Propulsion

Biological propulsion systems use an animal's muscles as the power source, and limbs such as wings, fins or legs as the propulsors.

A technological system uses an engine or motor as the power source (commonly called a powerplant), and wheels and axles, propellers, or a propulsive nozzle to generate the force.

In common speech, propulsion is associated with spatial displacement more strongly than locally contained forms of motion, such as rotation or vibration.

The propulsion system often consists of a combination of an engine or motor, a gearbox and wheel and axles in standard applications.

Maglev vehicles are claimed to move more smoothly and quietly and to require less maintenance than wheeled mass transit systems.

Nuclear reactors producing steam are used to propel warships and icebreakers, and there have been attempts to utilize them to power commercial vessels.

[3] Recent development in liquified natural gas (LNG) fueled engines are gaining recognition for their low emissions and cost advantages.

However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle.

Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south stationkeeping and orbit raising.

Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators.

Drag may also present a problem in flight, and the aerodynamically efficient body shapes of birds 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.