Thrust

The force that drives a jet plane forward is the thrust of its engine or engines. A jet engine produces forward thrust by accelerating gas backward. The forward thrust is a reaction to the backward force of the jet. Because of this, jet engines and rockets also are called reaction engines. A spinning propeller thrusts an airplane forward by accelerating the air backward.

Thrust is one of four forces that act on every powered aircraft. The other forces are drag, lift, and weight. Drag acts in the opposite direction to thrust. Weight is the force of gravity acting on the mass of the plane, pulling it down­ward. Lift is the upward force produced by a plane’s wings, a helicopter’s rotor blades, or the lifting gas of an airship.

Moving in a Circle

A force that acts in the same direction as an object’s movement can make it go

FORCE IN SPACE

Outside the atmosphere there is no air, so there is no lift or drag. Once a spacecraft is in space, it uses rockets to produce the thrust needed to change speed or direction. Without drag, a spacecraft does not have to keep firing its rockets to maintain its speed. Not all spacecraft travel completely outside the atmosphere. Spacecraft orbiting at a low altitude pass through the outer atmosphere.

The atmosphere causes drag, which slows spacecraft down, and they slowly lose altitude. To maintain orbit, they fire rockets to move back up to a higher orbit.

Spacecraft turn in space by firing small rockets or gas jets called thrusters. A spacecraft will continue to turn, even when a thruster stops firing, because there is nothing to stop it from doing so. A second thruster has to be fired in the oppo­site direction to stop it from turning.

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faster in a straight line. A force also can make something move in a circle. A turning force is also called torque or moment—the force applied by a wrench to tighten a nut, for example, is torque.

Earth’s surface and everything on it constantly moves in a circle. Spacecraft orbiting Earth are moving in a circle, too, or in an elongated circle shape

called an ellipse. An object can be made to move in a circle by a force directed toward the center of the circle. This sort of force is called centripetal force.

If a ball is whirled around at the end of a piece of string, the pull of the string provides the centripetal force that keeps the ball flying in a circle. The centripetal force that keeps everything on Earth’s surface and keeps orbiting spacecraft moving in a circle is gravity.

О The Moon stays in orbit around Earth because of Earth’s gravity, which is a centripetal force.