Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Puncture Proof
Name: Derek
Status: student
Age: 10
Location: N/A
Country: N/A
Date: 1999 


Question:
Why can a person lay on a bed of nails & not be injured? What is this scientific theory called? Where can I obtain information to demonstrate this theory for my science fair? What do I ask at the library?


Replies:
This is just a matter of the amount of force per unit area. Certainly if you were to put all of your weight on just one nail, it would injure you. If you put all your weight on two nails, each would have to bear just half the weight. If we increase to two hundred nails, each nail is carrying just a small fraction of your weight, at which point no serious damage is done. If you increase the nail density even more, you approach a flat surface.

If you should ever consider using a bed of nails, make sure that you do this under the supervision of a responsible adult. Check the bed of nails to make sure that all the nails are at the same height. If one is slightly taller, it can hurt you. Also, you need to think about how you will get on and off the bed. As you change your position, you do not want to have all of your weight on just a few nails. This can happen as you sit up or roll over the very edge.

---Nathan A. Unterman---


The phenomenon to which you refer has to do with pressure. Pressure is the weight of an object divided by the total area over which that weight is distributed. If a person sat down (or laid down) on one nail, hopefully you realize that it would puncture his skin. This is because the very small part of the skin that is contacting the nail is not strong enough to support his entire weight. If his weight is spread out over a sufficient number of nails, however, each little portion of skin will be supporting a very small fraction of his weight, and he will suffer no injury.

If you do a web search for "bed of nails," you are likely to find sufficient reference material for your project.

-Spencer
Spencer Pasero


You take the weight of a person and divide it by the number of nail points the person is laying on. If the weight per nail point is small enough, the person doesn't get punctured.

Tim Mooney



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory