Portal:Gravitation
Introduction
Gravity (from Latin gravitas, meaning 'weight'), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward (or gravitate toward) one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars – and for the stars to group together into galaxies – so gravity is responsible for many of the largescale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker on farther objects.
Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon. However, for most applications, gravity is well approximated by Newton's law of universal gravitation, which describes gravity as a force which causes any two bodies to be attracted to each other, with the force proportional to the product of their masses and inversely proportional to the square of the distance between them.
The effects of gravity
Newton conceived of gravitation when he considered the trajectory of a projectile. A projectile under the influence of gravity travels along a trajectory that is a conic section. The projectile follows either an elliptical path or a hyperbolic path, depending on whether its total mechanical energy is less than or greater than that necessary for escape velocity, respectively. In the pathological case where the projectile's total mechanic energy is exactly equal to that necessary for escape velocity, the projectile follows a parabolic trajectory. At low speeds and over small distances (small enough that the surface of the Earth can be considered flat), the elliptical trajectory of a projectile can be more easily approximated as a parabolic trajectory.
When Newton heard the sound of an apple falling on the ground, he asked himself 'might the same cause (which he called gravitation') for the motion of the apple, also explain the motion of the moon?. This was the first statement of the universal law of gravitation.
Newton's concept
 Every object in the Universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses and inversely proportional to the square of the separation between the two objects. (See also inversesquare law.)
 Two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Strictly speaking, this law applies only to pointlike objects. If the objects have spatial extent, the true force has to be found by integrating the forces between the various points.
The law expressed as an equation:
where:
 = gravitational force between two objects
 = mass of first object
 = mass of second object
 = distance between the center of mass of the objects
 = universal constant of gravitation
The Gravitational equation that is given above is similar to that of Coulomb's Law which deals with the electrostatic force between two charged particles.
Einstein's concept
Einstein's theory of general relativity (1915) stated that the presence of mass, energy, and momentum causes spacetime to become curved. Because of this curvature, the paths that objects in inertial motion follow can "deviate" or change direction over time. This deviation appears to us as an acceleration towards massive objects, which Newton characterized as being gravity. In general relativity however, this acceleration or free fall is actually inertial motion. So objects in a gravitational field appear to fall at the same rate due to their being in inertial motion while the observer is the one being accelerated. (This identification of free fall and inertia is known as the Equivalence principle.)
The Einstein field equations can be written briefly in abstract index notation as rank 2 symmetric tensors:
where is the curvature of spacetime and is the stressenergy within it. Many exact solutions of the Einstein field equations are known. The solutions to the field equations are metrics of spacetime. These metrics describe the structure of spacetime given the stressenergy and coordinate mapping used to obtain that solution.
Did you know
...That black holes were first proposed in 1783?
...That the ergosphere is a region located outside of a rotating black hole?
...That in theoretical physics, a kugelblitz is a concentration of light so intense that it forms an event horizon and becomes selftrapped?
...That the concept of antigravity was first introduced in H. G. Wells' The First Men in the Moon?
...That the mass of the black hole in GRO J0422+32 is the smallest yet found for any black hole?
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Where to start
Galileo was the first to demonstrate and then formulate the equation for the distance d traveled by a falling object under the influence of gravity g for a time t:
He used a wood molding, "12 cubits long, half a cubit wide and three fingerbreadths thick" as a ramp with a straight, smooth, polished groove to study rolling balls ("a hard, smooth and very round bronze ball"). He lined the groove with "parchment, also smooth and polished as possible". He inclined the ramp at various angles, effectively slowing down the acceleration enough so that he could measure the elapsed time. He would let the ball roll a known distance down the ramp, and used a water clock to measure the time taken to move the known distance; this clock was
 "a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results.".
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Key Topics
Mass
We can distinguish several meanings for mass in physics:
 Inertial mass is a measure of an object's inertia: its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily. Now we know that this mass is actually a measure of the total energy of the object and is equal , where E is total energy of the object and c is speed of light in a vacuum. Not only inertial but also all gravitational effects regarding this object are proportional to the object's inertial mass and that's why it is exactly the same as "gravitational mass", "passive" and "active". Before discovery of Einstein's theory of gravitation the "gravitational masses" were considered separate physical entities.
 Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller gravitational force than an object with a larger passive gravitational mass. This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive "gravitational mass" (inertial mass) remains unchanged.

Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less "active gravitational mass" than the Earth.
 Note that Einstein wrote, in his paper on General Relativity that inertial mass and gravitational mass must be equivalent. "We see that our extension of the principle of relativity implies the necessity of the law of the equality of inertial and gravitational mass."Einstein (1916)
 The amount of matter in an object.
 A sideeffect of the interaction between Higgs' Bosons.