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What goes up..

Gerald Haigh feels the pull of strong forces

Gravity is always waiting for you. Step out of an aeroplane - as this free fall parachutist did a few seconds ago - and you immediately start to accelerate towards the ground. Gravity even sets the rate at which you will accelerate and it is the same for all falling objects, 32ft per second per second. So after one second you will be falling at roughly 32ft per second. After two seconds your speed has risen to 64ft per second.

If the force of gravity worked unhindered, you would continue to accelerate at the same rate until you hit the ground. In our atmosphere, though, gravity is held in check by the resistance of the air. So, although our parachutist accelerates like a racing car for a second or two, the drag of the air builds up, and her rate of acceleration gradually slows.

Eventually the force of the air resistance balances out the force of gravity and she stops accelerating. This state is known as "terminal velocity". She now feels as if she is lying on a noisy, buffeting sort of cushion, because she is travelling towards the ground at about 120 miles per hour, which is the terminal velocity for a human in the conventional free fall position. At this speed a skydiver who jumps from, say, 16,000ft has perhaps a minute in which to enjoy the sensation.

The free fall position - arms out in front, head up, back arched - has been arrived at by trial and error. The hurricane of air rushing past the body can induce all manner of potentially dangerous spins and tumbles. Were you to open your parachute while in a spin, the lines might become twisted or wrapped around you and the parachute might only partially open. The arched position keeps the body stable - try dropping a paper plate one way up and then the other to see how it works.

Once she is confident with the basic position, the skydiver finds that by changing her arm and leg positions she can increase her vertical speed, and she can "track" forward or back.

Free fall is exciting - but the skydiver must concentrate. She is falling at nearly 200ft every second. At 1,000ft up the earth still looks a long way down, but she will cover that distance in less than six seconds and the slightest fumble or mistake could be fatal. She will open her parachute at about 3,000ft or 4,000ft, leaving her a margin for error. If her main parachute fails, she will need time to open the reserve 'chute. She won't depend on looking at the ground to gauge how far she has to fall, she'll use an altimeter and a stopwatch.

Once her parachute is open it increases the deceleration because of way the air "drags" on it. Soon a new balance is achieved between the force of gravity and the air resistance. There is now a much slower terminal velocity - perhaps 10ft per second. Expert handling of the parachute will reduce this at the last moment, so our skydiver will land on her feet.

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