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ExploreEnergy and forces

The world, a little closer

Gravity.

Every mass pulls on every other. Change how heavy two bodies are and how far apart they sit, and watch the pull set their dance.

Explore the scene Interactive article

01 / The experiment

The pull between two bodies.

Change the second mass and the distance, then reveal the well beneath them.

Interactive scene

Preparing your experiment…

The explanation below is available while the scene loads.

Circular orbits. The period follows Kepler's law exactly; sizes follow mass but are not to scale.The dipped sheet is an analogy for how strongly each place pulls, not a picture of anything real.

02 / Three pairs

Who circles whom.

The heavier the partner, the closer the balance point sits to it.

Mass ratio 1 to 330,00001

Sun and Earth

The Earth does almost all the circling. The Sun does move, by less than its own radius, which is how planets round other stars are found.

Balance point
Inside the Sun
Orbit
1 year
Who moves
The Sun, by a hair
Mass ratio 1 to 8102

Earth and Moon

The centre of mass is inside the Earth but well off its centre, so the Earth wobbles round it once a month while the Moon swings wide.

Balance point
1,700 km below the ground
Orbit
27 days
Who moves
Both, visibly
Mass ratio 1 to 803

Pluto and Charon

Charon is so large compared with Pluto that they orbit a point in the space between them. A double world rather than a planet and moon.

Balance point
In empty space between
Orbit
6.4 days
Who moves
Both, plainly
The big idea

Gravity grows with mass and fades with the square of distance. Two bodies always orbit their shared centre of mass, never simply one another.

F = G m₁m₂ / r²

Everything pulls

Every object with mass attracts every other. You pull on the Earth exactly as hard as it pulls on you; the difference is that the Earth is so heavy that your pull barely moves it, while its pull keeps you firmly on the ground. Gravity is by far the weakest of nature’s forces, but it reaches out forever and it never pushes, only pulls, so over enough mass it wins.

Stronger with mass, weaker with distance

Two things set the strength of the pull. More mass means more pull, in direct proportion: double a body’s mass and it pulls twice as hard. More distance means less pull, and quickly: double the distance and the pull drops to a quarter, triple it and the pull drops to a ninth. That is why the Sun, a third of a million times heavier than the Earth, holds us in orbit from 150 million kilometres away, and why a mountain beside you pulls so little that you never notice.

Both bodies move

Watch the scene with the second mass set small. The big body sits almost still while the small one circles it. Now make them equal. Neither sits still: both swing round a point exactly halfway between them. That point is the centre of mass, and both bodies always orbit it, the heavy one on a small circle and the light one on a large one. The Moon does not orbit the Earth’s centre; both orbit a point about 1,700 km beneath the Earth’s surface, and the Earth wobbles around it once a month.

Try it: set the second mass to Twins, then bring them close. Then slide the distance out and watch the orbit slow.

Falling forever

An orbit is a fall that keeps missing. The Moon is falling toward the Earth all the time; it also happens to be moving sideways fast enough that the ground curves away beneath it as quickly as it falls. Throw a ball harder and it lands farther away; throw it at eight kilometres a second and it never lands at all. Every satellite is a thrown ball moving that fast.

Farther out, gravity is weaker, so a body does not need to move as fast to stay in orbit, and it has farther to go. Both effects make distant orbits slow. Kepler found the exact pattern four centuries ago: the square of the period grows with the cube of the distance.

The well

Reveal the sheet and the two bodies sit in dips, deep near each mass and shallow far away. It is a picture, not a mechanism, but a useful one. The depth at any point shows how strongly something there would be pulled inward and how much energy it would take to climb out. A planet’s orbit is a marble rolling round the side of a bowl, never quite reaching the bottom.