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 proportional but opposite force on that system.
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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 from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable pitch 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 some 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:
where:
 T is the thrust generated (force)
 is the rate of change of mass with respect to time (mass flow rate of exhaust);
 v is the speed of the exhaust gases measured relative to the rocket.
For vertical launch of a rocket the initial thrust must be more than the weight.
Each of the three Space Shuttle Main Engines can 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.^{[1]} 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).
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