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Propulsion is the generation offorce by any combination of pushing or pulling to modify the translationalmotion of an object, which is typically arigid body (or an articulated rigid body) but may also concern afluid.[1] The term is derived from two Latin words:pro, meaning before orforward; andpellere, meaningto drive.[2] Apropulsion system consists of a source of mechanical power, and apropulsor (means of converting this power into propulsive force).
Plucking a guitar string to induce avibratory 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 agravitational 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 fordeep 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).
Biological propulsion systems use an animal's muscles as the power source, and limbs such aswings,fins orlegs as the propulsors. Atechnological system uses anengine or motor as the power source (commonly called apowerplant), andwheels and axles,propellers, or apropulsive nozzle to generate the force. Components such asclutches orgearboxes 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 asbullets, propelled at high speed are known asprojectiles; larger objects propelled at high speed, often intoballistic flight, are known asrockets ormissiles.
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), whichdrives the crankshaft (rotational motion), the crankshaft thendrives 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 arotatingbaseball 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 fromNewtonian mechanics, but are not commonly spoken of in this language.
An aircraft propulsion system generally consists of anaircraft engine and some means to generate thrust, such as apropeller or apropulsive nozzle.
An aircraft propulsion system must achieve two things. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. The greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.[2]
Someaircraft, like airliners andcargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount, which is why high-bypass turbofans and turboprops are commonly used on cargo planes and airliners.[2]
Some aircraft, likefighter planes or experimental high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Moderncombat aircraft usually have an afterburner added to alow bypass turbofan. Futurehypersonic aircraft may use some type oframjet or rocket propulsion.[2]
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Ground propulsion is any mechanism for propelling solid bodies along the ground, usually for the purposes oftransportation. The propulsion system often consists of a combination of anengine or motor, agearbox andwheel and axles in standard applications.
Maglev (derived frommagneticlevitation) is a system of transportation that usesmagnetic levitation to suspend, guide and propel vehicles with magnets rather than using mechanical methods, such aswheels, axles and bearings. With maglev a vehicle is levitated a short distance away from a guide way using magnets to create both lift and thrust. Maglev vehicles are claimed to move more smoothly and quietly and to require less maintenance than wheeledmass transit systems. It is claimed that non-reliance on friction also means that acceleration and deceleration can far surpass that of existing forms of transport. The power needed for levitation is not a particularly large percentage of the overall energy consumption; most of the power used is needed to overcome air resistance (drag), as with any other high-speed form of transport.
Marine propulsion is the mechanism or system used to generatethrust to move aship orboat across water. Whilepaddles andsails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of a motor or engine turning apropeller, or less frequently, in jet drives, animpeller. Marine engineering is the discipline concerned with the design of marinepropulsion systems.
Steam engines were the first mechanical engines used in marine propulsion, but have mostly been replaced bytwo-stroke orfour-stroke diesel engines, outboard motors, andgas turbine engines on faster ships.Nuclear reactors producing steam are used to propelwarships andicebreakers, and there have been attempts to utilize them to power commercial vessels.Electric motors have been used onsubmarines andelectric boats and have been proposed for energy-efficient propulsion.[3] Recent development in liquified natural gas (LNG) fueled engines are gaining recognition for their low emissions and cost advantages.
Spacecraft propulsion is any method used to acceleratespacecraft and artificialsatellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through asupersonic de Laval nozzle. This sort ofengine is called arocket engine.
All current spacecraft use chemical rockets (bipropellant orsolid-fuel) for launch, though some (such as thePegasus rocket andSpaceShipOne) have usedair-breathing engines on theirfirst stage. Most satellites have simple reliable chemical thrusters (oftenmonopropellant rockets) orresistojet rockets fororbital station-keeping and some usemomentum wheels forattitude control. Soviet bloc satellites have usedelectric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south stationkeeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have usedion thrusters andHall-effect thrusters (two different types of electric propulsion) to great success.
A cable car is any of a variety of transportation systems relying on cables to pull vehicles along or lower them at a steady rate. The terminology also refers to the vehicles on these systems. The cable car vehicles are motor-less and engine-less and they are pulled by a cable that is rotated by a motor off-board.
Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, includingrunning,swimming,jumping andflying. Animals move for a variety of reasons, such as to find food, a mate, or a suitablemicrohabitat, 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 theArctic 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 asfrogs) are likely to have costly but very fast locomotion. The study of animal locomotion is typically considered to be a sub-field ofbiomechanics.
Locomotion requiresenergy to overcomefriction,drag,inertia, andgravity, though in many circumstances some of these factors are negligible. Interrestrial 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 naturalbuoyancy 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 inflight, and theaerodynamically efficient body shapes ofbirds highlight this point. Flight presents a different problem from movement in water however, as there is no way for a living organism to have lowerdensity 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 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 afluid (eitherwater orair).[4] 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.
Media related toPropulsion at Wikimedia Commons
