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Thrust is a reaction force described quantitatively by Newton's Second and Third Law. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system.

Mathematically this means that the total force experienced by a system accelerating a mass m, is equal and opposite to the mass m times the acceleration a experienced by that mass:

F = −m·a

## ExamplesEdit

An aircraft generates forward thrust when the spinning propellers blow air, or eject expanding gases from a jet engine to the back of the aircraft. The forward thrust is proportional to the (mass of the air) multiplied by (average velocity of the airstream).

Similarly, a ship generates forward thrust (or reverse thrust) when the propellers are turned to accelerate water backwards (or forwards). The resulting thrust pushes the ship in the equal and opposite direction to the sum of the momentum change in the water flowing through the propeller.

A rocket (and all mass attached to it) is propelled forward by a thrust force equal to, and opposite of, the time-rate of momentum change experienced by the exhaust mass accelerating out from the combustion chamber through the rocket nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled. 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 MN, and each of its two Solid Rocket Boosters 14.7 MN, together 34.8 MN. Compare with the mass at lift-off of 2,040,000 kg, hence a weight of 20.0 MN.

The simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N each.