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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


Bed of Nails
Figure I. Figure II. Figure III.

Description

Equipment: Bed of nails (1,449 nails were nailed into 7/8" thick plywood. The plywood is 30" by 68" in size. The bed consists of 63 rows of 23 nails, each nail spaced 1" apart in a square grid. The nails are 3.125" long.) The holes for the nails were pre-drilled to make sure the nails would be straight.

Large wooden block (37 lb. block with (21.75")X(13.5")X(4.5") dimensions)
Large 5 pound hammer

Step 1: The bed of nails is shown to the audience. No students are allowed to lay on the bed of nails; only adults may do this. A volunteer teacher is selected and carefully placed onto the bed of nails. (See Figure I.) For safety, a towel is placed underneath the teacher's neck and ankles. Remarkably, the teacher is unharmed! (See Figure II.)

Step 2: A large wooden block is placed on top of the teacher and one of the demonstrators uses a large hammer to hammer the block. Again, the teacher is unharmed. (See Figure III.) Finally, the teacher is removed from the bed of nails.

Description

Basic Ideas: A nail can certainly be harmful, but a certain amount of force can be applied to a nail head before pain is induced. (Try pushing on a nail very lightly with your finger, you'll find that there is no pain).

A human's weight, although substantial, can be spread out over many nails, thus avoiding an apparent disaster.

Step 1: When laid very carefully on the bed on nails, the teacher's weight is more or less evenly distributed on the nails. Since each nail is only experiencing a small amount of the teacher's weight, the teacher experiences no pain.

Step 2: Since the wooden block is so massive, the blows from the hammer are almost entirely absorbed by the block. The teacher feels very little added pressure from the block. The tiny force that the teacher does feel is not enough to cause pain.

Description

Basic Ideas: A human's skin can endure approximately 2 pounds of force from an ordinary nail before feeling any pain. With the added protection of clothing, the skin can stand even more.

An average human of 150 pounds, when spread out among approximately 600 nails, will feel only about 0.25 pounds of force per nail.

Step 1: Since the teacher's weight is applying only about 0.25 pounds of pressure per nail, and the threshold of pain is approximately 2 pounds of pressure, it is clear that the teacher will remain unharmed. It can be argued that the teacher's weight is not evenly distributed on all the nails due to the weight distribution of a human, but even tripling the amount of weight on one specific nail (giving a total force of about 0.75 pounds) is not enough to cause pain. And don't forget that the extra protection of clothing is significant.

Step 2: When the hammer is hit against the block, the teacher will experience an additional force. The block is so massive however (the block weighs 37 pounds) that almost all of the impulse will be absorbed by the block. The teacher will experience very little additional force from the hammering of the block. The exact calculation and verification of this is left as an exercise for the reader.

Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia