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Physical

Gravity

Gravity is the basic force of nature that draws objects with mass toward each other and gives things their weight.

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The idea

Gravity is the pull that every object with mass has on every other object. Physicists consider it one of the four fundamental interactions of nature. It is the reason a dropped ball falls and the reason you have weight. The pull is stronger between objects with more mass. It gets weaker as objects move farther apart, but it never fully stops.

On Earth, gravity pulls everything toward the center of the planet. The pull is weakest at the equator, partly because Earth spins. Falling objects speed up by about 9.780 meters per second each second at the equator and about 9.832 at the poles. Gravity also helps make water waves and tides, and it guides the way plants grow.

Compared with electrical forces, gravity is extremely weak, so it generally makes no noticeable difference between individual subatomic particles. For planets, stars and galaxies, though, it is the most important force. Gravity determines the motion of satellites, planets, stars and entire galaxies, and it even bends the path of light. It keeps the Moon and satellites circling Earth, and the planets circling the Sun. In the early universe, gravity pulled clouds of hydrogen gas together until they formed stars.

Check yourself

Question 1 of 3

At which scale is gravity the most important force?

Pick one. The first pick counts.

Composition

what it's made of

  • Mass — Gravity comes mainly from mass, so heavier objects pull harder.
  • Distance — The pull weakens with the square of the distance between two objects.
  • Gravitational constant — A tiny fixed number, G, sets the strength of the pull. Henry Cavendish's experiment of 1797–1798 gave the first accurate value, though he stated his result as the density of the Earth.
  • Weight — The force of gravity on an object, which we feel as heaviness.
  • Curved spacetime — In Einstein's picture, mass bends spacetime and objects follow the curves.

Connection

how it relates

Under Motion

Cause and effect

  1. Water Cyclestrength 80 of 100

    Gravity pulls rain and snow to the ground and makes rivers flow downhill to the sea.

  2. Climate Patternsstrength 60 of 100

    Gravity substantially contributes to weather patterns, which add up over time to climate.

  3. Ocean Currentsstrength 60 of 100

    Gravity makes colder, denser seawater sink beneath lighter water, which helps drive deep currents.

  4. Rock Cyclestrength 50 of 100

    Gravity pulls loose rock and sediment downhill, where it can pile up into new layers.

Change

how and why it evolves

Past
In his 1638 book Two New Sciences, Galileo showed that falling objects pick up speed in a regular pattern. Isaac Newton later described gravitation as a universal force. In 1915, Albert Einstein's general relativity explained gravity as the curving of spacetime.
Now
Today, Einstein's theory is used when absolute precision is needed, while Newton's law is accurate enough for nearly all ordinary calculations.
Next
Scientists will likely keep searching for a theory of quantum gravity that links gravity with quantum mechanics.

Knowledge you can walk. Read the idea, then step up, across or down.

Gravity

Gravity is the basic force of nature that draws objects with mass toward each other and gives things their weight.

Learn, discuss, create, buy or invest

The idea

Gravity is the pull that every object with mass has on every other object. Physicists consider it one of the four fundamental interactions of nature. It is the reason a dropped ball falls and the reason you have weight. The pull is stronger between objects with more mass. It gets weaker as objects move farther apart, but it never fully stops.

On Earth, gravity pulls everything toward the center of the planet. The pull is weakest at the equator, partly because Earth spins. Falling objects speed up by about 9.780 meters per second each second at the equator and about 9.832 at the poles. Gravity also helps make water waves and tides, and it guides the way plants grow.

Compared with electrical forces, gravity is extremely weak, so it generally makes no noticeable difference between individual subatomic particles. For planets, stars and galaxies, though, it is the most important force. Gravity determines the motion of satellites, planets, stars and entire galaxies, and it even bends the path of light. It keeps the Moon and satellites circling Earth, and the planets circling the Sun. In the early universe, gravity pulled clouds of hydrogen gas together until they formed stars.

Check yourself

3 quick questions, instant feedback.