Blubber bullets
How fat would you have to be to be bulletproof, so that your fat layer would prevent a bullet fired from an ordinary handgun from reaching your vital organs? I recently read it was about 500 kilograms, but find this hard to believe.
鈥� The damage a bullet does to its target is measured in two ways: the depth of penetration and the amount of tissue damage per centimetre of penetration. These two figures are normally found by firing live rounds into blocks of a thick, viscous gel that is formulated to have the same physical properties, such as viscosity, density, as human flesh.
A 9-millimetre handgun round 鈥� the most common type 鈥� is quoted in the Compendium of Modern Firearms by K. Dockery and R. Talsorian (Games, 1991) as being able to penetrate approximately 60 centimetres of human flesh before it stops, doing an average of 1 cubic centimetre of damage per centimetre of penetration. In reality the distance penetrated is often much less, because rounds frequently hit bones or simply pass through the target. This data is also based on a general body tissue average. Because fat is approximately 10 per cent softer and less dense than muscle, the figure of 60 cm may be too little.
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Although being bullet-proof may sound advantageous, carrying a 60-cm-thick layer of body fat obviously comes with its own health hazards.
Thomas Lambert
Baslow, Derbyshire, UK
鈥� A human body would never be entirely bulletproof when you take into consideration tissues and appendages such as the hands, feet, eyes, ears and male genitals. Even if the skin was sufficiently thick to stop a bullet, the shock wave could seriously harm internal organs and the network of nerves below the skin, an effect which shot pellets exploit. Pellets from a shotgun can kill a human without penetrating the skin.
A bullet鈥檚 depth of penetration in a body depends on a number of factors, such as the bullet鈥檚 energy, diameter, mass, shape and material. Bullets from rifles and handguns may range from approximately 5 to 15 millimetres in diameter and from 70 to 7000 joules in energy. A typical police handgun bullet has a diameter of 9 mm and an initial energy of 500 joules. Penetration depth is measured in a gelatine block, and the police handgun bullet typically penetrates about 30 cm of gelatine at a distance of 5 metres from the barrel.
To estimate how much such a fat layer would weigh, start with the surface area of the person 鈥渦nderneath鈥�. There are several formulae to calculate the body surface area; I will use the Mosteller formula, which gives an individual鈥檚 body surface area in square metres as the square root of the product of their height in centimetres and their weight in kilograms, all divided by 60. For a man 175 cm tall and weighing 75 kg, this yields a body surface area of 1.91 m2. So in order to cover this area with a 30-cm-thick layer of fat with a density of 1 gram per cm3, we would need at least 573 kilograms. When you add this to the weight of the body, you find that a typical bulletproof person would weigh about 650 kilograms.
Hans-Ulrich Mast
Erding, Germany
This week鈥檚 questions
High brow
Why do people have eyebrows?
Ben Holmes
Edmonton, Canada
Ice makers
In the summer of 1809 the poet Byron and his friends visited Venice and ate ice cream in the cafes. How was ice cream made when there were no refrigerators?
N. M. MacNaughtan
By email, no address supplied
Pop the question
When I was a child in the 1950s I had a toy pop-pop boat, so called because of the popping noise it made as it moved across the water. I have recently found them in toyshops again, and I want to know how they work.
The boat is about 12 centimetres long and contains a flat, horizontal water boiler of a thin pliable material that looks like shim brass. It is about 30 by 35 millimetres on top and 3 mm deep. The boiler has two pipes that connect underneath it at the front, run along the bottom of the hull, and exit at the stern below the waterline. They are plain pipes with no valves or other mechanisms.
You fill the pipes and boiler completely with water and float the boat on a pond. Then you place a small candle under the boiler and soon the boiler emits a popping sound as the top membrane starts to vibrate up and down two or three times per second. This causes water to pulse in and out through the pipes and propels the boat forward. The boat does not use up the water in the boiler and operates as long as the candle stays alight. What is the physics behind this?
The photo shows a recent model, although my own 50-year-old version, while a little dilapidated, continues to work perfectly well.
Trevor Thompson
Ferny Hills, Queensland, Australia