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Union University Department of Physics

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Science Guys >November 2000

November 2000

How can a person lay on a bed of nails?

****WARNING!**** Reclining on a bed of nails violates all OSHA standards, and should only be attempted by physicists and swamis. If you try this stunt, be it known that we, the Science Guys, did not encourage you to do so.

Most children know of someone who has stepped on a nail and sustained a serious puncture wound. We naturally think of nails as being able to pierce the skin. How can someone lying on a bed of nails not have hundreds of holes in their skin? Actually, this is a timely question because we happen to have constructed a bed of nails at Union last month.

The pressure on the skin determines whether or not a nail pierces the skin. We first need to distinguish between force and pressure. Force is a push or a pull, while pressure is defined as the force per unit area or force divided by the area over which the force is exerted (P = F/A).

The force pressing the person onto a bed of nails is the weight of the individual. If a single nail had the entire weight of a person pressing on it, the pressure would be tremendous because the area over which the force is applied would be very small. However, a bed of nails consists of literally thousands of nails. While a nail is thought to be very pointed, it actually has a somewhat flattened area on the tip.

Consider that the average force exerted by a hammer striking a nail is about 100 pounds. The sharp nail tip has an area of one-hundredth of a square inch. So the pressure the nail tip exerts on the wood is 100 pounds divided by 1/100 square inch for an amazing 10,000 pounds per square inch! Certainly such a concentrated force could pierce the skin. Contrast this to the pressure between a man's feet and the floor. A 180 pound man is supported on two shoes of a total area about 100 square inches. This is merely 1.8 pounds per square inch. (As ladies know, walking on new linoleum can produce pockmarks because of weight being concentrated across the small area of the high heel.)

Assuming the one-hundredth of a square inch area for a nail tip, then for every hundred nails you have one square inch of area. Suppose 2000 evenly spaced nails support you. The combined nail points would have a total surface area of 20 square inches. If you assume that an adult man weighs 160 pounds, then his weight is spread over 20 square inches of nails, and the pressure on him is only 8 pounds per square inch of nail tips. If each nail tip is only 1/100 of a square inch, then each nail supports only eight hundredths of a pound of weight. While this might not be very comfortable, it is manageable, and a person can tolerate each nail pushing on their skin with only an ounce or so of force. The resulting force per unit area or pressure is not sufficient to pierce the skin.

The trick to lying on a bed of nails is getting onto or off of the bed of nails in such a way that most of the nails start or stop touching you at about the same time. This keeps the weight force spread over a large number of nails. As you can see, positioning yourself on a bed of nails takes some planning, and is definitely one of those "do not try this at home" tricks!