Bed of Nails


The bed of nails is an effective demonstration of the physics of pressure and is relatively straightforward to construct. A wooden base is required, which we have built in two sections to make easier the storage of the device when not in use. Strong hand rails are constructed along the sides of the base, in order to assist people in getting onto the bed. The base is about 2cm thick. A matric of holes is drilled in the base so that the spacing between the adjacent holes is 2.5cm. The diameter of the drilled holes is a couple of mm less than the diameter of the nails used, which are 10cm long iron nails around 4mm in diameter. In our bed, there are 1762 nails in total. The nails need not be made artificially blunt, since the distributed weight of a person lying on the bed will safeguard injury from even quite sharp nails. The nails have to be pre-selected so that their lengths do not vary by more than about 1mm. Nails of best match length are hammered into the central region of the bed where the person's back and legs are likely to be. The demands on matching the lengths of nails around the sides of the bed are obviously much lesser.




In use, the bed is placed on a flat surface and the hand rails used by the volunteer to lower themselves down onto the bed. It is best to sit on the nails first, taking up most of the bodyweight with the handrails, and then gently lowering the back, and legs onto the bed. A small pillow behind the head will avoid discomfort, and the experience of lying flat out should be painless. We have placed a wooden platform on a volunteer and had a 100kg person stand on the platform! A soft building block can also be cracked apart with a axe or hammer resting between two supports mounted on a wooden platform resting on a volunteer, although it is best for the person lying down to wear goggles in case of flying debris.
The physics behind the bed of nails is relatively straightforward. Consider an adult lying down on the bed. The weight of an average adult is around 70kg. Now, since there are over 1700 nails, that means that on each nail there is about (70/1700)kg of weight; that is, roughly 0.04kg, i.e. only 40g. This is a measure of the effective pressure of the system, or the 'weight per nail'. If you imagine taking even the a rather soft object weighing 40g and balancing it on the sharp point of a nail, nothing much will happen. So the human body will be fine. Even if the weight is doubled, or trebled to 120g, the nails will not pierce through clothes or skin. Things begin to get a bit risky when the weight on any one nail approaches 200g.