Gravity is the physical interaction associated with mass and energy that produces attraction between objects and, in modern physics, is described as a manifestation of spacetime curvature caused by the distribution of mass-energy. In everyday mechanics, gravity is usually introduced as a force that attracts objects towards one another.
Near the surface of Earth, the gravitational force acting on an object is commonly represented by F = mg, where F is gravitational force, m is mass and g is the local gravitational acceleration. More generally, Newton's law of universal gravitation gives F = Gm₁m₂ divided by r², where G is the gravitational constant, m₁ and m₂ are the interacting masses and r is the distance between their centres of mass. Gravity is responsible for falling objects, planetary orbits, satellite trajectories, tides, the weight of objects, and much of the large-scale structure of astronomical systems.
The historical development of the concept of gravity involved a major transition from qualitative descriptions of falling objects to mathematical physical laws. Aristotle associated natural motion with the intrinsic properties of bodies, whereas Galileo investigated falling motion experimentally and established important relationships concerning acceleration. Isaac Newton subsequently formulated universal gravitation in the seventeenth century, showing that the same mathematical interaction could account for falling bodies and celestial orbital motion.
Newton's theory treated gravity as a force acting between masses and successfully explained planetary motion when combined with his laws of motion. In the twentieth century, Albert Einstein's general theory of relativity provided a deeper description in which gravity is not treated as an ordinary force in the Newtonian sense but as the curvature of spacetime produced by matter and energy. Newtonian gravity remains an extremely accurate and useful approximation for many engineering, educational and astronomical calculations where relativistic effects are negligible.
Near Earth's surface, gravity produces an acceleration conventionally denoted by g, with a standard reference value of approximately 9.80665 metres per second squared. The actual local value varies because Earth is rotating, Earth is not a perfect sphere, and the distribution of mass within the planet is not uniform. Gravity is therefore slightly different at different latitudes and elevations. The effective downward acceleration experienced by an object also incorporates effects associated with Earth's rotation.
In elementary calculations, g is frequently rounded to 9.8 m/s² or sometimes 10 m/s² for convenience. Gravitational acceleration is independent of an object's mass when air resistance is neglected, which means that two objects released simultaneously in the same gravitational field undergo the same ideal free-fall acceleration. This does not mean that gravity exerts the same force on different masses, because gravitational force is proportional to mass.
Gravity has a central role in orbital mechanics because an orbit can be understood as continuous free fall combined with sufficient tangential velocity. A satellite in a circular orbit is constantly accelerating towards the centre of the body it orbits, but its tangential motion causes its trajectory to curve around the body rather than allowing it simply to fall onto its surface.
The orbital speed required for a circular orbit depends on the central body's mass and the orbital radius, and gravitational attraction supplies the centripetal acceleration required for the orbit. Escape velocity is another gravitationally determined quantity: it is the minimum ideal speed required for an object to escape permanently from a body's gravitational field without additional propulsion, ignoring atmospheric drag and other complications. For a spherical body in Newtonian mechanics, escape velocity is related to gravitational potential and is proportional to the square root of 2GM divided by r.
Gravity also produces gravitational potential energy. For objects close to Earth's surface, the change in gravitational potential energy is commonly approximated by ΔU = mgΔh, where Δh is the change in vertical height. This relationship is an approximation based on the gravitational field being effectively constant over the distance involved.
For larger distances from Earth or other astronomical bodies, gravitational potential energy is more accurately represented by U = −GMm/r when the zero of potential energy is chosen at infinite separation. The negative sign reflects the convention that a bound gravitational system has negative potential energy relative to the chosen zero. Raising an object increases its gravitational potential energy because external work is required against gravity. When the object falls, gravitational potential energy can be converted into kinetic energy, assuming negligible losses and appropriate system boundaries.
Gravity is often confused with weight, but the two concepts are not identical. Mass is a measure of an object's inertia and, in classical physics, its gravitational mass determines the strength of its gravitational interaction. Weight is the force associated with gravity acting on that mass and is commonly calculated near Earth's surface as W = mg.
An object's mass remains essentially unchanged when it is transported from Earth to the Moon, while its weight changes because the local gravitational field is weaker on the Moon. Apparent weight can also differ from ordinary gravitational force because acceleration of the reference frame and contact forces influence what a scale measures. Astronauts in orbit are therefore often described as experiencing weightlessness even though Earth's gravity remains substantial at orbital altitude; they are in continuous free fall and lack the supporting contact force that ordinarily produces a scale reading.
On astronomical scales, gravity governs the formation and behaviour of stars, planets, moons, galaxies and larger structures. Gas clouds can collapse under self-gravity, with gravitational potential energy being converted into thermal energy during contraction. In stars, gravity compresses material sufficiently for pressure and other physical processes to balance gravitational contraction. Gravity controls the trajectories of planets and comets, contributes to tidal phenomena, and plays a fundamental role in the formation of accretion discs around compact objects.
Extremely strong gravitational fields occur near neutron stars and black holes, where general-relativistic effects become essential. Gravitational time dilation means that clocks at different gravitational potentials do not necessarily accumulate the same amount of proper time relative to one another. Such effects are not merely theoretical curiosities; relativistic corrections are required for high-precision systems such as satellite-based navigation.
Gravity has limitations as a simple explanatory model when it is treated exclusively as a constant downward force. The approximation g = 9.8 m/s² is appropriate for many near-surface problems but cannot accurately describe situations involving large distances, significant changes in altitude, rapid rotation, strong gravitational fields or relativistic velocities. Air resistance, buoyancy, Earth's rotation and local geological variations can also influence observed motion even though they are not part of the simplest gravitational model.
General relativity further shows that the Newtonian force picture is not fundamental in all regimes. Nevertheless, Newtonian gravity remains indispensable because it provides an accurate, computationally efficient description across a very large range of ordinary physical situations. Understanding the distinction between gravitational force, gravitational field, gravitational acceleration, gravitational potential, gravitational potential energy and weight is essential for interpreting both elementary and advanced applications of gravity.
