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Dead before you hit the ground

There's a fairly common belief that if you happen to fall from a great height, you'll be "dead before you hit the ground". The reasons given probably stem from fear of your imminent death, or a generalised terror, leading to shock, heart attack, or even asphyxiation. On one hand, it is kind of reassuring to believe that you won't feel any pain if you fall from a great height. The reality is that it's the huge deceleration (as you suddenly stop) that kills you.

It's really hard to die while you are in "free fall", ie, falling freely through the atmosphere. One scenario in which you can die in free fall is that you are so high up (say above 100,000 feet or about 30 km) that the intense cold and lack of oxygen will kill. But even this scenario can be survived. On August 16, 1960, US Air Force Captain Joe Kittinger rode a helium balloon to 102,800 feet. The temperature was -79°C. It turns out that the air pressure is low enough at 62,000 feet to have boiled the water in his blood - and at 102,800 feet, the air pressure is actually a lot lower. He was kept warm by many layers of warm clothing. He was kept alive by the thin newly developed MC3 pressure suit covering his entire body, and a tank strapped to his body feeding him pure oxygen to breathe.

He jumped out of his open gondola, and began falling. By 90,000 feet, he had reached about 1,149 km/hr - faster than the speed of sound. He fell in free fall for about four-and-a-half minutes. His speed gradually reduced to around 200 km/hr as he dropped though the increasingly-thicker air. His parachute opened around 14,000 feet. There was a sudden jerk as his speed suddenly dropped to around 21 kph. He landed about 12 minutes later, with no permanent injuries. He still holds two records - the only person to break the sound barrier without being in a craft, and the highest parachute jump.

The important thing to realise from this tale is that the act of falling freely does not kill you. You might become so terrified as you fall (which sounds very reasonable to me) that you might psychologically "freeze" and become unaware of your environment, but the actual fall does not kill you.

On the other hand, if you stop suddenly from 200 km/hr over a distance of a few centimetres, everything in your body effectively weighs 7,500 times more than normal. Your 1.5 kg brain briefly weighs 10 tonnes. In that brief instant, cells are burst open and blood vessels are torn asunder. The aorta (the huge main artery coming out of the heart) will usually rip loose from the heart. For a few beats, your heart continues to pump blood into the space around the heart and lungs, while no blood goes to your brain. But most of the blood vessels in your brain have also instantaneously torn loose. After that brief instant, your "weight" returns to normal - but blood is now eating its way through your irreparably damaged brain. This is what medical people mean to when they refer to "massive internal bleeding".

There was a case of a parachutist who survived a fall (with broken limbs) when her parachute did not open. But the ground was very soft, and according to the fire officer, "She left a good 12 inch hole into the ground".

In general, most fit humans can just barely survive a sudden deceleration of about 40 G. In a "typical" car crash at around 100 kph, the people inside can probably survive, if they are wearing seat belts. The crush zone of the car is about 1.2 metres - that's by how much the car shrinks. The airbag and/or seatbelt slow you down over another 0.4 metres. If the deceleration was even (which it is not), the victims would suffer decelerations of around 25 - 30 G. But the deceleration is usually not uniform, so there are very brief peak decelerations of 40 - 60 G.

There are no decelerations in free fall. But it's not the fall that kills you - it's the Big Crunch at the end that brings you to a dead stop.

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