Teresa Craft, age 13, of Monroe, La., for her question:
HOW LARGE ARE PARACHUTES?
Parachutes look somewhat like large umbrellas and they are used to slow down the fall of a person or an object from aircraft or from any other great height. Parachutes designed for human use are between 24 and 28 feet across when extended and those used for cargo are sometimes 100 feet across.
For many years parachutes were made of silk. But nylon, which is stronger and cheaper, is used more generally today.
The most commonly worn parachute is the seat pack. Others attach to the wearer's chest or back.
Parachutes are worn on a harness that consists of a series of straps fitting around the shoulders and legs of the parachutist. The harness acts as a support during descent. Special straps called risers are attached to the shoulder portion of the harness. They hold the lines or shrouds which are attached to the canopy, the umbrella like part of the parachute.
Also attached to one of the harness straps is a ring for pulling the rip cord. When this cord is pulled, the parachute springs out of the pack and the air forces it open.
As soon as the canopy opens, the air slows the descent so quickly that the parachutist jerks sharply. To reduce the force of this opening shock, manufacturers have designed a special ribbon parachute. This style has holes or slots in the canopy that allows some of the air to flow through, thus reducing the area of resistance. These openings also reduce the amount of swaying during the descent of the parachute.
Manufacturers have developed still another type of parachute called the vortex ring parachute. It has four cloth sections that rotate during descent. The rotating sections function much like the rotating wings of a helicopter and allow better control of the parachute.
Parachutes descend at the rate of about 15 feet per second or slightly faster.
Parachute jumps from less than 500 feet above the ground are dangerous because this height doesn't allow enough distance for the parachute to open.
It is important for the parachutist to have some control over his parachute as he comes in for a landing. He must be able to judge wind speed, altitude and direction. Parachutists land with such force that they can easily sprain their ankles or break some bones, so great care just be taken.
The first successful parachute jump was made from a tower in 1783 by a French physicist named Sebastien Lenormand. Then in 1797, another Frenchman, this one named Andre Garnerin, made the first parachute jump from a balloon.
The operation of the parachute is based on simple principles. There are two forces that act on any falling object: air resistance and gravity. Gravity pulls the object quickly toward the earth but air resists the objects's movement.
A falling object with a great, flat surface offers a greater area of resistance to the air than a thin, sharp surface. Therefore, an object shaped like a saucer falls more slowly than one shaped like a needle.