View source for Gravitation



{{Redirect|Gravity}}
{{Otheruses4|the scientific notion}}
{{wiktionary}}
'''Gravitation''' is a natural [[phenomenon]] by which all objects with [[mass]] attract each other, and is one of the [[fundamental forces]] of physics. In everyday life, gravitation is most commonly thought of as the agency that gives objects [[weight]]. It is responsible for keeping the Earth and the other planets in their [[orbit]]s around the Sun; for keeping the [[Moon]] in its orbit around the Earth, for the formation of [[tides]]; for [[convection]] (by which hot fluids rise);  for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the [[Earth]], the [[Sun]], and most macroscopic objects in the [[universe]]; without it, matter would not have coalesced into these large masses and [[life]], as we know it, would not exist. 

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 most cases. 

The terms '''gravitation''' and '''gravity''' are mostly interchangeable in everyday use, but in scientific usage a distinction may be made. "Gravitation" is a general term describing the attractive influence that all objects with mass exert on each other, while "gravity" specifically refers to a [[force]] that is supposed in some theories (such as Newton's) to be the cause of this attraction. By contrast, in [[general relativity]] gravitation is due to spacetime curvatures that cause [[inertia]]lly moving objects to accelerate towards each other. 
 
[[Image:Solar sys8.jpg|right|350px|thumb|Gravitation keeps the planets in orbit about the Sun. (Not to scale)]]
== History of gravitational theory ==
{{main|History of gravitational theory}}

=== Early history ===
Efforts to understand gravity began in ancient times. [[Indian philosophy|Philosophers]] in [[History of science in early cultures#India|ancient India]] explained the phenomenon from the 8th century BC.<ref>Dick Teresi (2002), ''Lost Discoveries: The Ancient Roots of Modern Science - from the Babylonians to the Maya'', Simon & Schuster, New York, ISBN 0-684-83718-8:
<br>{{quote|"Two hundred years before [[Pythagoras]], philosophers in northern India had understood that gravitation held the solar system together, and that therefore the sun, the most massive object, had to be at its centre."}}</ref> According to [[Kanada]], founder of the [[Vaisheshika]] school, "[[Mass|Weight]] causes falling; it is [[Wiktionary:imperceptible|imperceptible]] and known by [[inference]]."<ref>[[Subhash Kak|S. Kak]] (2003). [http://arxiv.org/abs/physics/0310001 Indian Physics: Outline of Early History''], p. 22. ''[[arXiv]]''. [[Louisiana State University]].</ref>

In the 4th century BC, the [[Greek philosophy|Greek philosopher]] [[Aristotle]] believed that there was no [[result|effect]] without a [[cause]], and therefore no [[Motion (physics)|motion]] without a [[force]]. He hypothesized that everything tried to move towards its proper place in the [[Celestial spheres|crystalline sphere]]s of the heavens, and that physical bodies fell toward the center of the [[Earth]] in proportion to their [[weight]].

[[Brahmagupta]], in the ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' (628 AD), responded to critics of the [[heliocentrism|heliocentric]] system of [[Aryabhata]] (476–550 AD) stating that "all heavy things are attracted towards the center of the earth" and that "all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow, that of fire to burn, and that of wind to set in motion... The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."<ref>[[Brahmagupta]] (628 AD). ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ("''The Opening of the Universe''").</ref><ref>[[Al-Biruni]] (1030). ''Ta'rikh al-Hind'' (''Indica'').</ref>

In the 9th century, the eldest [[Banū Mūsā]] brother, [[Ja'far Muhammad ibn Mūsā ibn Shākir|Muhammad ibn Musa]], in his ''Astral Motion'' and ''The Force of Attraction'', hypothesized that there was a force of attraction between heavenly bodies,<ref>K. A. Waheed (1978). ''Islam and The Origins of Modern Science'', p. 27. Islamic Publication Ltd., Lahore.</ref> foreshadowing [[Newton's law of universal gravitation]].<ref>[[Robert Briffault]] (1938). ''The Making of Humanity'', p. 191.</ref> In the 1000s, the [[Persian]] [[Islamic science|scientist]] [[Ibn al-Haytham]] (Alhacen), in the ''Mizan al-Hikmah'', discussed the theory of attraction between [[mass]]es, and it seems that he was aware of the [[Magnitude (mathematics)|magnitude]] of [[acceleration]] due to gravity.<ref>Dr. Nader El-Bizri, "Ibn al-Haytham or Alhazen", in Josef W. Meri (2006), ''Medieval Islamic Civilization: An Encyclopaedia'', Vol. II, p. 343-345, [[Routledge]], New York, London.</ref>
In 1121, [[Al-Khazini]], in ''The Book of the Balance of Wisdom'', differentiated between [[force]], [[mass]], and [[weight]],<ref>[[Donald Routledge Hill]] (1993), ''Islamic Science and Engineering'', p. 61, [[Edinburgh University Press]]. ([[cf.]] Salah Zaimeche PhD (2005), [http://www.muslimheritage.com/uploads/Merv.pdf Merv], p. 5, Foundation for Science Technology and Civilization.)</ref> and discovered that gravity varies with the distance from the centre of the Earth,<ref name=Abattouy>Professor Mohammed Abattouy (2002). "The Arabic Science of weights: A Report on an Ongoing Research Project", ''The Bulletin of the Royal Institute for Inter-Faith Studies'' '''4''', p. 109-130.</ref>
though he believed that the weight of heavy bodies increase as they are farther from the centre of the Earth.<ref>N. Khanikoff, ed. and trans. (1858-1860), "Analysis and Extracts of ... Book of the Balance of Wisdom, An Arabic Work on the Water-Balance, Written by 'Al-Khâzinî in the Twelfth Century", chap. 5, sect. 3.1, ''Journal of the American Oriental Society'' '''6''', p. 36.</ref>  All these early attempts at trying to explain the force of gravity were philosophical in nature and it would be [[Isaac Newton]] that gave the first correct desciption of [[gravity]].

=== Scientific revolution ===
Modern work on gravitational theory began with the work of [[Galileo Galilei]] in the late 16th century and early 17th century.  In his famous (though probably apocryphal) experiment dropping balls from the [[Leaning Tower of Pisa|Tower of Pisa]], and later with careful measurements of balls rolling down [[Inclined plane|inclines]], Galileo showed that gravitation accelerates all objects at the same rate.  This was a major departure from Aristotle's belief that heavier objects are accelerated faster.  (Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.)  Galileo's work set the stage for the formulation of Newton's theory of gravity.

=== Newton's theory of gravitation ===
{{main|Newton's law of universal gravitation}}
In 1687, English mathematician [[Sir Isaac Newton]] published ''[[Philosophiae Naturalis Principia Mathematica|Principia]]'', which hypothesizes the [[inverse-square law]] of universal gravitation.  In his own words, “I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.” 

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.  Calculations by [[John Couch Adams]] and [[Urbain Le Verrier]] both predicted the general position of the planet, and Le Verrier's calculations are what led [[Johann Gottfried Galle]] to the discovery of Neptune.

Ironically, it was another discrepancy in a planet's orbit that helped to point out flaws in Newton's theory.  By the end of the 19th century, it was known that the orbit of [[Mercury (planet)|Mercury]] could not be accounted for entirely under Newton's theory, but all searches for another perturbing body (such as a planet orbiting the [[Sun]] even closer than Mercury) had been fruitless.  The issue was resolved in 1915 by [[Albert Einstein]]'s new [[General Relativity|General Theory of Relativity]], which accounted for the discrepancy in Mercury's orbit.

Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are still made using Newton's theory because it is a much simpler theory to work with than [[General Relativity]], and gives sufficiently accurate results for most applications.

===General relativity===
{{main|Introduction to general relativity}}
In '''[[general relativity]]''', the effects of gravitation are ascribed to [[spacetime]] [[curvature]] instead of a force.  The starting point for general relativity is the [[equivalence principle]], which equates free fall with inertial motion.  The issue that this creates is that free-falling objects can accelerate with respect to each other.  In [[Newtonian physics]], no such acceleration can occur unless at least one of the objects is being operated on by a force (and therefore is not moving inertially).

To deal with this difficulty, Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. (This type of path is called a [[geodesic (general relativity)|geodesic]].)  More specifically, Einstein discovered the [[field equation]]s of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him.  The [[Einstein field equations]] are a set of 10 [[simultaneous equation|simultaneous]], [[Nonlinearity|non-linear]], [[differential equation]]s.  The solutions of the field equations are the components of the [[metric tensor (general relativity)|metric tensor]] of spacetime.  A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.  

Notable solutions of the Einstein field equations include:
* The [[Schwarzschild solution]], which describes spacetime surrounding a [[spherical symmetry|spherically symmetric]] non-[[rotation|rotating]] uncharged massive object.  For compact enough objects, this solution generated a [[black hole]] with a central [[gravitational singularity|singularity]].  For radial distances from the center which are much greater than the [[Schwarzschild radius]], the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
* The [[Reissner-Nordström black hole|Reissner-Nordström solution]], in which the central object has an electrical charge.  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]].
* The [[Kerr metric|Kerr solution]] for rotating massive objects. This solution also produces black holes with multiple event horizons.
* The [[Kerr-Newman metric|Kerr-Newman solution]] for charged, rotating massive objects. This solution also produces black holes with multiple event horizons.
* The [[physical cosmology|cosmological]] [[Robertson-Walker coordinates|Robertson-Walker solution]], which predicts the expansion of the [[universe]].

General relativity has enjoyed much success because of how its predictions of phenomena which are not called for by the theory of gravity have been regularly confirmed.  For example:
* General relativity accounts for the anomalous [[apsis|perihelion]] [[precession]] of the planet [[Mercury (planet)|Mercury]].
* The prediction that time runs slower at lower potentials has been confirmed by the [[Pound-Rebka experiment]], the [[Hafele-Keating experiment]], and the [[GPS#Relativity|GPS]].
* The prediction of the deflection of light was first confirmed by [[Arthur Eddington]] in 1919, and has more recently been strongly confirmed through the use of a [[quasar]] which passes behind the [[Sun]] as seen from the [[Earth]]. See also [[gravitational lensing]].
* The [[time delay of light]] passing close to a massive object was first identified by [[Irwin Shapiro]] in 1964 in interplanetary spacecraft signals.
* [[Gravitational radiation]] has been indirectly confirmed through studies of binary [[pulsar]]s.
* The expansion of the universe (predicted by the [[Alexander Friedmann]]) was confirmed by [[Edwin Hubble]] in 1929.

===Gravity and quantum mechanics===

{{main|Graviton|Quantum gravity}}
Several decades after the discovery of general relativity it was realized that general relativity is incompatible with [[quantum mechanics]].<ref>{{cite book | author=Randall, Lisa | title=Warped Passages: Unraveling the Universe's Hidden Dimensions | publisher=Ecco | year=2005 | id=ISBN 0-06-053108-8}}</ref> It is possible to describe gravity in the framework of [[quantum field theory]] like the other [[fundamental forces]], with the attractive force of gravity arises due to exchange of [[virtual particle|virtual]] [[gravitons]], in the same way as the electromagnetic force arises from exchange of virtual [[photons]].<ref>{{cite book |last= Feynman |first= R. P. |coauthors= Morinigo, F. B., Wagner, W. G., & Hatfield, B. |title= Feynman lectures on gravitation |publisher= Addison-Wesley |year= 1995 |isbn=0201627345 }}</ref><ref>{{cite book | author=Zee, A. |title=Quantum Field Theory in a Nutshell | publisher = Princeton University Press | year=2003 | id=ISBN 0-691-01019-6}}</ref> This reproduces general relativity in the [[classical limit]]. However, this approach fails at short distances of the order of the [[Planck length]],<ref>{{cite book | author=Randall, Lisa | title=Warped Passages: Unraveling the Universe's Hidden Dimensions | publisher=Ecco | year=2005 | id=ISBN 0-06-053108-8}}</ref> where a more complete theory of [[quantum gravity]] (or a new approach to quantum mechanics) is required. Many believe the complete theory to be [[string theory]],<ref>{{cite book | author=Greene, Brian | title=The elegant universe: superstrings, hidden dimensions, and the quest for the ultimate theory | publisher=Vintage Books |location = New York| year=2000 | id=ISBN 0375708111}}</ref> or more currently [[M Theory]].

== Specifics ==
===Earth's gravity===
{{main|Earth's gravity}}
Every planetary body (including the Earth) is surrounded by its own gravitational field, which exerts an attractive force on all objects. Assuming a spherically symmetrical planet (a reasonable approximation), the strength of this field at any given point is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body. 

The strength of 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 9.81&nbsp;m/s² (32.2 ft/s²) as the [[Standard gravity|standard average]]. This means that, ignoring air resistance, an object falling freely near the earth's surface increases its velocity with 9.81&nbsp;m/s (32.2 ft/s or 22 mph) for each second of its descent. Thus, an object starting from rest will attain a velocity of 9.81&nbsp;m/s (32&nbsp;ft/s) after one second, 19.6&nbsp;m/s (64&nbsp;ft/s) after two seconds, and so on, adding 9.8&nbsp;m/s to each resulting velocity. 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 acceleration of the Earth by this same force is negligible, when measured relative to the system's [[center of mass]].

===Equations for a falling body===
[[Image:Falling ball.jpg|thumb|right|100px|Ball falling freely under gravity. See text for description.]]
{{main|Equations for a falling body}}
The kinematical and dynamical equations describing the trajectories of falling bodies are considerably simpler if the gravitational force is assumed constant. This assumption is reasonable for objects falling to [[Earth]] over the relatively short vertical distances of our everyday experience, but does not hold over larger distances, such as spacecraft trajectories, since the acceleration due to Earth's gravity is much smaller at large distances.

Under an assumption of constant gravity, [[Newton’s law of gravitation]] simplifies to ''F'' = ''mg'', where ''m'' is the [[mass]] of the body and ''g'' is a constant vector with an average magnitude of 9.81&nbsp;m/s². The acceleration due to gravity is equal to this ''g''. An initially-stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. The image on the right, spanning half a second, was captured with a stroboscopic flash at 20 flashes per second. During the first 1/20th of a second the ball drops one unit of distance (here, a unit is about 12 mm); by 2/20ths it has dropped at total of 4 units; by 3/20ths, 9 units and so on. 

Under the same constant gravity assumptions, the [[potential energy]], ''E<sub>p</sub>'', of a body at height ''h'' is given by ''E<sub>p</sub>'' = ''mgh'' (or ''E<sub>p</sub>'' = ''Wh'', with ''W'' meaning weight). This expression is valid only over small distances ''h'' from the surface of the Earth. Similarly, the expression <math>h = v^2/2g</math> for the maximum height reached by a vertically projected body with velocity ''v'' is useful for small heights and small initial velocities only. In case of large initial velocities we have to use the principle of conservation of energy to find the maximum height reached. This same expression can be solved for ''v'' to determine the velocity of an object dropped from a height ''h'' immediately before hitting the ground, <math>v=\sqrt{2gh}</math>, assuming negligible air resistance.

===Gravity and astronomy===
{{main|Gravity (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, [[quasar]]s and even the theory of [[dark matter]].  Although we have not traveled to all the planets nor to the Sun, we know their masses.  These masses are obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its [[orbit]] because of the force of gravity acting upon it. Planets orbit stars, stars orbit [[galactic center]]s, [[galaxy|galaxies]] orbit a center of mass in clusters, and clusters orbit in [[supercluster]]s.  The force of gravity is proportional to the mass of an object and inversely proportional to the square of the distance between the objects.

===Gravitational radiation===
{{main|Gravitational wave}}

In general relativity, gravitational radiation is generated in situations where the curvature of [[spacetime]] is oscillating, such as is the case with co-orbiting objects.  The gravitational radiation emitted by the [[solar system]] is far too small to measure.  However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as [[PSR 1913+16]].  It is believed that [[neutron star]] mergers and [[black hole]] formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as [[LIGO]] have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.

==Alternative theories==
{{main|Alternatives to general relativity}}
===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 theory|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
* [[Tensor-vector-scalar gravity]] (TeVeS) (2004), a relativistic modification of MOND by [[Jacob Bekenstein]]

==See also== 
{{portal|Gravitation}}
<div style="-moz-column-count:3; column-count:3;">
* [[Anti-gravity]], the idea of neutralizing or repelling gravity
* [[Artificial gravity]]
* [[Escape velocity]], the minimum velocity needed to fly away from a massive space object
* [[General relativity]]
* [[g-force]], a measure of [[acceleration]]
* [[Gravitational field]]
* [[Gravitational wave]]s
* [[Gravitational binding energy]]
* [[Gravity Research Foundation]]
* [[Divergence theorem#Gravity|Gravity and the divergence theorem]]
* [[Kepler's laws of planetary motion#Kepler's third law|Kepler's third law of planetary motion]]
* [[Newton's laws of motion]]
* [[n-body problem|''n''-body problem]]
* [[Pioneer anomaly|The Pioneer spacecraft anomaly]]
* [[Scalar Gravity]]
* [[Speed of gravity]]
* [[Standard gravitational parameter]]
* [[Standard gravity]]
* [[Weight]]
* [[Weightlessness]]
* [[Lagrange Points]]
* [[Gravity assist]]
</div>

==Notes==
<div class="references-small">
* {{fnb|1}} Proposition 75, Theorem 35: p.956 - I.Bernard Cohen and Anne Whitman, translators: Isaac Newton, ''The Principia'': Mathematical Principles of Natural Philosophy. Preceded by ''A Guide to Newton's Principia'', by I. Bernard Cohen. University of California Press  1999 ISBN 0-520-08816-6 ISBN 0-520-08817-4 
* {{fnb|2}} [[Max Born]] (1924), ''Einstein's Theory of Relativity'' (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)
</div>

==References==
{{reflist|2}}
{{refbegin}}
*{{cite book | last = Halliday | first =  David | coauthors = Robert Resnick; Kenneth S. Krane | title = Physics v. 1 | location = New York | publisher =  John Wiley & Sons | year = 2001 | id = ISBN 0-471-32057-9 }}
*{{cite book | last = Serway | first =  Raymond A. | coauthors =  Jewett, John W. | title = Physics for Scientists and Engineers | edition = 6th ed. | publisher = Brooks/Cole | year = 2004 | id = ISBN 0-534-40842-7 }}
*{{cite book | last = Tipler | first =  Paul | title = Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics | edition = 5th ed. | publisher = W. H. Freeman | year = 2004 | id = ISBN 0-7167-0809-4 }}
{{refend}}

==External links==
* [http://www.lightandmatter.com/html_books/1np/ch10/ch10.html Chapter 10. Gravity], from Light and Matter: educational materials for physics and astronomy
* [http://einstein.stanford.edu/ Gravity Probe B Experiment]  The Official Einstein website from Stanford University
* [http://geophysics.mines.edu/cgem Center for Gravity, Electrical, and Magnetic Studies]
* [http://static.scribd.com/docs/8deo8fwbo2y96.swf Gravity for kids] (flash)
* [http://www.newton.dep.anl.gov/askasci/phy99/phy99x10.htm Ask a scientist], Physics Archive
* [http://www.howstuffworks.com/question232.htm How stuff works]: How does gravity work?
* [http://www.physorg.com/news85310822.html Alternative theory of gravity explains large structure formation -- without dark matter] [[PhysOrg.com]]
* [http://www.fourmilab.ch/gravitation/foobar/ Do it yourself, gravitation experiment]

{{FundamentalForces}}

[[Category:Fundamental physics concepts]]
[[Category:Gravity]]
[[Category:Introductory physics]]
[[Category:Celestial mechanics]]

<!--interwiki-->

[[als:Gravitation]]
[[ar:ثقالة]]
[[bn:মহাকর্ষ]]
[[zh-min-nan:Tāng-le̍k]]
[[bs:Gravitacija]]
[[br:Gravitadur]]
[[bg:Гравитация]]
[[ca:Gravetat]]
[[cs:Gravitace]]
[[cy:Disgyrchiant]]
[[da:Gravitation]]
[[de:Gravitation]]
[[et:Gravitatsioon]]
[[el:Βαρύτητα]]
[[es:Gravedad]]
[[eo:Gravito]]
[[fa:گرانش]]
[[fr:Gravitation]]
[[ga:Imtharraingt]]
[[ko:중력]]
[[hr:Gravitacija]]
[[io:Graveso]]
[[id:Gravitasi]]
[[ia:Gravitate]]
[[it:Forza di gravità]]
[[he:כבידה]]
[[kn:ಗುರುತ್ವ]]
[[la:Gravitas (physica)]]
[[lv:Gravitācija]]
[[lb:Gravitatioun]]
[[lt:Gravitacija]]
[[hu:Gravitáció]]
[[ms:Graviti]]
[[nl:Zwaartekracht]]
[[ja:万有引力]]
[[no:Tyngdekraft]]
[[nn:Gravitasjon]]
[[nov:Gravitatione]]
[[pl:Grawitacja]]
[[pt:Gravidade]]
[[ro:Gravitaţie]]
[[qu:Llasaturaku]]
[[ru:Гравитация]]
[[scn:Gravitati]]
[[simple:Gravitation]]
[[sk:Gravitácia]]
[[sl:Težnost]]
[[sr:Гравитација]]
[[fi:Painovoima]]
[[sv:Gravitation]]
[[th:ความโน้มถ่วง]]
[[vi:Lực hấp dẫn]]
[[chr:ᏄᏓᎨᏒ (Gravitation)]]
[[tr:Kütleçekim]]
[[uk:Гравітація]]
[[ur:کششِ ثقل]]
[[yi:גראוויטאציע]]
[[zh:万有引力]]

{{WikiDoc Help Menu}}
{{WikiDoc Sources}}