Motion and Its Causes

Force and Newton’s Laws of Motion

What about pins, needles, and beds of nails?

A pin or needle has a very small area at its sharpened tip, so even a small force on it will create a large pressure at the tip, enough to penetrate cloth or your skin.

Have you ever seen the bed of nails demonstration? A large number of nails are pounded into a board that is about the size of a person’s back. The person very carefully lies on the bed. Sometimes a second bed is placed on his chest. A concrete block is put on this bed and a sledgehammer is used to shatter the block.

The person is not harmed. How can that be? Even though the nails have very tiny tips and would certainly penetrate the skin if there were only one, because there are dozens of nails in the bed, the person’s weight is distributed over many nails, and the force of each nail is quite small. The pressure is not enough to penetrate the skin.

Anyone doing this demonstration has to take precautions. The parts of the body not on the lower bed have to be supported at the height of the nails. If the concrete block is broken the person must wear safety goggles, and a sheet of wood should be held to keep any concrete from flying into the person’s face.


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