Imagine you are a sky diver about to step out of a plane and fall through the air. What forces will you experience during your fall? How can you use these forces to combine an exhilarating experience with a safe landing?
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Sky diving relies on two principles of physics. First, if there is nothing to support you, the force of gravity will cause you to accelerate downward. Second, all fluids—including air—produce a drag force that opposes the motion of an object moving through the fluid. The speed of a falling object increases until the drag force equals the force of gravity. At this point, the net force is zero and the sky diver falls at a constant, terminal speed.
The actual terminal speed depends on several factors. For example, an object with greater mass has a greater gravitational force on it, increasing its terminal speed. Thinner air, such as air at very high altitudes, increases velocity by decreasing the drag force. A sky diver can also partially control the drag force, and terminal speed, by changing shape. Opening the parachute dramatically increases the drag force and lowers the terminal speed to about 4.5 meters per second—a speed suitable for landing.