Facts for Kids

Gravity is the pull that makes things come together. It is a very simple idea that works from tiny rocks to giant stars: any object with mass makes a space around it that pulls other objects inward. Because of gravity the Earth holds us and the Moon, and gravity helped clouds of gas join to make stars long ago.

Scientists use different ways to describe gravity. For everyday things, Isaac Newton gave a clear rule. For very big things like stars and bending space, Albert Einstein gave a deeper picture. Both help us understand how the universe hangs together.

Gravity is an invisible pull between objects that have mass. It always pulls objects toward each other, so it is called an attractive force. A useful rule from Newton says the pull gets stronger when objects are heavier and weaker when they are farther apart. In a short form you can write this idea as

\[F = G \frac{m m'}{r^2}\]

Here, \(F\) means the pull (force), \(G\) is a fixed number that tells how strong gravity is, \(m\) and \(m'\) are the two masses (how much “stuff” each object has), and \(r\) is the distance between their centers. Another idea, the equivalence principle, means that the way mass resists motion and the way mass feels gravity are the same—so all things fall the same way when air does not get in the way.

People in different places thought about why things fall long before modern science. Aristotle, a Greek thinker, said each kind of thing wants to be in its 'natural place' and that heavier things fall faster. Later, builders and thinkers like Archimedes studied the center of weight in shapes to help with lifting and machines.

Across Asia and the Middle East some writers thought of Earth pulling things inward. For example, Brahmagupta used a word like attraction toward Earth, and Al-Biruni and Al-Khazini argued that matter is drawn to Earth’s center. These ideas slowly changed over centuries as people tested and compared results.

In the 1500s and 1600s experiments began to challenge old ideas about falling. Observers like Simon Stevin and experimenters who used ramps or swinging pendulums showed that, without air, heavy and light objects fall the same way. Isaac Newton put these ideas together and said the same force that makes apples fall also makes the Moon orbit Earth.

Newton wrote the neat rule shown earlier, and it let people calculate how planets move and how objects pull on one another. His law works extremely well for most things we meet in daily life and for planets in the sky.

General relativity is Albert Einstein’s idea that tells us gravity comes from the way big things bend the fabric of space and time. Long ago, astronomers noticed Mercury’s path around the Sun moved a tiny bit more each century than expected. People once guessed another hidden planet, but none was found. Einstein’s theory from 1915 explained Mercury’s motion by showing that the Sun curves the space and time around it, and Mercury follows that curve.

Think of a stretched sheet with a heavy ball on it: the sheet dips and smaller balls roll toward the dip. That picture helps show how mass makes a curve in space and time that we call gravity.

Gravity pulls gas together to make stars. As gas falls inward it gets denser and hotter. If the lump is small, it may become a brown dwarf or a big planet. If it is heavy enough, the center gets hot enough for nuclear fusion and a star is born. Inside a steady star, hydrostatic equilibrium means gravity pulls inward while pressure from heat pushes outward and they balance.

When stars use up their fuel, different endings happen: small cores cool into white dwarfs, bigger ones can become neutron stars, and the most massive can collapse into a black hole. Colliding stars and collapsing cores also send out gravitational waves, ripples in space-time that scientists can now detect. On the largest scale, invisible dark matter acts by gravity to pull gas into big halos, helping galaxies form and grow.

Orbit means falling around something without hitting it. A planet moves in a curved path around the Sun because the Sun’s gravity pulls it inward while the planet keeps moving forward. The Moon and our satellites do the same around Earth. In fact, both objects in space are falling toward each other a little; the path they follow depends on how close and how heavy they are.

Long ago, Galileo and Newton described these motions well. Later, Einstein showed that for very heavy or very fast situations we need the curved-space idea. Orbits can be simple ellipses, but when many bodies pull on each other they can become more complicated.