The mutually attractive force between two objects due to their masses; expressed by Newton's law of gravitation F = Gm1m2/r2, where F is the force between objects of mass m1 and m2 separated by distance r, and G is the gravitational constant. The direction of force is along a line joining the two bodies. It is the weakest of all forces, important only on a large scale. The form of Newton's law was verified experimentally by determining the force between heavy spheres placed close together, as in the experiment to measure G by British scientist Henry Cavendish (1798). Newtonian theory is adequate for predictions of planetary motion to high accuracy. The improved theory is general relativity, in which gravitation is viewed as a distortion of spacetime. Attempts to produce a gravitation theory consistent with quantum theory have been unsuccessful. Gravity refers to the intensity of gravitation at the surface of the Earth or some other celestial body.
Gravitation is a property by which all objects attract each other. Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in many cases.
Gravitation is the reason for the very existence of the Earth, the Sun, and other celestial bodies; Gravitation is also responsible for keeping the Earth and the other planets in their orbits around the Sun, the Moon in its orbit around the Earth, for the formation of tides, and for various other natural phenomena that we observe.
History of gravitational theory
Early (pre-Newtonian) history
Since the time of the Greek philosopher Aristotle in the 4th century BC, there have been many attempts to understand and explain gravity. He hypothesized that everything tried to move towards their proper place in the crystalline spheres of the heavens, and that physical bodies fell toward the center of the Earth in proportion to their weight. Another example of an attempted explanation is that of the Indian astronomer Brahmagupta who, in AD 628 , wrote that "bodies fall towards the earth as it is in the nature of the earth to attract bodies, just as it is in the nature of water to flow".
Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th century and early 17th century. In his famous experiment dropping balls at the Tower of Pisa and later with careful measurements of balls rolling down inclines, Galileo showed that gravitation accelerates all objects at the same rate. (Galileo correctly postulated air resistance as the reason that lighter objects appear to fall more slowly.) Galileo's work set the stage for the formulation of Newton's theory of gravity.
Newton's theory of gravitation
In 1687, English mathematician Sir Isaac Newton published the famous Principia, which hypothesizes the inverse-square law of universal gravitation. and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth;
Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted by the actions of the other planets.
Ironically, it was another discrepany in a planet's orbit that helped to doom Newton's theory. By the end of the 19th century, it was known that the oribit of Mercury could not be accounted for entirely under Newton's theory, and all searches for another perturbing body (such as a planet orbiting the Sun even closer than Mercury) has come up empty. This issue was resolved in 1915 by Albert Einstein's new general relativity theory; this theory accounted for the discrepancy in Mercury's orbit.
Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are based on Newton's work due its being a much simpler theory to work with.
General relativity
In this theory Einstein proposed that inertial motion occurs when objects are in free-fall instead of when they are at rest with respect to a massive object such as the Earth (as is the case in classical mechanics). The problem is that in flat spacetimes such as those of classical mechanics and special relativity, there is no way that inertial observers can accelerate with respect to each other, as free-falling bodies can do as they are each accelerated towards the center of a massive object.
To deal with this difficulty, Einstein proposed that spacetime is curved by the presence of matter, and that free-falling objects are following the geodesics of the spacetime. Notable solutions of the Einstein field equations include:
The Schwarzschild solution, which describes spacetime surrounding a spherically symmetric non-rotating uncharged massive object. For charges with a geometrized length which are less than the geometrized length of the mass of the object, this solution produces black holes with two event horizons.Specifics
Earth's gravity
Every planetary body, including the Earth, is surrounded by its own gravitational field, which exerts an attractive force on any object. The gravitational field is numerically equal to the acceleration of objects under its influence, and its value at the Earth's surface, denoted g, is approximately 9.80665 m/s² or 32.17405 ft/s². This means that, ignoring air resistance, an object falling freely near the earth's surface increases in speed by 9.80665 m/s (around 22 mph) for each second of its descent. According to Newton's 3rd Law, the Earth itself experiences an equal and opposite force to that acting on the falling object, meaning that the Earth also accelerates towards the object. However, because the mass of the Earth is huge, the measurable acceleration of the Earth by this same force is negligible.
Equations for a falling body
Under normal earth-bound conditions, when objects move owing to a constant gravitational force a set of kinematical and dynamical equations describe the resultant trajectories. This assumption is reasonable for objects falling to earth over the relatively short vertical distances of our everyday experience, but is very much untrue over larger distances, such as spacecraft trajectories, because the acceleration far from the surface of the Earth will not in general be g.
Gravity and astronomy
The discovery and application of Newton's law of gravity accounts for the detailed information we have about the planets in our solar system, the mass of the Sun, the distance to stars and even the theory of dark matter. In space an object maintains its orbit because of the force of gravity acting upon it.
Gravity versus gravitation
It is important to note that gravitation is not gravity. Gravitation is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects are theorized to exert on each other to cause gravitation. Although these terms are used interchangeably in everyday use, it is important to note that in theories other than Newton's, gravitation is caused by factors other than gravity. For example, in general relativity, gravitation is due to spacetime curvatures which causes inertially moving object to tend to accelerate towards each other. Another (but discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other.
Applications
A vast number of mechanical contrivances depend in some way on gravity for their operation.
Gravity is used in geophysical exploration to investigate density contrasts in the subsurface of the Earth.
Alternative theories
Historical alternative theories
Aristotelian theory of gravity Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire universe. Nordström's theory of gravitation (1912, 1913), an early competitor of general relativity. Whitehead's theory of gravitation (1922), another early competitor of general relativity.Recent alternative theories
Brans-Dicke theory of gravity (1961) Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter. Rosen bi-metric theory of gravity In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations. The new and highly controversial Process Physics theory attempts to address gravity The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation. Nonsymmetric gravitational theory (NGT) (1994) by John Moffat The satirical theory of Intelligent falling (2002, in its first incarnation as "Intelligent grappling") Tensor-vector-scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein
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