View source for Gravitational field

[[Category:Done 2018-05-18]] 
[[File:Earth_Western_Hemisphere.jpg|300px|thumb|Figure 1. The Earth, and every other massive object, has a gravitational field that causes other massive objects to interact with it.<ref>Wikimedia Commons [Online], Available: http://upload.wikimedia.org/wikipedia/commons/7/7b/Earth_Western_Hemisphere.jpg</ref>]]

<onlyinclude>A '''gravitational field''' is a field induced by any object with [[mass]], which will interact with other massive objects by applying a [[force]] on it. Gravitational fields are expressed in [[Newton]]s per [[kilogram]] (N/kg), which is the same unit as [[acceleration]].</onlyinclude> This means that any massive object present in another's gravitational field will accelerate towards it, and vice versa. The field of an object is given by the equation:<ref name=Knight>R. D. Knight, "Little g and Big G" in ''Physics for Scientists and Engineers: A Strategic Approach,'' 3nd ed. San Francisco, U.S.A.: Pearson Addison-Wesley, 2008, ch.13, sec.4, pp. 359-360</ref>

<center><math>g=-\frac{GM}{\left|R\right|^2}\hat{R}</math></center>

where:

* <math>G</math> is the [[gravitational constant]]
* <math>M</math> is the mass of the object, 
* <math>\left|R\right|</math> is the distance away from the center of the object
* <math>\hat{R}</math> is the unit direction vector, pointing towards the object (which explains the negative sign)
* <math>g</math> is the [[acceleration due to gravity]]


Using the equation above provides the value for gravitational field strength. Specifically, using the mass and radius of the Earth, the equation shows that the acceleration due to gravity near the surface of the Earth is: <math>g=9.8\frac{m}{s^2}</math> (this value changes the further or closer an object is relative to the centre of the Earth). 

This value means that all objects will fall at the same rate near the surface of the Earth (ignoring [[air drag]] and minor changes due to altitude and [[latitude]]).

Since the gravitational field of the Earth is [[conservative force|conservative]] it must have a [[gravitational potential energy|potential energy]] associated with it. This means that any object at a certain height from the surface of the Earth has a potential energy, which can be translated into [[kinetic energy]] as the object falls. Humans can make use of falling objects such as water to transform this potential energy into something useful, like [[electricity]]. [[Hydroelectric facilities]] make use of the moving water in a river in order to create electricity, which depends on this potential energy in order to operate. 

==References==
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