. When you and the floor fall together, the floor doesn't push on your feet, and that is what floating is.
radius of the Earth = 6,400 km
height of the space station = 400 km
distance from Earth's centre = 6,800 km
how much farther than the ground? 6,800 / 6,400 = 1.06 times
pull weakens by that number squared: 1.06 x 1.06 = 1.13
gravity at the station = 1 / 1.13 of gravity on the groundabout 0.89, so roughly 90% of what you feel right nowWhat counts is your distance from the Earth's centre. Going up 400 km only moves you 6% farther from it.
Check yourself
The space station is falling toward the Earth every moment it is in orbit, and gravity at its height is still about 90% of what you feel on the ground.
Show the answer
True
True, and both halves of it surprise people. Gravity barely weakens over 400 km, so the station is pulled almost as hard as you are. It stays up because it also moves sideways at about 8 km/s, so it keeps falling past the Earth instead of into it. Take that sideways speed away and it drops straight down.
SHAPE
Why big things are round
Gravity pulls every part of a body toward its centre. On a small body, the strength of rock easily resists that pull, so it can be any shape. On a big enough body, the weight of the material overwhelms the rock: over time high parts sink, low parts fill in, and you get the one shape where everything is as close to the centre as it can be. That shape is a sphere. The changeover happens at a few hundred kilometres across.
Earth's highest mountain is under 9 km tall on a ball 12,700 km wide. Mars has weaker gravity, about 38% of Earth's, and carries a volcano more than 20 km high.
Who wins: rock or gravity?
Small bodies
Asteroids, comets and small moons, up to tens of kilometres across. Their gravity is feeble, and you could jump off some of them. Rock wins, so they look like potatoes, peanuts and rubble piles.
Large bodies
Large moons, planets and stars. Gravity wins and pulls them into spheres. Spin makes them bulge slightly at the equator, but no planet is a cube and no star is a potato.
Check yourself
Would gravity have made it round?
- The Sun
- A comet nucleus 5 km across
- Our Moon, about 3,500 km across
- Phobos, a moon of Mars about 22 km across
- Ceres, a dwarf planet about 940 km across
- An asteroid 300 metres across
Show the answer
Round: The Sun, Our Moon, about 3,500 km across, Ceres, a dwarf planet about 940 km across
Irregular: A comet nucleus 5 km across, Phobos, a moon of Mars about 22 km across, An asteroid 300 metres across
STRUCTURE
The rich get richer
The young universe was filled with gas spread almost perfectly evenly. Almost. A patch that was very slightly denser than its surroundings had slightly more gravity, so it pulled in a little extra gas. That made it denser still, so it pulled harder. Given hundreds of millions of years, this runaway turned faint ripples into gas clouds, stars, galaxies and clusters of galaxies, with nearly empty voids between them.
Gravity is by far the weakest of nature's forces: a fridge magnet beats the whole Earth's pull on a paperclip. But it only ever attracts, never cancels out, and it reaches across any distance. On the largest scales nothing else competes.
Gravity in three jobs
- It steers. Sideways speed plus a constant fall gives an orbit: moons around planets, planets around stars, stars around the centre of a galaxy.
- It shapes. Above a few hundred kilometres across, it crushes bodies into spheres.
- It gathers. It amplifies tiny differences in density until smooth gas becomes clouds, and clouds become stars. That is where the next lesson begins.
Check yourself
Picture two patches of gas in the early universe. Patch A is 1% denser than its surroundings. Patch B is exactly average. What does gravity do over the next few hundred million years?
- It evens things out, so A thins down to average
- Both collapse at the same rate, since gravity pulls on every gram of gas equally
- Nothing: a 1% difference is too small for gravity to act on
- A pulls in nearby gas and grows denser, while the space around it empties
Show the answer
A pulls in nearby gas and grows denser, while the space around it empties
Right. A small head start means slightly more pull, which means more gas, which means more pull. Gravity makes differences grow. That is how a nearly smooth universe ended up lumpy.
Lesson recap
- Gravity grows with mass and weakens with distance: twice as far means a quarter of the pull. It never reaches zero.
- An orbit is a continuous fall with enough sideways speed (about 8 km/s near the Earth) to keep missing the ground.
- Astronauts float because they are falling with their station, not because gravity is absent.
- Bodies larger than a few hundred kilometres are round because their own gravity overwhelms the strength of rock.
- Gravity amplifies tiny density differences, turning nearly smooth gas into clouds, stars and galaxies.