
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system.〔 http://www.grc.nasa.gov/WWW/k12/airplane/thrust1.html 〕 The force applied on a surface in a direction perpendicular or normal to the surface is called thrust. Force, and thus thrust, is measured in the International System of Units (SI) as the newton (symbol: N), and represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared. In mechanical engineering, force orthogonal to the main load (such as in parallel helical gears) is referred to as thrust. ==Examples== A fixedwing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine.〔http://www.grc.nasa.gov/WWW/k12/airplane/newton3.html〕 The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variablepitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed. Birds normally achieve thrust during flight by flapping their wings. A motorboat generates thrust (or reverse thrust) when the propellers are turned to accelerate water backwards (or forwards). The resulting thrust pushes the boat in the opposite direction to the sum of the momentum change in the water flowing through the propeller. A rocket is propelled forward by a thrust force equal in magnitude, but opposite in direction, to the timerate of momentum change of the exhaust gas accelerated from the combustion chamber through the rocket engine nozzle. This is the exhaust velocity with respect to the rocket, times the timerate at which the mass is expelled, or in mathematical terms: :$\backslash mathbf=\backslash mathbf\backslash frac$ where T is the thrust generated (force), $\backslash frac$ is the rate of change of mass with respect to time (mass flow rate of exhaust), and v is the speed of the exhaust gases measured relative to the rocket. For vertical launch of a rocket the initial thrust at liftoff must be more than the weight. Each of the three Space Shuttle Main Engines could produce a thrust of 1.8 MN, and each of the Space Shuttle's two Solid Rocket Boosters 14.7 MN, together 29.4 MN. Compare with the mass at liftoff of 2,040,000 kg, hence a weight of 20 MN. By contrast, the simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N each. In the airbreathing category, the AMTUSA AT180 jet engine developed for radiocontrolled aircraft produce 90 N (20 lbf) of thrust. The GE90115B engine fitted on the Boeing 777300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," has a thrust of 569 kN (127,900 lbf). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thrust」の詳細全文を読む スポンサード リンク
