I would like to know two answers. First, I would like to know how those swamis lie on a bed of nails, and I would also like to know how they do that weird rope trick thing. –Anonymous
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The trick, when you think about it, is obvious. You’d impale yourself if you put all your weight on a single nail, but it’s a different story when your weight is spread across hundreds. What you’ve got here is a demonstration of pressure as force per unit of area. According to one calculation, a 70-kilogram individual lying on a grid of nails spaced at 2.5-centimeter intervals would exert a downward force of only 40 grams per nail, not enough to break the skin. In fact, there’s enough safety margin that real daredevils have a volunteer stand on them or pile on concrete blocks (sometimes atop of an additional bed of nails, points down), which the volunteer then whacks with a sledgehammer. No damage, although you have to wonder whether doing this for a roomful of freshmen who just got their physics midterms back isn’t pushing your luck.
Despite the precautions, things can go wrong. Physics legend Jearl Walker–you remember Jearl, the guy who used to plunge his hand into a vat of molten lead to demonstrate the Leidenfrost effect–tells of performing physics demonstrations as part of motivational talks for IBM salespeople (“If I can plunge my hand in a vat of molten lead, the least you pups can do is sell some computers”). The bed-of-nails demo was customarily preceded by a stunt in which Jearl “fell” off the stage to the floor below. (I’m not sure what principle of physics this demonstrated, but it’s a can’t-miss principle of comedy.) One day Jearl took his pratfall a bit too hard, breaking a rib. Demonstrating the show-must-go-on spirit that’s the hallmark of true scientists, he gamely proceeded to do his bed-of-nails act, in which the local IBM boss would stand on his chest, sandwiching him between two beds of nails. Unfortunately for Jearl, this particular IBM boss weighed 230 pounds. “When he stood up on the top bed of nails, the pain in my chest went ballistic,” Jearl relates. “I could hardly breathe.” Somehow he got through it, then dragged himself to a doctor’s office and got patched up. He was back giving bed-of-nails demonstrations the following week.
