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Thrust

Thrust is a reaction (physics)|reaction force (physics)|force described quantitatively by Isaac Newton|Newton's Newton's laws of motion|Second and Third Laws. When a system expels or acceleration|accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system.

Examples

A fixed-wing aircraft generates forward thrust when air is pushed in the opposite direction of flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating turbine pushing air out the back of a jet engine, or by ejecting hot gases with 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 reversal|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. In a bird,this is normally acheived by flapping the primary feathers during flight. Also aiding in increasing the angle of attack to preys.. 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's mass is propelled forward by a thrust force equal to, and opposite of, the time-rate of momentum change of the exhaust mass accelerated from the combustion chamber through the rocket engine nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms:
-

\mathbf{T}=\frac{dm}{dt}\mathbf{v}

where:
- '''T''' is the thrust generated (force); some texts use the symbol F_th
-

\frac {dm} {dt}

is the rate of change of mass with respect to time (mass flow rate of exhaust);
- '''v''' is the exhaust velocity. Of course, for a launch the thrust at lift-off should be more than the weight, and with a fair margin, because a "slow launch" would be very inefficient. Each of the three Space shuttle main engines can produce a thrust of 1.8 meganewton|MN, and each of its two Space Shuttle Solid Rocket Booster|Solid Rocket Boosters 14.7 MN, together 29.4 MN. Compare with the mass at lift-off 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 air breathing category, the AMT-USA AT-180 jet engine developed for radio-controlled aircraft produce 90 N (20 Lb_f) of thrust. The GE90-115B engines fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," have a tested thrust of 569 kN (127,900 lb_f). Thrust is also a common dance move or action. This is when you move your stomach forward and your arms back repeatedly.

References

Related Images

- Forces on an aircraft

Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL

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