Editing Gravity (section)

==On Earth==
[[File:Falling ball.jpg|thumb|upright=0.45|An initially-stationary object that is allowed to fall freely under gravity drops a distance that is proportional to the square of the elapsed time. This image spans half a second and was captured at 20 flashes per second.]]
{{main|Gravity of Earth}}
Every planetary body (including the Earth) is surrounded by its own gravitational field, which can be conceptualized with Newtonian physics as exerting an attractive force on all objects. Assuming a spherically symmetrical planet, the strength of this field at any given point above the surface is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.
[[File:Gravity action-reaction.gif|thumb|left|If an object with comparable mass to that of the Earth were to fall towards it, then the corresponding acceleration of the Earth would be observable.]]
The strength of the gravitational field is numerically equal to the acceleration of objects under its influence.<ref>{{cite book |title=Companion to the History of Modern Science |first1=G.N. |last1=Cantor |first2=J.R.R. |last2=Christie |first3=M.J.S. |last3=Hodge |first4=R.C. |last4=Olby |publisher=Routledge |year=2006 |isbn=978-1-134-97751-2 |page=448 |url=https://books.google.com/books?id=gkJn6ciwYZsC&pg=PA448 |access-date=22 October 2017 |archive-date=17 January 2020 |archive-url=https://web.archive.org/web/20200117131121/https://books.google.com/books?id=gkJn6ciwYZsC&pg=PA448 |url-status=live }}</ref> The rate of acceleration of falling objects near the Earth's surface varies very slightly depending on latitude, surface features such as mountains and ridges, and perhaps unusually high or low sub-surface densities.<ref>{{Cite APOD|date = 15 December 2014|title = The Potsdam Gravity Potato|access-date = }}</ref> For purposes of weights and measures, a [[standard gravity]] value is defined by the [[International Bureau of Weights and Measures]], under the [[International System of Units]] (SI).

The force of gravity experienced by objects on Earth's surface is the [[Euclidean vector|vector sum]] of two forces:<ref name=HWM>{{cite book
|last1 = Hofmann-Wellenhof |first1 = B.
|last2 = Moritz |first2 = H.
|title = Physical Geodesy
|publisher = Springer
|edition = 2nd
|isbn = 978-3-211-33544-4
|year = 2006
|quote = § 2.1: "The total force acting on a body at rest on the earth's surface is the resultant of gravitational force and the centrifugal force of the earth's rotation and is called gravity.
}}</ref> (a) The gravitational attraction in accordance with Newton's universal law of gravitation, and (b) the centrifugal force, which results from the choice of an earthbound, rotating frame of reference. The force of gravity is weakest at the equator because of the [[centrifugal force]] caused by the Earth's rotation and because points on the equator are farthest from the center of the Earth. The force of gravity varies with latitude, and the resultant acceleration increases from about 9.780&nbsp;m/s<sup>2</sup> at the Equator to about 9.832&nbsp;m/s<sup>2</sup> at the poles.<ref name="Boynton">{{cite conference |last=Boynton |first=Richard |date=2001 |title=''Precise Measurement of Mass'' |book-title=Sawe Paper No. 3147 |publisher=S.A.W.E., Inc. |location=Arlington, Texas |url=http://www.space-electronics.com/Literature/Precise_Measurement_of_Mass.PDF |access-date=22 December 2023 |archive-date=27 February 2007 |archive-url=https://web.archive.org/web/20070227132140/http://www.space-electronics.com/Literature/Precise_Measurement_of_Mass.PDF |url-status=dead }}</ref><ref>{{cite web |url=http://curious.astro.cornell.edu/question.php?number=310 |title=Curious About Astronomy? |website= Cornell University |accessdate=22 December 2023 |archive-date=28 July 2013 |archiveurl=https://web.archive.org/web/20130728125707/http://curious.astro.cornell.edu/question.php?number=310}}</ref>

=== Gravity wave ===
{{main|Gravity wave}}
Waves on oceans, lakes, and other bodies of water occur when the gravitational equilibrium at the surface of the water is disturbed by for example wind.<ref>{{Cite book |last=Young |first=I. R. |title=Wind generated ocean waves |date=1999 |publisher=Elsevier |isbn=978-0-08-043317-2 |edition=1st |series=Elsevier ocean engineering book series |location=Amsterdam ; New York}}</ref> Similar effects occur in the [[Atmospheric wave|atmosphere]] where equilibrium is disturbed by thermal [[weather fronts]] or mountain ranges.<ref>{{Cite journal |last1=Fritts |first1=David C. |last2=Alexander |first2=M. Joan |date=March 2003 |title=Gravity wave dynamics and effects in the middle atmosphere |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2001RG000106 |journal=Reviews of Geophysics |language=en |volume=41 |issue=1 |page=1003 |doi=10.1029/2001RG000106 |bibcode=2003RvGeo..41.1003F |issn=8755-1209}}</ref>