Editing Propulsion (section)

{{Short description|Means of creating force leading to movement}}
{{Redirect|Powerplant|facilities that generate electricity|Power station|other uses|Power plant (disambiguation)}}
{{more citations needed|date=August 2017}}
[[File:Armadillo Aerospace Pixel Hover.jpg|thumb|[[Armadillo Aerospace]]'s quad [[rocket]] vehicle showing [[shock diamond]]s in the exhaust plume from its propulsion system]]
'''Propulsion''' is the generation of [[force]] by any combination of pushing or pulling to modify the translational [[motion]] of an object, which is typically a [[rigid body]] (or an articulated rigid body) but may also concern a [[fluid]].<ref>{{cite book|title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=0-85045-163-9 |publisher=Frederick Fell, Inc. |publication-place=New York |date=1974 |edition=1st American |page=216}}</ref> The term is derived from two Latin words: ''[[wikt:pro|pro]]'', meaning'' before'' or ''forward''; and ''[[wikt:pellere|pellere]]'', meaning ''to drive''.<ref name=":0">{{cite web| url = http://www.grc.nasa.gov/WWW/k-12/airplane/bgp.html| title = Beginner's Guide to Propulsion NASA}}</ref> 
A '''propulsion system''' consists of a source of mechanical power, and a ''propulsor'' (means of converting this power into propulsive force). 

Plucking a guitar string to induce a [[vibration|vibratory]] translation is technically a form of propulsion of the guitar string; this is not commonly depicted in this vocabulary, even though human muscles are considered to propel the fingertips. The motion of an object moving through a [[gravitational field]] is affected by the field, and within some frames of reference physicists speak of the gravitational field generating a force upon the object, but for [[general relativity|deep theoretic reasons]], physicists now consider the curved path of an object moving freely through space-time as shaped by gravity as a natural movement of the object, unaffected by a propulsive force (in this view, the falling apple is considered to be unpropelled, while the observer of the apple standing on the ground is considered to be propelled by the reactive force of the Earth's surface). 

[[Biology|Biological]] propulsion systems use an animal's muscles as the power source, and limbs such as [[wing]]s, [[fin]]s or [[leg]]s as the propulsors. A [[technology|technological]] system uses an [[engine|engine or motor]] as the power source (commonly called a '''powerplant'''), and [[wheel and axle|wheels and axles]], [[propeller]]s, or a [[propulsive nozzle]] to generate the force. Components such as [[clutch]]es or [[gearbox]]es may be needed to connect the motor to axles, wheels, or propellers. A technological/biological system may use human, or trained animal, muscular work to power a mechanical device.

Small objects, such as [[bullet]]s, propelled at high speed are known as [[projectile]]s; larger objects propelled at high speed, often into [[ballistic flight]], are known as [[rocket]]s or [[missile]]s. 

Influencing rotational motion is also technically a form of propulsion, but in speech, an automotive mechanic might prefer to describe the hot gasses in an engine cylinder as propelling the piston (translational motion), which ''drives'' the crankshaft (rotational motion), the crankshaft then ''drives'' the wheels (rotational motion), and the wheels propel the car forward (translational motion). In common speech, propulsion is associated with spatial displacement more strongly than locally contained forms of motion, such as rotation or vibration. As another example, internal stresses in a [[rotation|rotating]] [[Baseball (ball)|baseball]] cause the surface of the baseball to travel along a sinusoidal or helical trajectory, which would not happen in the absence of these interior forces; these forces meet the technical definition of propulsion from [[Newtonian mechanics]], but are not commonly spoken of in this language.