The Bed of NailsI 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 DemoThe demo has two players (the "victim" and the "Hammer"), and 5 pieces of equipment:
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:
Haunted Labs 2010In 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 ClassroomThe 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 & ImagesSome other captured video of the demo. Double click to start videos.
ExplanationShow 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"):
ConstructionA few pictures of construction, for those who would like their own bed of nails! Click to enlarge.