Gravitation

2007-12-29T04:44:14Z

Deepblue:

'''Gravity''' is a [[phenomenon]] which attracts all objects within the [[universe]] to each other <ref>New Oxford American Dictionary, 2nd Edition</ref>. In modern [[physics]], it is explained by the secular [[Theory of relativity|General Theory of relativity]]. Before general relativity, gravitation was described by [[Sir Isaac Newton|Isaac Newton's]] naturalistic law of universal gravitation in his [[Philosophiæ Naturalis Principia Mathematica|''Principia Mathematica'']].

Everything in the universe that has mass attracts every other thing that has mass. How much depends on the size of the masses and the distance between them. For normal objects, this pull is minute, but you can measure the pull between a very large object like the [[Earth]] and another object like you by standing on the scales. Your [[weight]] is the measure of the pull of gravity between you and the planet you are standing on. This [[force]] depends on your [[mass]] and the mass of that planet, but it also depends on your [[distance]] from the center of the planet. The further you are from the planet's center, the weaker the pull between it and your body. If you double your distance, the force is one quarter. At ten times the distance, the force is one hundredth. It drops off with the square of the distance. This is called the [[Inverse Square Law.]] <ref>http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html</ref>The force never becomes zero, no matter how far you travel.

The law itself states that each particle in the universe attracts another particle with a force directly [[proportion|proportional]] to the product of their masses and inversely proportional to the [[exponentiation|square]] of the distance between them<ref>Serway and Beichner, Physics for Scientists and Engineers, Fifth Edition</ref>, or:

:<math>F = G \frac{m_1 m_2}{r^2}</math>

Where:

''F'' is the magnitude of the gravitational force.

''m''1 and ''m''2 are the object's masses.

''r'' is the distance that separates the objects' centers of mass.

''G'' is the [[gravitational constant]]: 6.67428x10-11 N m2 Kg -2

Gravitation is responsible for making objects accelerate towards each other as well as for the formation of the [[Earth]] and [[Sun]], the [[stars]] and the [[planets]].

An objects escape speed is the velocity required to escape the gravitational pull of a another object, usually a planet. It ca be expressed as:

:<math>v = \sqrt \frac{2 G M}{R}</math>

Where:

''v'' is necessary velocity

''M'' is the mass of the planet

''R'' is the radius of the planet

''G'' is the [[gravitational constant]]: 6.67428x10-11 N m2 Kg -2

==Gravity and Modern Physics==
Newton's [[Theory of Gravity]] was one of the earliest triumphs of modern [[physics]]. It now stands as both one of the most successful and most mysterious areas of that field. On one hand, the [[Theory_of_Relativity|General theory of Relativity]] is one of the most successful [[Scientific_Theory|scientific theories]] to date. On the other hand, how General Relativity might be reconciled with [[Quantum_mechanics|quantum physics]] during the first few milliseconds of the [[Big Bang]] remains an open question, and is one of the hotly contested areas among modern theoretical physicists. In the 1980s [[string theory]] was seen by many physicists as a more likely path towards a particular unification of gravity with the other fundamental forces (electromagnetism, the strong and weak nuclear forces), but the theory has been a failure.

==See also==
*[[gravitational gradient]]

== References ==
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<references />
[[category:physics]]

Communism

2007-12-29T04:43:09Z

Deepblue:

'''Communism''' is [[Karl Marx]]'s proposed establishment of a "classless society" by means of eliminating private property.
==Demise of Communism==
Between 1989 and 1991, many communist governments fell. The [[Berlin Wall]] in [[Germany]], which had become a symbol for the division between the West and communist states, was torn down largely in response to Ronald Reagan in 1989, and there was also a large revolution against Romanian dictator [[Nicolaie Ceausescu]]. In 1991, the USSR broke up into several countries - each which reformed to capitalism. Some of these remained under autocratic governments, but some have embraced [[democracy]].

Although Communism's influence has decreased dramatically in Europe, it still has a strong presence in Asia, where the most famous country governed by a communist one-party system is [[China]], but others (such as [[North Korea]], [[Laos]], and [[Vietnam]]) remain. There is considerable debate as to the extent to which these governments actually implement communist policies. China has not democratized (note especially the crushing of the [[Tiananmen Square protests of 1989]]), but its economic policies have been called "red capitalism" by some commentators, as there is a growing sector that behaves in a less-regulated, free market style. China's economy manufactures a wide variety of products that are sold to non-communist countries, and there is quite obvious tolerance of economic inequality, with some provinces struggling with poverty while others prosper. One of the only remaining Modern communist countries, [[Cuba]], is still kept under economic sanctions by the USA.

President Ronald Reagan speaking to the British House of Commons at the Palace of Westminster said,
{{Cquote|What I am describing now is a plan and hope for the long term -- the march of freedom and democracy which will leave [[Marxism-Leninism]] on the ash heap of history as it has left other tyrannies which stifle the freedom and muzzle the self-expression of the people.<ref>President Ronald Reagan [http://www.iri.org/history_ReaganSpeech.asp Speech to the British Parliament], June 8, 1982. Retrieved from International Republican Institute 05/24/07. </ref>}}

==Notable communists==

Notable communists include [[Joseph Stalin]], [[Friedrich Engels]], [[Vladimir Lenin]], [[Karl Marx]], and [[Fidel Castro]].

==See also==
*[[Capitalism]]
*[[Socialism]]
*[[Nazism]]

==Notes and references==
{{reflist|2}}

==External links==
*[http://www.memorialsighet.ro/en/colectie.asp?id=3 Memorial to the Victims of Communism and the Resistance]

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[[Category:Cults]]
[[Category:Liberal Falsehoods]]
[[Category:Communism|*]]

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Gravitation

Communism