The Physics of the Bed of Nails: No Woo Required
Posted by mattusmaximus on July 22, 2009
When I was at The Amazing Meeting 7 in Las Vegas recently, I participated in the first TAM Talent Show. My skit focused upon a favorite physics demonstration and gave me an excellent opportunity to teach some skepticism: the Bed of Nails. Having performed this demonstration about 40-50 times throughout my teaching career, I decided it was time to take things to the big stage :)
The demonstration is simple, and dangerous – IT SHOULD NOT BE ATTEMPTED WITHOUT PROPER SUPERVISION!!! In it, I lay down upon a bed of nails, have a second bed of nails placed atop my stomach & chest, have a concrete cinder block placed atop that, and – last but not least – have an assistant break the block with a 10-lb sledgehammer! Here’s some slow motion footage of the demo at the TAM Talent Show…
Moments later, I got up off the bed of nails with no damage at all – pretty impressive stuff! Here’s a photo of my chest seconds after the demo…
The bed of nails has a history of woo – there are some in the martial arts community who claim that their chi or “life force” protects them from injury. Others, such as gurus in India, claim that the only reason they are uninjured is because of the specific type of meditation they perform. But a simple application of Occam’s Razor is all that is necessary to explain what’s going on here – and it doesn’t have anything to do with chi, mysticism, or anything supernatural.
So what is going on with the bed of nails, and why am I not injured? It all boils down to some pretty simple physics…
1. Pressure: In physics, pressure is defined as force/area. Due to the large number of nails on each bed, both my weight and the force of impact of the hammer are distributed over a large number of sharp points (that is, a large area). As a result, the pressure at any one nail tip isn’t enough to break the skin or cut me. Also, since I’m a pretty big guy (a tad over 200-lbs), I have a large enough surface area that when I lay down I spread out and touch many more nails than a thinner person. If you’re too lean, it’s not a good idea to be laying down on the bed of nails!
2. Inertia: According to Newton’s First Law, all massive objects possess inertia – which is a resistance to changes in motion. Often this is stated that “objects at rest tend to remain at rest and objects in motion tend to remain in motion.” In other words, in this specific case, the concrete block is very massive, so because of its high inertia it won’t move much. This also means that the inertia of the block won’t allow the upper bed of nails to move much during impact, so the chances of me being stabbed are minimized. Think of it this way: which would be worse for me? Having the concrete block in the way of the sledgehammer, or having no concrete block at all?
3. Kinetic Energy & Conservation of Energy: When the sledgehammer is raised high above the block, it acquires a large amount of gravitational potential energy (GPE). As the head of the hammer falls, this GPE is converted into kinetic energy of motion (KE). Now all of that KE has to go somewhere, and the last place I want it going is into me (ouch!). If all goes according to plan, the kinetic energy of the hammer goes into breaking the concrete block, as evidenced by the chemical bonds holding the bits of concrete being broken and the subsequent pieces of block flying every which way. As a result, very little of the hammer’s KE is transferred into me. Unfortunately, I have performed the demo a few times where the block didn’t break, and it is very painful – I’m not stabbed because of #1 and #2, but it does hurt because the KE of the hammer goes into me. So, bottom line, make sure the concrete block breaks!
So there you have it, folks. No chi, no meditation, no mysticism required – just nothing more than good physics. Of course, if the New Age and “chi master” gurus really want to impress me with their supposed paranormal abilities, they’d perform the bed of nails demonstration with only a single nail :D
For some reason I don’t expect them to try this anytime soon, because when one of these “chi masters” actually does attempt to go against the laws of physics, they lose. Perhaps the best example of this is documented in the following Youtube video of a supposed “chi master” using his abilities to protect his arm from a self-inflicted machete slash. No kidding, the poor sap deluded himself into believing he could use magic to make his arm impervious to a sharp blade… wow.
If you’re squeamish, don’t watch this video – which kind of gives away the ending… “Mind, Body & Kick Ass Miracles – Master Cuts Arm w/ Sword”
There you go, folks. Physics beats the gurus – every time. I may be a bit crazy in doing the bed of nails demonstration, but I do it because I know the laws of physics – the non-supernatural laws of nature understood by science – will insure that I survive. I’ll take that over any amount of woo-nonsense any day.