Editing Propulsion (section)

==Animal==
{{main|Animal locomotion}}
[[Image:Beeinflightfromfront.jpg|thumb|upright=1.3|A [[bee]] in flight]]

Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including [[running]], [[aquatic locomotion|swimming]], [[jumping]] and [[Flying and gliding animals|flying]]. Animals move for a variety of reasons, such as to find food, a mate, or a suitable [[microhabitat]], and to escape predators. For many animals the ability to move is essential to survival and, as a result, selective pressures have shaped the locomotion methods and mechanisms employed by moving organisms. For example, migratory animals that travel vast distances (such as the [[Arctic tern]]) typically have a locomotion mechanism that costs very little energy per unit distance, whereas non-migratory animals that must frequently move quickly to escape predators (such as [[frog]]s) are likely to have costly but very fast locomotion. The study of animal locomotion is typically considered to be a sub-field of [[biomechanics]].

Locomotion requires [[energy]] to overcome [[friction]], [[drag (physics)|drag]], [[inertia]], and [[gravity]], though in many circumstances some of these factors are negligible. In [[Terrestrial planet|terrestrial]] environments gravity must be overcome, though the drag of air is much less of an issue. In aqueous environments however, friction (or drag) becomes the major challenge, with gravity being less of a concern. Although animals with natural [[buoyancy]] need not expend much energy maintaining vertical position, some will naturally sink and must expend energy to remain afloat. Drag may also present a problem in [[flight]], and the [[aerodynamic]]ally efficient body shapes of [[bird]]s highlight this point. Flight presents a different problem from movement in water however, as there is no way for a living organism to have lower [[density]] than air. Limbless organisms moving on land must often contend with surface friction, but do not usually need to expend significant energy to counteract gravity.

[[Newton's laws of motion|Newton's third law of motion]] is widely used in the study of animal locomotion: if at rest, to move forward an animal must push something backward. Terrestrial animals must push the solid ground; swimming and flying animals must push against a [[fluid]] (either [[water]] or [[air]]).<ref name="Bie">{{Cite book |last=Biewener |first=Andrew A. |url=https://books.google.com/books?id=yMaN9pk8QJAC&q=biomechanics+biewener |title=Animal Locomotion |date=2003-06-19 |publisher=OUP Oxford |isbn=978-0-19-850022-3 |language=en}}</ref> The effect of forces during locomotion on the design of the skeletal system is also important, as is the interaction between locomotion and muscle physiology, in determining how the structures and effectors of locomotion enable or limit animal movement.