### Question

Why can a person lay on a bed of nails and not be injured? What is this scientific theory called?

The 'secret' to this phenomenon is first to understand that it is the force per unit area of skin (i.e., pressure) that determines if a nail will pierce the skin.

The force is determined by the weight of the person lying on the bed of nails.

The area is determined by the effective area of the tip of the nail, or nails, in contact with the skin.

If there were only one nail, the entire force created by the weight of the body would be distributed over the very small area presented by the tip of the one nail. In this case, the force per unit area, that is, the ratio of the force to the area, would be very great (because the area is small) and would likely result in piercing of the skin, and injury.

However, when a 'bed' of nails is used, the same force produced by the weight of the body is distributed over perhaps hundreds of nails. Therefore, the force applied to any one nail is correspondingly reduced, with the result that the force per unit area at the tip of any one nail will (one intends) be well below the level required to pierce the skin. Since this applies to all the nails in the bed, no nail then penetrates the skin.
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA

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### Science Quote

'In a way science is a key to the gates of heaven, and the same key opens the gates of hell, and we do not have any instructions as to which is which gate. Shall we throw away the key and never have a way to enter the gates of heaven? Or shall we struggle with the problem of which is the best way to use the key?'

Richard Phillips Feynman
(1918-1988)