# Thrust

For the land-speed record breaking car, see ThrustSSC and Thrust2

For the computer game, see Thrust (computer game)

For the rapper, see Thrust (rapper)

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

Examples:

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Aeroforces.jpg
Forces on an aircraft

1. 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).

2. 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.

3. 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.

Examples:

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy