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Thrust

A force pushing an airplane, or any object, forward is called thrust. The thrust is produced by the engines of the airplane or by the flapping of a bird’s wings. The engines push fast-moving air out behind the plane, by either propeller or jet. The fast-moving air causes the plane to move forward, countering drag. Since the Wright brothers first flew in 1903, aeronautical engineers have created a multitude of airplane types, every one of which has dealt with the same four forces of weight, drag, lift, and thrust. All people have to deal with the challenges of stability with respect to these forces. Flying faster than the speed of sound has its own special demands, but the underlying forces of weight, drag, lift, and thrust remain the same.

In some sense, it is easier to fly in space, which is devoid of air, than it is to fly in air. However, spaceflight has its own special challenges. In space, one must deal with only two forces, weight and thrust. Thrust provides the force to lift a rocket into space. Once in orbit, a spacecraft no longer needs propulsion. Short bursts from smaller rockets are used to maneuver the spacecraft. To change its orientation, a spacecraft applies torque, a twisting force, by firing small rockets called thrusters or by spinning internal reaction wheels.

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Acceleration

Imagine a track meet. The runners all line up at the starting line. At this point, their velocity is 0—they aren’t moving. Then, the starting gun goes off, and the runners push off. They begin to increase their speed.

We say that they accelerate. To most people, acceleration means simply “speeding up.” In science, though, the word has a different meaning. It is the rate at which velocity changes. Remember that velocity involves the direction in which an object moves as well as its speed. So accelerating the object may involve changing its speed or changing its direction (or both).

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