Gravity, in mechanics, is the universal force of attraction that acts between all bodies of matter. It is the weakest known force in nature and plays no role in determining the internal properties of everyday matter. However, through its long reach and universal action, it controls the trajectories of bodies in the solar system and elsewhere in the universe, as well as the structures and evolution of stars, galaxies, and the cosmos. Isaac Newton proposed in 1687 that the same invisible force pulling an apple to the ground also keeps the Moon in orbit around Earth. His concept of universal gravitation stated that all objects with mass attract each other, with the pull weakening as they get farther apart. Albert Einstein 's theory of general relativity describes gravity as a geometric phenomenon arising from the curvature of space-time.

How does gravity affect objects on Earth?

Gravity is a universal force of attraction between all objects. On Earth, gravity gives objects weight, which is a downward force proportional to their mass. The acceleration of gravity on Earth's surface is about 9.8 meters (32 feet) per second squared, meaning an object's speed increases by about 9.8 meters per second every second it falls. Without gravity, Earth would not have tides. Projectile motion, like that of a ball, is also affected by Earth's gravity.

Why do objects fall to the ground when dropped?

Objects fall to the ground due to gravity, the universal force of attraction between all matter. Isaac Newton explained that the magnitude of this attractive force (F) is proportional to the masses of the objects ((m_1) and (m_2)) and inversely proportional to the square of the distance (R) between them: (F = G(m_1m_2)/R^2). On Earth, gravity gives all objects a downward acceleration of approximately 9.8 meters (32 feet) per second squared. This means an object's speed increases by about 9.8 meters per second every second it falls.

Who discovered the law of gravity?

Isaac Newton formulated the law of universal gravitation in 1687, stating that any two objects attract each other with a force proportional to their masses and inversely proportional to the square of the distance between them. Newton's work built upon earlier insights from Galileo Galilei , who studied acceleration, and Johannes Kepler , who described planetary motion. Newton tied their insights together into one theory that explained why objects move the way they do in both the sky and on the ground. His theory remained the leading explanation of gravity for centuries, until Albert Einstein expanded on it in the 20th century.

How does gravity keep planets in orbit around the Sun?

The Sun's gravity keeps planets in orbit, acting as a centripetal force that constantly pulls them towards it. Without this force, a planet would move in a straight line due to inertia. The gravitational force between the Sun and a planet depends on their masses and the distance between them. Einstein's theory of general relativity explains gravity as a result of the shape of space-time. Massive objects like the Sun curve space-time, causing planets to move along these curves, which we observe as orbits.

How does gravity change when you move further away from a planet?

The force of gravity weakens as you move away from a planet. Isaac Newton mathematically described this relationship in his law of universal gravitation. Gravity is proportional to the product of the masses and inversely proportional to the square of the distance between their centers. This means that if you double the distance from the planet, the gravitational force becomes one-fourth of what it was originally. This is known as the inverse square law.

gravity

Introduction
Development of gravitational theory
- Early concepts
- Newton’s law of gravity
  - Weight and mass
  - Interaction between celestial bodies
- Potential theory
  - Effects of local mass differences
  - Weighing the Earth
Acceleration around Earth, the Moon, and other planets
- Variations in g
  - Changes due to location
  - Changes with time
- Measurements of g
  - Unit of gravity
  - Absolute measurements
  - Relative measurements
- Gravimetric surveys and geophysics
- The Moon and the planets
Gravitational theory and other aspects of physical theory
- Field theories of gravitation
- Gravitational fields and the theory of general relativity
- The paths of particles and light
- Gravitational radiation
Some astronomical aspects of gravitation
Experimental study of gravitation
- The inverse square law
- The principle of equivalence
- The constant of gravitation
- The variation of the constant of gravitation with time
- Fundamental character of G

References & Edit History Quick Facts & Related Topics

Images & Videos

How gravity works on Earth and in the universe

Diagram showing the Moon orbiting Earth, with arrows illustrating gravitational forces between them and how gravity alters the Moon's natural path, resulting in its curved orbit around the planet.

Diagram showing how gravity decreases with distance from Earth, with a series of labeled distances extending from the planet. For each distance, values are given for gravitational acceleration and the weight of a 100-pound person at that location.

variation in Earth's gravitational field

View a demonstration to understand Albert Einstein's general theory of relativity

experimental evidence for general relativity

How are gravitational waves applied in science and in everyday life?

Illustration of a space-based long-range interferometer system for detecting gravitational waves, showing three spacecraft in triangular formation exchanging laser beams, with labeled components including interferometers, solar arrays, thrusters, and the path of gravity waves, as well as Earth's position and orbital path in the background.

At a Glance

gravity summary

gravity

physics

Also known as: g-force, gravitation

Written by

Kenneth L. NordtvedtAll

Fact-checked by

Britannica Editors

Last updated

Nov. 12, 2026 •History

News •

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gravity, in mechanics, the universal force of attraction acting between all bodies of matter. It is by far the weakest known force in nature and thus plays no role in determining the internal properties of everyday matter. On the other hand, through its long reach and universal action, it controls the trajectories of bodies in the solar system and elsewhere in the universe and the structures and evolution of stars, galaxies, and the whole cosmos. On Earth all bodies have a weight, or downward force of gravity, proportional to their mass, which Earth’s mass exerts on them. Gravity is measured by the acceleration that it gives to freely falling objects. At Earth’s surface the acceleration of gravity is about 9.8 meters (32 feet) per second per second. Thus, for every second an object is in free fall, its speed increases by about 9.8 meters per second. At the surface of the Moon the acceleration of a freely falling body is about 1.6 meters per second per second.

The works of Isaac Newton and Albert Einstein dominate the development of gravitational theory. Newton’s classical theory of gravitational force held sway from his Principia, published in 1687, until Einstein’s work in the early 20th century. Newton’s theory is sufficient even today for all but the most precise applications. Einstein’s theory of general relativity predicts only minute quantitative differences from the Newtonian theory except in a few special cases. The major significance of Einstein’s theory is its radical conceptual departure from classical theory and its implications for further growth in physical thought.

The launch of space vehicles and developments of research from them have led to great improvements in measurements of gravity around Earth, other planets, and the Moon and in experiments on the nature of gravitation.

Development of gravitational theory

Early concepts

Newton argued that the movements of celestial bodies and the free fall of objects on Earth are determined by the same force. The classical Greek philosophers, on the other hand, did not consider the celestial bodies to be affected by gravity, because the bodies were observed to follow perpetually repeating nondescending trajectories in the sky. Thus, Aristotle considered that each heavenly body followed a particular “natural” motion, unaffected by external causes or agents. Aristotle also believed that massive earthly objects possess a natural tendency to move toward Earth’s center. Those Aristotelian concepts prevailed for centuries along with two others: that a body moving at constant speed requires a continuous force acting on it and that force must be applied by contact rather than interaction at a distance. These ideas were generally held until the 16th and early 17th centuries, thereby impeding an understanding of the true principles of motion and precluding the development of ideas about universal gravitation. This impasse began to change with several scientific contributions to the problem of earthly and celestial motion, which in turn set the stage for Newton’s later gravitational theory.

The 17th-century German astronomer Johannes Kepler accepted the argument of Nicolaus Copernicus (which goes back to Aristarchus of Samos) that the planets orbit the Sun, not Earth. Using the improved measurements of planetary movements made by the Danish astronomer Tycho Brahe during the 16th century, Kepler described the planetary orbits with simple geometric and arithmetic relations. Kepler’s three quantitative laws of planetary motion are:

Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.

Britannica Quiz

Physics and Natural Law

The planets describe elliptic orbits, of which the Sun occupies one focus (a focus is one of two points inside an ellipse; any ray coming from one of them bounces off a side of the ellipse and goes through the other focus).
The line joining a planet to the Sun sweeps out equal areas in equal times.
The square of the period of revolution of a planet is proportional to the cube of its average distance from the Sun.

During this same period the Italian astronomer and natural philosopher Galileo Galilei made progress in understanding “natural” motion and simple accelerated motion for earthly objects. He realized that bodies that are uninfluenced by forces continue indefinitely to move and that force is necessary to change motion, not to maintain constant motion. In studying how objects fall toward Earth, Galileo discovered that the motion is one of constant acceleration. He demonstrated that the distance a falling body travels from rest in this way varies as the square of the time. As noted above, the acceleration due to gravity at the surface of Earth is about 9.8 meters per second per second. Galileo was also the first to show by experiment that bodies fall with the same acceleration whatever their composition (the weak principle of equivalence).

gravity

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Development of gravitational theory

Early concepts