Table of Mechanics Demonstration

List of Mechanics Equipment & Supplies

Lecture Demonstrations

Bed of Nails, 1K30.10


BedOfNailsSetup01-400.jpg BedOfNailsSetup02-400.jpg


A 2′ by 4.5′ bed of nails is made up of 80d nails placed in a 1" grid. The bed contains 1375 aluminum nails which have been leveled and dulled. A second bed of nails (1.5′ x 2.5′) made up of 425 aluminum nails placed in a 1" grid can be placed on top of the person lying on the larger bed of nails. For extra excitement, break a cinder block that is placed, centered and stable, on top of the small bed of nails.



ID Number

Large Bed of Nails

ME, Floor Item


Small Bed of Nails

Nestled within larger bed


Cinder Blocks

ME, Floor Item


sledge hammer

With the beds


Safety Pillow

With the beds


Safety Shield

With the beds


Safety Boards

With the beds


Safety Glasses

Within Safety Drawer


Important Setup Notes:

Setup and Procedure:

  1. Place larger bed of nails onto level floor. Place the bed on two cinder blocks for extra height and or comfort
  2. Have volunteer stand with back to bed, feet about 6" from base
  3. Have two people, one on either side of the volunteer, grab the arms of the volunteer and slowly lower volunteer onto bed
  4. After volunteer is laying on bed, place the pillow beneath his or her head

When Breaking a block:

  1. Put face shield on volunteer
  2. Gently place smaller bed of nails nail-side-down on top of the volunteer so that it is stable, centered on the volunteer's torso
  3. Place a board, leaning with its bottom on the volunteer's lap leaning onto small bed so that the volunteer's groin is covered from falling debris
  4. Place cinder block vertically-oriented in a stable position, centered on the back of the smaller bed of nails
  5. With a lite overarm stroke, use sledge hammer to smash cinder block, while wearing safety glasses
  6. Remove any debris from the bed and the volunteer
  7. Gently remove small bed from volunteer's torso
  8. Have two people, one on either side of the volunteer, grab an arm and gently assist the volunteer off the bed of nails

Cautions, Warnings, or Safety Concerns:


Several physics principles are involved here: force, pressure, inertia, and energy.

The total force on the bed of nails (the volunteer's weight)is spread out among all the nails that are in contact with the volunteer. Thus, each nail supports only a faction of the total combined weight of the volunteer and the cinder bock. In fact, with our setup, one can stand on the top of the small bed of nails instead of having a brick on top. The total force is still spread out among the nails, and the reaction force from any individual nail is still greatly reduced. For example, let's assume the volunteer has an average human weight of 175 pounds. Our lower bed has 1375 nails, however not all of them are in contact with the person so let's assume that 60% of them are. Then the weight is spread out among approximately 825 nails. Therefore, the volunteer will only feel about 0.21 pounds(corresponding to a mass of ~3.4 ounces or ~95 grams) of force per nail on average. Some nails will poke the volunteer harder than others (i.e. since the weight of the body isn't evenly distributed, more weight will be supported by some nails than others). With a person standing on top, the force per nail is roughly doubled to 0.41 pounds of force per nail applied by each bed. The volunteer will feel slightly unconformable but will not be close to the average pressure threshold of pain which is approximately 23 pounds per square inch for an adult.

When the sledge hammer smashes the block, the volunteer will experience an additional force. The upper bed only has 425 nails which are, for the most part, all in contact with the volunteer's torso. The force imparted by the hammer is distributed across all of these nails. However, the cinder block is so massive (a weight of ~37 pounds or mass of ~590 ounces or ~17 kg) that almost all of the imparted force will be absorbed by the block. The block's relatively large inertia opposes much of the motion of the hammer. In addition, a large majority of the hammer's kinetic energy is taken away by the chunks of block that fly away during the impact. Thus, the force from the hammer imparted upon the volunteer is significantly reduced. This is why it is important that the block breaks. If it doesn't, all of the kinetic energy is available to be transferred.








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