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

Asked by: Marcia

Answer

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

'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'