The goal of a turbofan engine is to produce **thrust** to drive the airplane forward. Thrust is generally measured in pounds in the United States (the metric system uses Newtons, where 4.45 Newtons equals 1 pound of thrust). A "pound of thrust" is equal to a force able to accelerate 1 pound of material 32 feet per second per second (32 feet per second per second happens to be equivalent to the acceleration provided by gravity). Therefore, if you have a jet engine capable of producing 1 pound of thrust, it could hold 1 pound of material suspended in the air if the jet were pointed straight down. Likewise, a jet engine producing 5,000 pounds of thrust could hold 5,000 pounds of material suspended in the air. And if a rocket engine produced 5,000 pounds of thrust applied to a 5,000-pound object floating in space, the 5,000-pound object would accelerate at a rate of 32 feet per second per second.

Thrust is generated under Newton's principle that "every action has an equal and opposite reaction." For example, imagine that you are floating in space and you weigh 100 pounds on Earth. In your hand you have a baseball that weighs 1 pound on Earth. If you throw the baseball away from you at a speed of 32 feet per second (21 mph / 34 kph), your body will move in the opposite direction (it will **react**) at a speed of 0.32 feet per second. If you were to continuously throw baseballs in that way at a rate of one per second, your baseballs would be generating 1 pound of continuous thrust. Keep in mind that to generate that 1 pound of thrust for an hour you would need to be holding 3,600 pounds of baseballs at the beginning of the hour. If you wanted to do better, the thing to do is to throw the baseballs harder. By "throwing" them (with of a gun, say) at 3,200 feet per second, you would generate 100 pounds of thrust.

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