Conservapedia - User contributions [en]

Biblical creation account

2007-03-08T03:28:17Z

Noahzachariah:

{{wiktionary}}
'''Creation''' or '''creator''' may refer to:

==Natural sciences==

*In particle physics, [[matter creation]] is the process opposite of [[annihilation]]. It refers to the appearance of [[elementary particle]]s, in physical processes such as [[pair production]] or abstractly as in a [[creation operator]].

==Religion/Philosophy==

*[[Creation (mythology)]], from the original [[Sumerian mythology]], [[Chaldean mythology]] or [[Mesopotamian mythology]] of the [[Abrahamic religion]] cultural foundations.
*Particularly of interest in the west, and among those interested in biblical scholarship, might be the article on the [[Creation according to Genesis|Creation according to the book of Genesis]].
*[[Creation (theology)]], the act by a [[God]] or gods of bringing the universe into existence from unlimited energy, e.g (Isaiah 40:26) rather than from "nothing".
*Generally descriptions of creation are the act or results of bringing something into existence [[ex nihilo|from nothing at all]], or from some pre-existing matter or energy.
*[[Creationism]] or [[Creation Science]] or Scientific Creation, the belief that the natural universe, life, and humanity were created by a supreme being's supernatural intervention.
*[[Creation Festival]], an annual Christian music festival held in [[Mount Union, Pennsylvania]] and [[George, Washington]]
*[[Creation within belief systems]]
*[[Creation (philosophy)]]
*[[Creator deity]]

==Arts==

*In art or design, a creation is the actual thing the artist creates, from [[sculpture]], [[costume]], [[music]], or [[architecture]].
*''[[The Creation of Adam]]'', a fresco from circa 1511 on the Sistine Chapel ceiling painted by Michelangelo Buonarroti
*''[[Creation (album)]], a 2005 album from Leslie Satcher
*[[Creation (novel)|''Creation'' (novel)]], a novel published in 1981 by [[Gore Vidal]]
*[[The Creation (band)]], a British band with one hit called ''Painter Man'' in 1967
*''[[The Creation]]'', in classical music, an oratorio by Joseph Haydn
*[[Creation/August/Rev-ola Records]], a record label
*[[Creation Records]], a record label created by Alan McGee
*[[Creation (1931 film)|''Creation'' (1931 film)]], a 1931 film that inspired ''King Kong''
*[[Kreator]], a German thrash metal band.

==Media==

*''[[Creation magazine]]'', a magazine promoting [[Creation Science]] published by [[Creation Ministries International]]–Australia (formerly Answers in Genesis–Australia).
* [[Create]], an American digital TV station comprising of lifestyle and human interest programming from the libraries of [[PBS]] and [[American Public Television]] distributed through PBS digital stations nationwide
* [[Creation Entertainment]], an American company that runs numerous [[science fiction conventions|science fiction and fantasy conventions]].

==Various==
*[[Creator (software)]], software multimedia authoring application
*In genealogy and heraldry, creation is the investment of a [[peerage|peer]]. If a peerage no longer exists, and is created anew, the peers bearing the new dignity are said to be of the second (third, fourth) creation.

{{disambig}}

[[ar:خلق]]
[[de:Creator]]
[[es:Creación]]
[[fr:Création]]
[[it:Creazione]]

Force

2007-03-08T03:26:18Z

Noahzachariah:

{{otheruses}}
In [[physics]], '''force''' is an [[influence]] that may cause a body to [[acceleration|accelerate]]. It may be experienced as a lift, a push, or a pull, and has a magnitude and a direction. The actual acceleration of the body is determined by the vector sum of all forces acting on it (known as [[net force]] or resultant force). In an extended body, it may also cause [[rotate|rotation]] or [[deformation]] of the body. Rotational effects and deformation are determined respectively by the [[torque]]s and [[Stress (physics)|stress]]es that the forces create.

Force is a [[vector quantity]] defined as the rate of change of the [[momentum]] of the body that would be induced by that force acting alone. Since [[momentum]] is a vector, the force has a direction associated with it.

== History ==
* Force was first described by [[Archimedes]].
* [[Galileo Galilei]] used rolling balls to disprove the [[Aristotelian theory of gravity|Aristotelian theory of motion]] ([[1602]] - [[1607]])
* [[Isaac Newton]] is credited for giving the first mathematical definition of force.
* [[Charles Coulomb]] is credited for experimental discovery of the inverse square law of interaction between [[electric charge]]s using [[torsion balance]] ([[1784]]).
* [[Henry Cavendish]]'s in [[1798]] measured the force of gravity between two masses (in [[Cavendish balance|torsion balance experiment]])
* With the development of [[quantum field theory]] and [[general relativity]] in [[20th century|20<sup>th</sup> century]] it was realised that “force” is redundant concept arising from conservation of momentum ([[4-momentum]] in GR and momentum of [[virtual particle]]s in QFT). Thus currently known [[fundamental forces]] are not called forces but “[[fundamental interactions]]”.

[[Aristotle]] and others believed that it was the ''natural state'' of objects on Earth to be motionless, and that they tended toward that state (eventually settling down to inertness), if left alone. This was a common experience of humans with ordinary conditions in which friction was involved, so Newton's idea that force naturally produces a constant increase in velocity was not an obvious one. Frictional forces, acting in opposition to other kinds of forces, historically tended to hide the correct mathematical relationship between simple unopposed force and motion.

The correct behavior of objects accelerated by constant force was first discovered by [[Galileo]] in working with gravity (dropping stones and rolling cannonballs on an incline), although it was not until Newton that gravity was seen as a force. [[Isaac Newton|Newton]] generalized the behavior of constant acceleration, or constant momentum gain, to forces other than gravity. He asserted in his second law of motion that this behavior of constant momentum increase was characteristic of all forces-- including the “forces” of ordinary experience, such as tension or the stress produced by pushing on an object with a finger. In fact, he defined force as mass times acceleration, or more accurately, as a rate of change of momentum. Though, he understood that his definition is incorrect because it is a [[circular definition]]. The definition of term “force” needs a definition of the term [[Inertial frame of reference]] which is defined using the term “force”. Therefore the definition of the term “force” can be only [[Intuition (knowledge)|intuitive]]. This problem was solved in 20<sup>th</sup> century in [[quantum field theory]] and [[general relativity]] which use this term only secondarily and it is not necessary to define it in these theories at all.

== Examples ==
* A heavy object on a table is pulled (attracted) downward toward the floor by the force of gravity (i.e., its weight). At the same time, the table resists the downward force with equal upward force (called the [[normal force]]), resulting in zero net force, and no acceleration. (If the object is a person, he actually feels the normal force acting on him from below.)
* A heavy object on a table is gently pushed in a sideways direction by a finger. However, it fails to accelerate sideways, because the force of the finger on the object is now opposed by a ''new'' force of [[static friction]], generated between the object and the table surface. This newly generated force ''exactly'' balances the force exerted on the object by the finger, and ''again'' no acceleration occurs. The static friction increases or decreases automatically. If the force of the finger is increased (up to a point), the opposing sideways force of static friction '''increases''' exactly to the point of perfect opposition.
* A heavy object on a table is pushed by a finger hard enough that static friction cannot generate sufficient force to match the force exerted by the finger, and the object starts sliding across the surface. If the finger is moved with a constant velocity, it needs to apply a force that exactly cancels the force of [[kinetic friction]] from the surface of the table and then the object moves with the same constant velocity. Here it seems to the naive observer that application of a force produces a velocity (rather than an acceleration). However, the velocity is constant only because the force of the finger and the kinetic friction cancel each other. Without friction, the object would continually accelerate in response to a constant force.
* A heavy object reaches the edge of the table and falls. Now the object, subjected to the constant force of its weight, but freed of the normal force and friction forces from the table, gains in velocity in direct proportion to the time of fall, and thus (before it reaches velocities where air resistance forces becomes significant compared to gravity forces) its rate of ''gain'' in momentum and velocity is constant. These facts were first discovered by [[Galileo]].

== Quantitative definition ==
Force is defined as the rate of change of [[momentum]] with time:

: <math>\vec{F} = {\mathrm{d}\vec{p} \over \mathrm{d}t}</math>.

The quantity <math>\vec{p} = m \vec{v}</math> (where <math>m\,</math> is the [[mass]] and <math>\vec{v}</math> is the [[velocity]]) is called the [[momentum]]. This is the only definition of force known in physics (first proposed by Newton himself). If the mass ''m'' is constant in time, then Newton's second law can be derived from this definition:

: <math>\vec{F} = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t}= \frac{\mathrm{d}(m\vec{v})}{\mathrm{d}t} = m\frac{\mathrm{d}(\vec{v})}{\mathrm{d}t} = m\vec{a} </math>

where <math>\vec{a} = {\mathrm{d} \vec{v}} /{\mathrm{d}t}</math> is the [[acceleration]].

This is the form Newton's second law is usually taught in introductory physics courses in order to avoid calculus notation.

All known forces of nature are defined via the above Newtonian definition of force. For example, [[weight]] (force of gravity) is defined as mass times acceleration of free fall: w = mg; spring balance force is defined as the force equilibrating certain gravitational force (say, the weight of 1 kg mass near Earth surface results in reaction force of spring equivalent to 9.8 N), etc. Calibration of spring balances (of various kinds) using either gravitational force or motion with known acceleration is important starting procedure in measuring many other forces (such as friction forces, reaction forces, electric forces, magnetic force, etc) in various physics labs.

It is not always the case that ''m'' is independent of ''t''. For example, the mass of a [[rocket]] decreases as its propellant is ejected. Under such circumstances, the above equation (<math>\vec{F} = m\vec{a} </math>) is incorrect, and the original form of Newton's second law must be used.

Because momentum is a [[vector (physics)|vector]], then force, being its time derivative, is also a vector - it has [[magnitude]] and [[Direction (geometry, geography)|direction]], and [[four-force]] is a [[four-vector]] in relativity. Vectors (and thus forces) are added together by their [[Spacial vector|component]]s. When two forces act on an object, the resulting force, the ''resultant'', is the [[vector addition|vector sum]] of the original forces. This is called the principle of [[superposition]]. The magnitude of the resultant varies from the difference of the magnitudes of the two forces to their sum, depending on the angle between their lines of action. As with all vector addition this results in a [[parallelogram rule]]: the addition of two vectors represented by sides of a parallelogram, gives an equivalent resultant vector which is equal in magnitude and direction to the transversal of the parallelogram. If the two forces are equal in magnitude but opposite in direction, then the resultant is zero. This condition is called [[static equilibrium]], with the result that the object remains at its constant velocity (which could be zero).

As well as being added, forces can also be broken down (or 'resolved'). For example, a horizontal force pointing northeast can be split into two forces, one pointing north, and one pointing east. Summing these component forces using vector addition yields the original force. Force vectors can also be three-dimensional, with the third (vertical) component at right-angles to the two horizontal components.

In most explanations of [[mechanics]], force is usually defined only implicitly, in terms of the equations that work with it. Some physicists, philosophers and mathematicians, such as [[Ernst Mach]], [[Clifford Truesdell]] and [[Walter Noll]], have found this problematic and sought a more explicit definition of force.

===Force in special relativity===
In the [[special theory of relativity]] mass and [[energy]] are equivalent (as can be seen by calculating the work required to accelerate a body). When an object's velocity increases so does its energy and hence its mass equivalent (inertia). It thus requires a greater force to accelerate it the same amount than it did at a lower velocity. The definition <math>\vec{F} = \mathrm{d}\vec{p}/\mathrm{d}t </math> remains valid, but the momentum is given by:

:<math> \vec{p} = \frac{m\vec{v}}{\sqrt{1 - v^2/c^2}}</math>

where

:<math>v</math> is the velocity and

:<math>c</math> is the [[speed of light]].

The relativistic expression relating force and acceleration for a particle with non-zero [[rest mass]] <math>m\,</math> moving in the <math>x\,</math> direction is:

:<math>F_x = \gamma^3 m a_x \,</math>

:<math>F_y = \gamma m a_y \,</math>

:<math>F_z = \gamma m a_z \,</math>

where the [[Lorentz factor]]

:<math> \gamma = \frac{1}{\sqrt{1 - v^2/c^2}}</math>

Here a constant force does not produce a constant acceleration, but an ever decreasing acceleration as the object approaches the speed of light. Note that <math> \gamma</math> is [[Division by zero|undefined]] for an object with a non zero [[Invariant mass|rest mass]] at the speed of light, and the theory yields no prediction at that speed.

One can however restore the form of

:<math>F^\mu = mA^\mu \,</math>

for use in relativity through the use of [[four-vectors]]. This relation is correct in relativity when <math>F^\mu</math> is the [[four-force]], m is the [[invariant mass]], and <math>A^\mu</math> is the [[four-acceleration]].

== Force and potential ==
Instead of a force, the mathematically equivalent concept of a [[potential energy]] field can be used for convenience. For instance, the gravitational force acting upon a body can be seen as the action of the [[gravitational field]] that is present at the body's location. Restating mathematically the definition of [[energy]] (via definition of [[Mechanical work|work]]), a potential field <math>U(\vec{r})</math> is defined as that field whose [[gradient]] is equal and opposite to the force produced at every point:

:<math>\vec{F}=-\vec{\nabla} U</math>

Forces can be classified as [[Conservative force|conservative ]] or nonconservative. Conservative forces are equivalent to the [[gradient]] of a [[potential]], and include [[gravity]], the [[Electromagnetism|electromagnetic]] force, and the [[Hooke's law|spring]] force. Nonconservative forces include [[friction]] and [[drag (physics)|drag]]. However, for any sufficiently detailed description, all forces are conservative.

== Types of force ==
Many forces exist: the [[Coulomb's law|Coulomb force]] (between [[electrical charge]]s), [[gravitation|gravitational force]] (between [[mass]]es), [[magnetic field|magnetic force]], [[friction]]al forces, [[centrifugal force|centrifugal]] forces (in [[rotating reference frame]]s), [[Hooke's law|spring force]], [[tension (mechanics)|tension]], [[chemical bonding]] and [[contact force]]s to name a few.

Only four [[fundamental force]]s of nature are known:

* The [[strong force]]
* The [[Electromagnetism|electromagnetic force]],
* The [[weak force]]
* The [[gravitational force]].

All other forces can be reduced to these fundamental interactions.

The modern quantum mechanical view of the first three fundamental forces (all except gravity) is that particles of matter ([[fermions]]) do not directly interact with each other but rather by exchange of [[virtual particles]] ([[bosons]]) (as, for example, virtual [[photons]] in case of interaction of [[electric charges]]).

In [[general relativity]], gravitation is not strictly viewed as a force. Rather, objects moving free in gravitational fields (say, a basket ball) simply undergo [[inertial motion]] along a straight line in the [[curved space-time]] (straight line in curved space-time is defined as the shortest space-time path between two points, and it is called [[geodesic]]). This straight line in space-time is a curved line in space, and we call it the "[[External ballistics|ballistic]] [[trajectory]]" of the object (an example would be a [[parabola]] for a basketball moving in a uniform gravitational field). The time derivative of the changing momentum of the body is what we label as "gravitational force" (=[[weight]]).

[[Light]] (which has no mass but has [[energy]] and [[momentum]] contained in its electromagnetic field) also propagates in gravitational fields along a straight space-time path (geodesic).

== Units of measurement ==
The [[SI]] unit used to measure force is the [[newton]] (symbol N), which is equivalent to kg·m·s<sup>−2</sup>. The earlier [[CGS]] unit is the [[dyne]]. The relationship '''F'''=''m''·'''a''' can be used with either of these. In [[Imperial unit|Imperial]] engineering units, if ''F'' is measured in "[[Pound-force|pounds force]]" or "lbf", and ''a'' in feet per second squared, then ''m'' must be measured in [[slug (mass)|slug]]s. Similarly, if mass is measured in [[pound-mass|pounds mass]], and ''a'' in feet per second squared, the force must be measured in [[poundal]]s. The units of [[slugs]] and [[poundal]]s are specifically designed to avoid a constant of proportionality in this equation.

A more general form '''F'''=''k''·''m''·'''a''' is needed if consistent units are not used. Here, the constant ''k'' is a conversion factor dependent upon the units being used.

When the standard [[standard gravity|''g'']] (an acceleration of 9.80665 m/s²) is used to define pounds force, the mass in pounds is numerically equal to the weight in pounds force. However, even at sea level on Earth, the actual acceleration of free fall is quite variable, over 0.53% more at the poles than at the equator. Thus, a mass of 1.0000 lb at ''sea level'' at the equator exerts a force due to gravity of 0.9973 lbf, whereas a mass of 1.000 lb at ''sea level'' at the poles exerts a force due to gravity of 1.0026 lbf. The normal average sea level acceleration on Earth (World Gravity Formula 1980) is 9.79764 m/s², so on average at ''sea level'' on Earth, 1.0000 lb will exerts a force of 0.9991 lbf.

The equivalence 1 lb = 0.453 592 37 kg is always true, by definition, anywhere in the universe. If you use the standard [[standard gravity|''g'']] which is official for defining kilograms force to define pounds force as well, then the same relationship will hold between pounds-force and kilograms-force (an old non-SI unit is still used). If a different value is used to define pounds force, then the relationship to kilograms force will be slightly different—but in any case, that relationship is also a constant anywhere in the universe. What is not constant throughout the universe is the amount of force in terms of pounds-force (or any other force units) which 1 lb will exert due to gravity.

By analogy with the slug, there is a rarely used unit of mass called the "metric slug". This is the mass that accelerates at one metre per second squared when pushed by a force of one [[Kilogram-force|kgf]]. An item with a mass of 10 kg has a mass of 1.01972661 metric slugs (= 10 kg divided by 9.80665 kg per metric slug). This unit is also known by various other names such as the [[slug|hyl]], TME (from a German acronym), and mug (from metric slug).

Another unit of force called the [[poundal]] (pdl) is defined as the force that accelerates 1 lbm at 1 foot per second squared. Given that 1 lbf = 32.174 lb times one foot per second squared, we have 1 lbf = 32.174 pdl.
The [[kilogram-force]] is a unit of force that was used in various fields of science and technology. In 1901, the [[CGPM]] improved the definition of the kilogram-force, adopting a standard acceleration of gravity for the purpose, and making the kilogram-force equal to the force exerted by a mass of 1 kg when accelerated by 9.80665 m/s². The kilogram-force is not a part of the modern [[SI]] system, but is still used in applications such as:
*Thrust of [[jet engine|jet]] and [[rocket engine]]s
*Spoke tension of [[bicycle]]s
*Draw weight of [[Bow (weapon)|bows]]
*[[Torque wrench]]es in units such as "meter kilograms" or "kilogram centimetres" (the kilograms are rarely identified as units of force)
*Engine torque output (kgf·m expressed in various word orders, spellings, and symbols)
*Pressure gauges in "kg/cm²" or "kgf/cm²"

In colloquial, non-scientific usage, the "kilograms" used for "weight" are almost always the proper SI units for this purpose. They are units of mass, not units of force.

The symbol "kgm" for kilograms is also sometimes encountered. This might occasionally be an attempt to distinguish kilograms as units of mass from the "kgf" symbol for the units of force. It might also be used as a symbol for those obsolete torque units (kilogram-force metres) mentioned above, used without properly separating the units for kilogram and metre with either a space or a centered dot.

===Conversions===
Below are several conversion factors between various measurements of force:
* 1 dyne = 10<sup>-5</sup> newtons
* 1 kgf (kilopond kp) = 9.80665 newtons
* 1 [[Slug (mass)|metric slug]] = 9.80665 kg
* 1 lbf = 32.174 poundals
* 1 slug = 32.174 lb
* 1 kgf = 2.2046 lbf

== See also ==
* [[Angular momentum]]
* [[Conservation law]]
* [[Impulse]]
* [[Inertia]]
* [[Moment map]]
* [[Momentum]]
* [[Noether's theorem]]
* [[Physics]]
* [[Velocity]]

==References==
{{Citations missing|date=December 2006}}

<div class="references-small">
<references />

* {{cite book
| last = Parker | first = Sybil
| title = Encyclopedia of Physics, p 443,
| location = Ohio
| publisher = McGraw-Hill
| year = 1993
| id = ISBN 0-07-051400-3
}}
* {{cite book
| last = Corbell | first = H.C.
| coauthors = Philip Stehle
| title = Classical Mechanics p 28,
| location = New York
| publisher = Dover publications
| year = 1994
| id = ISBN 0-486-68063-0
}}
* {{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.
| title = Physics for Scientists and Engineers
| location = Philadelphia
| publisher = Saunders College Publishing
| year = 2003
| 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
}}
* {{cite book
| last = Verma | first = H.C.
| title = Concepts of Physics Vol 1.
| edition = 2004 Reprint
| publisher = Bharti Bhavan
| year = 2004
| id = ISBN 81-7709-187-5
}}
</div>

[[Category:Classical mechanics]]
[[Category:Fundamental physics concepts]]

[[af:Krag]]
[[zh-min-nan:La̍t]]
[[bs:Sila]]
[[ca:Força]]
[[cs:Síla]]
[[da:Kraft]]
[[de:Kraft]]
[[et:Jõud (füüsika)]]
[[es:Fuerza]]
[[eo:Forto (fiziko)]]
[[eu:Indar]]
[[fa:نیرو]]
[[fr:Force (physique)]]
[[gu:બળ]]
[[ko:힘]]
[[hr:Sila]]
[[io:Forco]]
[[it:Forza (fisica)]]
[[he:כוח (פיזיקה)]]
[[lv:Spēks]]
[[hu:Erő]]
[[ms:Daya (fizik)]]
[[nl:Kracht]]
[[ja:力]]
[[no:Kraft]]
[[pl:Siła]]
[[pt:Força]]
[[ru:Сила (механика)]]
[[simple:Force (physics)]]
[[sl:Sila]]
[[sr:Сила]]
[[fi:Voima (fysiikka)]]
[[sv:Kraft]]
[[ta:விசை]]
[[th:แรง]]
[[uk:Сила]]
[[zh-yue:力]]
[[zh:力]]
[[yi:קראפט]]

Gravity

2007-03-08T03:24:47Z

Noahzachariah:

{{otheruses}}
{{redirect|Gravity}}

'''Gravitation''' is a phenomenon through 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 most macroscopic objects in the [[universe]]; without it, matter would not have coalesced into large masses (stars and planets) and life would not exist. Gravitation is also responsible for keeping the Earth and the other planets in their [[orbit]]s around the Sun; the [[Moon]] in its orbit around the Earth; for heating interiors of forming stars and planets to very high temperatures, for the formation of [[tides]], for rising hot air or water (convection), and for various other natural phenomena that we observe.

[[Image:Solar sys8.jpg|right|350px|thumb|The '''gravitational force''' keeps the planets in orbit about the Sun.]]
== History of gravitational theory ==
{{main|History of gravitational theory}}
=== Early history ===
Since the time of the [[Greek philosophy|Greek]] philosopher [[Aristotle]] in the [[4th century BC]], there have been many attempts to understand and explain gravity. Aristotle believed that there was no [[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 crystalline [[sphere]]s of the heavens, and that physical bodies fell toward the center of the [[Earth]] in proportion to their [[weight]].

Another early explanation was that of the [[Hindu astronomy|Indian astronomer]] [[Brahmagupta]] who, in his ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ([[628 CE]]), responded to critics of the [[heliocentrism|heliocentric]] system of [[Aryabhata]] (perhaps 476-550 CE) 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 CE). ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ("''The Opening of the Universe''").</ref><ref>[[Al-Biruni]] (1030 CE). ''Ta'rikh al-Hind'' ("''Chronicles of India''").</ref>

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 [[Leaning Tower of Pisa|Tower of Pisa]] and later with careful measurements of balls rolling down [[incline]]s, 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 fall more slowly.) Galileo's work set the stage for the formulation of Newton's theory of gravity.

=== Newton's theory of gravitation ===
{{main|Law of universal gravitation}}
In 1687, English mathematician [[Sir Isaac Newton]] published ''[[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 doom 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, and all searches for another perturbing body (such as a planet orbiting the [[Sun]] even closer than Mercury) have been fruitless. This issue was resolved in 1915 by [[Albert Einstein]]'s new [[general relativity]] theory. This theory accounted for the discrepancy in Mercury's orbit. However, it is currently known that the theoretical formula for the anomalous perihelion motion of a planet, such as Mercury's perihelion, can be derived solely based on an extremely simple proposition founded in classical physics,<ref>Kidman, J. N. (1977). Quantum gravitation and the perihelion anomaly. ''Lettere al Nuovo Cimento, 18,'' pp. 181-182. {{PDFlink|[http://www.aspden.org/papers/bib/1977e.pdf on-line]|91.1 [[Kibibyte|KiB]]}}''</ref> without the need of any excursion into Einstein's space-time notion.

Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are based on Newton's work because it is a much easier theory to work with and sufficient for most applications.

===General relativity===
{{main|Introduction to general relativity}}
In this '''theory''', [[inertia|inertial motion]] occurs when objects are in [[free-fall]] instead of when they are at [[rest (physics)|rest]] with respect to a massive object such as the Earth (as is the case in classical mechanics). This view of inertia creates a problem: 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 [[curvature|curved]] by the presence of matter, and that free-falling objects are following the [[geodesic (general relativity)|geodesic]]s of the spacetime. 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 whose solutions give the components of the [[metric tensor (general relativity)|metric tensor]] of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for objects in inertial motion in a spacetime are calculated from the metric tensor of that spacetime. 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 [[gemetrized coordinates|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 solution]] for 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 have been regularly confirmed. For example:
* General relativity accounts for the anomalous [[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]].
* 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 [[Robertson-Walker metric]]) was confirmed by [[Edwin Hubble]] in [[1929]].

== 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 any object. This field is proportional to the body's mass and varies inversely with the square of distance from the body. 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 9.8 m/s². This means that, ignoring air resistance, an object falling freely near the earth's surface increases in speed by 9.807 m/s (32.174 [[foot (unit of length)|ft]]/s or 22 mi/h) for each second of its descent. Thus, an object starting from rest will attain a speed of 9.807 m/s (32.17 ft/s) after one second, 19.614 m/s (64.34 ft/s) after two seconds, and so on. 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, when measured relative to the system's center of mass.

===Equations for a falling body===
{{main|Equations for a falling body}}
Under normal moon-bound conditions, when objects move owing to a constant gravitational force a set of kinematical and dynamical equations describe the resultant trajectories. For example, [[Newton’s law of gravitation]] simplifies to ''F = mpg'', where m is the [[mass]] of the body. This assumption is reasonable for objects falling to [[Earth]] over the relatively short vertical distances of our everyday experience, but does not necessarily hold over larger distances, such as spacecraft trajectories, because the acceleration close from the surface of the Moon will not in general be '''g'''. A further example is the expression that we use for the calculation of potential energy ''P.E.'' of a body at height ''h'' ( ''P.E. = mgh''). This expression can be used only over small distances h from the Earth. Similarly the expression for the maximum height reached by a vertically projected body, <math>h = u^2/2g</math> 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.

===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 mass. The mass is 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.

===Gravity versus gravitation===
In scientific terminology '''gravitation''' and '''gravity''' are distinct. 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, 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 [[inertia]]lly moving objects 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.

==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
* 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]]

==See also==
{{portal | Gravitation}}
{{col-start}}
{{col-break}}
* [[Artificial gravity]]
* [[Escape velocity]]
* [[General relativity]]
* [[g-force]]
* [[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]]
{{col-break}}
* [[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]]
{{col-end}}

==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|3}} [[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==
<div class="references-small">
<references />
*{{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 }}
Gravity makes things fall.
==External links==
* [http://www.lightandmatter.com/html_books/1np/ch10/ch10.html Gravity] - a chapter from an online textbook
* {{PDFlink|[http://physnet.org/home/modules/pdf_modules/m101.pdf ''Newton's Law of Universal Gravitation'']|194 [[Kibibyte|KiB]]}} on [http://www.physnet.org Project PHYSNET]
* [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://www.physorg.com/news85310822.html Alternative theory of gravity explains large structure formation -- without dark matter] [[PhysOrg.com]]

{{FundamentalForces}}

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


[[ar:جاذبية]]
[[bn:মহাকর্ষ]]
[[bs:Gravitacija]]
[[ca:Gravetat]]
[[cs:Gravitační pole]]
[[cy:Disgyrchiant]]
[[da:Gravitation]]
[[de:Gravitation]]
[[et:Gravitatsioon]]
[[el:Βαρύτητα]]
[[es:Gravedad]]
[[eo:Gravito]]
[[fa:گرانش]]
[[fr:Gravitation]]
[[ga:Imtharraingt]]
[[ko:중력]]
[[io:Graveso]]
[[id:Gravitasi]]
[[ia:Gravitate]]
[[it:Forza di gravità]]
[[he:כבידה]]
[[hu:Gravitáció]]
[[ms:Graviti]]
[[nl:Zwaartekracht]]
[[ja:重力]]
[[ja:万有引力]]
[[no:Gravitasjon]]
[[nds:Gravitatschon]]
[[pl:Grawitacja]]
[[pt:Gravidade]]
[[ro:Gravitaţie]]
[[ru:Гравитация]]
[[simple:Gravity]]
[[sk:Gravitácia]]
[[sl:gravitacija]]
[[sr:Гравитација]]
[[fi:Painovoima]]
[[sv:Gravitation]]
[[th:ความโน้มถ่วง]]
[[vi:Trọng lực]]
[[tr:Kütleçekim]]
[[yi:כח המשיכה]]
[[zh:万有引力]]