﻿ Fakir Physics

Fakir Physics

pressure - impulse - inelastic collisions - transformation of energy - faith in physics

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.