Powerplant
Propulsion is the generation of force by any combination of pushing and/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. A propulsion system is a machine system consists of at least two core elements: the powerplant, a source of mechanical power; and the propulsor, an actuator mechanism that converts the supplied power into propulsive force; often also with supplementary components such as transmission and control system that enhance the final outcome of the propulsion system.
Plucking a guitar string to induce a vibratory translation is technically a form of propulsion for 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 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).
Biological propulsion systems such as those in animals use muscle contractions as the powerplant, and appendages such as limbs, legs, wings, fins or tails as the propulsors, while microorganisms evolved sophisticated membrane protein complexes that act as molecular motors to power flagella as propulsors. Technological systems such as motor vehicles typically use one or more engines (motors) as the powerplant, and wheels and axles, propellers or a propulsive nozzle to generate the propulsion. Drivetrain components such as clutches, gearboxes and drive shafts may be needed to connect the motor to axles, wheels, or propellers. In additional to internal powerplants, a machine system may sometimes use external forces as power sources to act upon propulsors, such as the use of human- or animal power to propel non-motorized vehicles and watercrafts, the capture of wind power to drive sails or tethered power kites, and the use of gravity to drive the forward motion of gliders.
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 rotating 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.
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