Flying Off Earth Name: Tyler C. Status: student Age: 12 Location: N/A Country: N/A Date: 12/20/2004 Question: How come planes do not fly off the earth since it is round? Replies: A very good question Tyler! At first glance, it would seem that an airplane, with it own power source, could indeed fly in a straight line and wind up in space somewhere. This (thankfully) does not happen due to the Earth's gravity. Think of a rock tied to the end of a string. If you spin the rock around in a circle, the rock, due to inertia, will want to always continue on in a straight line. The string however, keeps if from doing so and makes the rock travel in a circular path. In the case of the airplane, gravity is the string. Bob Hartwell Tyler C. A force called gravity stops planes from flying off the Earth. Gravity always pulls the plane toward the Earth. As a plane moves forward, gravity makes it curve down. It is like when you have a ball on a string. You spin the ball in a big circle. The string pulls the ball toward your hand, so the ball moves in a circle rather than a straight line. Gravity makes the plane, and even the Moon, move in a circle around the Earth rather than a straight line away from the Earth. Dr. Ken Mellendorf Physics Instructor Illinois Central College They do not fly fast enough. Imagine you have a ball tied to a string, and you start to swing the ball around, in a circle. The faster you swing the ball, the harder it pulls on the string. Perhaps eventually the string gets pulled on so hard that it breaks, and the plane goes flying off. The ball is like your plane, the string is like the gravity of the earth. Usually the gravity of the earth is easily strong enough to hold the plane, it is always trying to pull the plane back to earth (and the plane fights this with the lift of its wings). But if the plane flies at about 11 km/sec (forget exact number, but its called "escape velocity", or escape speed), then the plane CAN just fly off to the moon etc. But we would call this a rocket. Planes fly much much slower than this. Steve Ross All objects in the Universe attract all other objects by the force of gravity. In the case of an aircraft moving at a constant altitude its forward momentum is balanced by the force of gravity of the earth -- other sources of gravity like the sun and moon are negligible. If some how you could "turn off" the gravitational attraction of the earth, the aircraft would fly in a straight line tangent to its motion circling the earth. So in fact the aircraft is constantly FALLING toward the earth. The force being compensated by the force in its direction of flight. That this is the case you can understand if you think about what happens if the aircraft turns its engines off. It falls toward the earth attracted by the earth's gravity. Vince Calder An airplane flies because the air traveling over the wings creates lift. Since the layer of air around the earth is only a few miles thick the airplanes cannot fly more than a few miles high, thus they cannot fly off the earth. Greg Bradburn Click here to return to the Physics Archives

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