Also in this week's column:

• What happens when you are executed by electrocution?
• What type of person is accident-prone?
• Why do you sometimes lose bowel function when scared?

From what height can you survive a dive into water?

Asked by Jack Crompton of Liverpool, UK

The cliff divers of Acapulco amaze tourists with their bravery and skill. But from how much higher could they plunge without suffering serious injury or death?

Surprisingly, it is difficult to be precise as to this height limit. While some unlucky ones die of falls in their bathtub, others have survived falls from incredible heights. For example, The Free Fall Research Page lists the case of World War II Russian airman, Lt I M Chisov. Chisov's Ilyushin IL-4 bomber was shot down by German fighters in January, 1942. Chisov fell 22,000 feet (6,705 metres) and hit the edge of a snow-covered ravine and rolled to the bottom. Although badly injured, Chisov survived. Although this is not cliff diving into water, it shows what is possible.

Intricately involved in any such calculation of maximum survival height is terminal velocity. Terminal velocity is the maximum speed of free fall of a human in air. Once terminal velocity is reached, no matter how much higher one falls from, they will not increase their speed in falling. Although there is some dispute about this figure, the terminal velocity of a human is estimated to be about 325 km per hour.

The speed of a diver from a 30 metre cliff is estimated to be only about 90 km per hour. This is only one-third or so of the terminal velocity.

Another factor too is plunging position. If the diver is plunging head-first, their speed will be somewhat faster than if they were falling spread-eagled due to less drag in the head-first position.

According to Linn Emrich, author of The Complete Book of Sky Sports, first published back in 1970, a 77 kg (170 lb) person would reach terminal velocity after about 14 seconds. They would fall nearly 10,000 feet (3,048 meters) in one minute. Cliff divers are not in the air for anywhere near 14 seconds. This is why they can dive and survive.

Interesting facts

Some living creatures have a terminal velocity that is not fatal. For example, ants can survive falls from heights that would be easily fatal for humans. But cliff diving ants would not be nearly as popular with tourists in Acapulco.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney.

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