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Free-fall
In a vacuum, an object in free-fall accelerates at a rate of 32 feet per second squared. This means that the velocity of an object increases by an additional 32 feet per second every second. For example, an object falling at a rate of 60 feet per second in free-fall will be falling just one second later at a rate of 60 + 32 = 92 feet per second. As long as an object is falling, its velocity will continue to increase.
Under real-world atmospheric conditions, air friction prevents the velocity from continuing to increase and a threshold is eventually reached. This threshold velocity is called terminal velocity. Several factors can affect the terminal velocity of a person in free-fall. These factors include the weight of the person and equipment, the physical build of the person in free-fall, jumpsuit size, and air pressure. Terminal velocity usually ranges from 98 to 120 miles per hour. At a terminal velocity of 109 miles per hour, a 5500-foot free-fall from an exit altitude of 8000 feet takes 39 seconds.
Skydivers follow a strict procedure to determine the amount of safe free-fall time. First, skydivers determine the approximate terminal velocity, based on previously known jumps. A person on the ground timing jumps usually determines this information. Second, the desired chute-opening altitude is subtracted from the exit altitude, giving the distance in feet of free-fall. Once the terminal velocity and free-fall distance have been determined, charts can be referenced to determine the time of free-fall.
After the free-fall time has expired, the parachute must be deployed. All parachutes consist of a canopy, pack, suspension lines and risers, and harness. Some parachutes have a smaller parachute, called the pilot chute, that deploys first. Its purpose is to pull the main chute out of the pack. Some parachute systems utilize a ballistic spreader gun, especially during jumps with low air speeds. A ballistic spreader gun is a gas powered chute-opening device that helps force the main chute open in a reliable fashion.
The velocity of a person after the chute deploys is called vertical descent velocity. A variety of factors determine this speed including canopy size and fabric porosity. Vertical descent velocity typically ranges from 17.5 feet per second to 25 feet per second. These speeds correspond to a person jumping from heights of 3 to 10.5 feet, respectively.
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