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The Bed of Nails

I have always regarded the Bed of Nails as The #1 Physics Demonstration of All Time and bill it as such to my students. As a general rule, my students don't know what the demo is going to be, just that I think it is the best one ever, and that it has the potential to do me bodily harm. On the day of the event, it is unveiled, we do the demo, I SURVIVE then we talk about the physics.

This demonstration is a classic example of the impulse-momentum theorem and comes at the end of our discussion of momentum.


The Demo

The demo has two players (the "victim" and the "Hammer"), and 5 pieces of equipment:
  • Safety glasses
  • 10 pound (4.5 kg) sledgehammer
  • 1/2 cinder block (8"x8")
  • The Chestplate of Doom
  • The Bed of Death
The Chestplate of Doom is a 16" x 16" square of plywood, pierced by 576 16 penny nails on 5/8" spacings.

The Bed of Death is a 2' x 4' piece of plywood, pierced by 2664 16 penny nails on 5/8" spacings.

The demo proceeds as follows:

  • I vamp and stall, feigning fear of this demo.
  • I toss my wallet and iPhone out into the audience, obstensibly to pass on to my family should I perish.
  • I (The Victim) lie down on the Bed of Death.
  • The Chestplate of Doom is placed on top of me.
  • A cinder block is placed in the center of the Chestplate of Doom.
  • The Hammer prepares to take a large roundhouse swing at the cinder block, but I scream "WAIT!"
  • I remind The Hammer that eye protection should be used, and The Hammer puts on safety goggles.
  • The Hammer takes a full roundhouse swing, and breaks the cinder block.
  • The debris and Chestplate of Doom are removed, and I stand up unscathed.
  • The Laws of Physics Prevail!

Haunted Labs 2010

In 2010, the Society of Physics Students put on their annual "Haunted Labs" haunted house in the SER Building at USU. The event is always extremely popular, and the lines are long, so they entertained those waiting with physics demos. I agreed to do the Bed of Nails. Over the course of about 2 hours, I took 20 hits. :-)

TED2012 -- The Classroom

The following composite video was prepared in December 2011, in the style of my usual short science videos, as part of my application to TED2012: The Classroom. Double click to start videos.


Video & Images

Some other captured video of the demo. Double click to start videos.


Explanation

Show what is going on here? The impulse-momentum theorem is about how you use an impulse (provided by a force) that changes the momentum of an object.

The classic example is rockets --- a rocket engine provides a force (the thrust) that makes a rocket go (increases the rocket's momentum).

In this demo we are doing the opposite -- using a force (the cinder block pushing on the hammer) to slow down a fast moving dangerous object (the hammer).

There are two ways to stop the hammer (to "provide an impulse"):

  1. Method 1 is to provide a large force in a small amount of time.
  2. Method 2 is to provide a small force over a large time
If I am going to survive the experiment, then the option I want is clearly Method 2 -- small forces! The cinder block slows the hammer down over time. Instead of hitting me directly and slowing down in about 0.001 seconds, the cinder blocks makes the slow down take much longer, about 0.5 seconds! This small change reduces the force by about 500x -- in the first case the force is equivalent to 7700 pounds stacked on top of you (~3 cars), and in the second case about 15 pounds!

Construction

A few pictures of construction, for those who would like their own bed of nails! Click to enlarge.

grid thumb drilling thumb nailing thumb
done thumb nails thumb back plate thumb
plate on thumb bed of nails thumb hand thumb
wheels thumb cover buckle thumb handles thumb
uncovered thumb chestplate thumb skull thumb

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