http://www.youtube.com/watch?v=hG7lGZqWFpM

You may have seen a bed of nails demonstrated. You may even have done it yourself. I’ve laid on a bed of nails before (it’s not desperately comfortable but it doesn’t hurt) though what this video does is a bit beyond what I think I’d like to try.

So why is it possible? It really comes down to pressure. Just how much force goes through each nail, and over what area is it spread? You’ll find that there are a lot of nails on a bed of nails, so, if you divide your weight by that number you’ll get a measure of how much force each has to support.

You could easily end up with your weight distributed on a thousand or so nails. If you have a mass of say 65 kg, or weigh 650 N, that gives a force of around 0.65 N on each nail. Call it a newton for the sake of round figures. However, what’s more important is the area over which the pressure is spread. Your skin will deform around the point of the nail, and so the area over which the nail acts might be larger than you think. But let’s say its about a millimetre by a millimetre, which is a millionth of a metre squared. That gives a pressure of about 1 N divided by a millionth of a metre squared, or about a million pascals. That’s not so big when you remember that air pressure is about a hundred thousand pascals – it’s only ten times this.

Think also about the pressure on your feet when you stand on tiptoe – or when you run – when the whole of your weight is supported by a small area of a single foot. A rough estimate gives me a similar sort of pressure to what the bed of nails will do. Your skin doesn’t get ripped to shreds when you run, so then you wouldn’t expect it to on the bed of nails. And I can vouch for that.

So, if you get the opportunity, try one – it’s fun.