Concrete block smashed on lecturer's chest with sledge hammer.
What it shows:
The lecturer (or someone else) lies on a bed-of-nails without discomfort, thus demonstrating the concept of pressure, which is the force per unit area. For added drama the person is sandwiched between two beds of nails with the added weight of a cinder block on top. The cinder block can be broken with a sledge hammer.
How it works:
The forces (weight of the body, cinder block, etc.) are distributed over the total area of all the nail points so that the pressure is not high enough for the nails to puncture the skin. Of course we know what would happen if one were to lie on just one nail! In this case, the pressure is extremely high because the area of just one nail point is quite small.
A 2 cm (3/4") thick, 45 cm by 75 cm (1.5' × 2.5') plywood board has approximately 1,000 16-penny nails pounded into it so that they extend through the board. The nails are spaced about 2 cm apart. A similar board, but with twice the thickness (actually, just two boards together) of wood, is used for the top of the sandwich. One or two thick books (preferably physics books) serve as a pillow under the head.
Addition of the heavy top board plus the weight of the cinder block make it seem like one is adding to the difficulty of the demonstration. In fact, the additional weight makes it easier. The inertia of all this mass helps absorb the blow of the sledge hammer. Additionally, some of the sledge hammer's kinetic energy is dissipated by the fragmentation of the cinder block. For safety purposes, a plastic shield (with a U-shaped cut-out for the neck) as well as safety goggles are used to protect the subject's face and eyes from flying cinder block debris. 1
Setting it up:
The bed-of-nails can be set up on top of the lecture bench or on the floor. The lecturer (or whoever is to be sandwiched between the nails) and the assistant wielding the sledge hammer should practice the choreography beforehand. Easing oneself onto the nails and figuring out how to lie "comfortably" should be practiced. Relaxing is the key--tensing parts of the body tends to make the bones protrude more in that area, thus leading to increased pressure.
If you really want to milk this one for all its worth, you can demonstrate to the audience (either before or after) that the nails are truly pointed--heave an orange at the bed-of-nails. After the demonstration, a board with a single spike sticking out of it can be shown to the audience. Ask if there are any volunteers to sit on it.
Besides getting some physics concepts across, this demonstration will help dispel any misconceptions the audience has already been tainted with. Lying on a bed of nails traditionally has some mystic quality or connotations attached to it and, if not that, the opposite--only the tough, "right stuff", macho man can do it. 2 It ranks up there with walking barefoot on hot coals and breaking six boards with a karate blow of the hand, but more about that later. So here is this nerdy physicist doing it! Have faith, physics works. In case you haven't guessed by now, we think this is a good demonstration.
figure 1: A supermarket tabloid reports on how macho man pulls off this stunt.
figure 2: Wolfgang Rueckner beats death by faith in physics. Please note that this picture is posed ... in the actual demonstration a plastic shield and goggles are used to protect the face and eyes from flying debris -- very important! (Photo by Laura Wulf)
1 M. Bucher, Am J Phys 56, 806-810 (1988) "The bed of nails revisited" gives a more complete analysis of the demonstration in terms of inelastic collisions, etc. More safety precautions are suggestted.
2 The cover article reproduced here is one example.