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Jet propulsion
Jet propulsion is thrust produced by passing a jet of matter (typically air or water) in the opposite direction to the direction of motion. By Newton's third law, the moving body is propelled in the opposite direction to the jet. It is most commonly used in the jet engine, but is also the favoured means of propulsion used to power various space craft.
A number of animals, including cephalopods, sea hares, arthropods, and fish have convergently evolved jet propulsion mechanisms.
==Physics==
Jet propulsion is most effective when the Reynolds number is high - that is, the object being propelled is relatively large and passing through a low-viscosity medium.
In biology, the most efficient jets are pulsed, rather than continuous: at least when the Reynolds number is greater than 6.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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