Gravity

Gravity, also called gravitation, is a basic force that pulls objects toward each other. It is one of the four main forces of the universe and helps shape everything around us. Gravity keeps us on the ground and plays a part in many things we see.

In the early universe, gravity brought together clouds of hydrogen and dark matter. This helped form stars, galaxies, and larger structures. Even though gravity gets weaker with distance, it never truly stops. We understand gravity using Einstein’s ideas about the curvature of spacetime caused by mass. For everyday use, Newton's law of universal gravitation works well. It tells us that every object pulls on every other object. The strength of this pull depends on how heavy the objects are and how far apart they are.

On Earth, gravity gives us weight and affects many natural things, like ocean tides and water waves. It also helps plants grow and guides how fluids move in living things. Scientists are still studying gravity to learn how it fits with other ideas about the very small parts of the universe.

Characterization

Gravity is the force that pulls objects toward each other. Every object with mass—like a planet or a star—pulls on other objects. The more mass an object has, and the closer two objects are, the stronger this pull becomes.

Gravity is one of the four main forces in the universe. It is very weak for tiny things, but it is very important for large objects like planets, stars, and galaxies. It keeps satellites moving around Earth and helps control how everything moves in space.

History

Main article: History of gravitational theory

People have wondered about gravity for thousands of years. In Ancient Greece, Aristotle thought objects move to certain places — like rocks falling down.

Later, during the Scientific Revolution, scientists tested ideas about gravity. They learned that all objects fall at the same speed if you ignore wind. This changed how people thought about how things move.

Then, Isaac Newton made a theory about how gravity keeps planets moving in circles and makes apples fall. His ideas worked very well for a long time. After that, Albert Einstein made a new theory called general relativity. It helped explain things Newton couldn’t, like how the planet Mercury moves. Einstein’s work helped scientists learn about black holes and space-time waves.

On Earth

Every planet, including Earth, has gravity that pulls objects toward it. This pull depends on how big the planet is and how far you are from its center. Near Earth, gravity makes things fall, and scientists use a standard number for this force in their work.

Gravity is weakest at the equator because Earth's spin creates a centrifugal force that works against gravity a little. As you move closer to the poles, gravity gets slightly stronger. Gravity also affects ocean waves and the movement of air in the sky.

Orbits

Main article: Orbit

Planets orbit the Sun in an ellipse because of gravity. The Moon and artificial satellites also orbit the Earth for the same reason. Gravity pulls objects together, keeping them moving in paths around larger bodies. This pull can make the paths of planets change a little.

Astrophysics

Main article: Star formation

Gravity helps form stars and shapes the universe. When clouds of hydrogen gas come together, gravity pulls them closer. If there is enough gas, it can become hot and dense enough to start burning, creating a star. After a star uses up its fuel, it can become different kinds of objects, like a white dwarf, a neutron star, or even a black hole where gravity is very strong.

Gravity also affects light. Very massive objects can bend the path of light passing near them. This effect helps scientists find hidden matter in space, called dark matter, which we cannot see but whose gravity affects how things move.

Models

Physicists use different models to explain gravity, depending on what they want to understand. One model is Newton's inverse square law. This model explains how objects pull toward each other based on their mass and distance. It works well in many situations, but it doesn’t explain why gravity happens.

Another model uses fields. A field shows how gravity affects every point in space. This helps us see how objects influence each other from far away.

A third model uses action principles. This describes gravity in a more abstract, mathematical way. It looks at how the energy of a system changes, instead of focusing on individual forces or fields.

Main article: Gravitational field

Main article: Action principles

General relativity

See also: Introduction to general relativity

In modern physics, general relativity is our best idea about how gravity works. It says that gravity bends space and time, because of mass and energy. Scientists test this idea, and it fits what we see in space very well.

General relativity fits with the rules of special relativity and matches what we see in the universe. It tells us that gravity is not a force, but the shape of space and time. This helps us understand how planets and stars move. Even though it is tricky, it works better than older ideas about gravity.