The Skeptical Teacher

Musings of a science teacher & skeptic in an age of woo.

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 dangerousIT 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.

11 Responses to “The Physics of the Bed of Nails: No Woo Required”

  1. […] physical feats: walking barefoot on broken glass, breaking five wooden boards with my fist, and laying between two beds of nails while a concrete cinder block is crushed on my chest. Talk about fun […]

  2. […] I’ve stated publicly on this blog that, whether you’re talking about breaking boards, laying on beds of nails, or walking barefoot on glass shards, such amazing physical feats have absolutely nothing to this […]

  3. Mary Baker said

    Sorry, but one small part of your presentation is very ignorant and bigoted. Maybe some gurus in India–the ones you read?–claim it is due to their personal meditation practices. But true yoganandas use the practice of lying on a bed of nails to stimulate energy channels in the body evenly and induce deep relaxation–it is precisely the physics you describe that makes it possible, and they are aware of that. It’s like having a zillion massaging fingernails poking you at the same time. I have tried it myself and when performed properly it really works. Pretty amazing results, actually.

    • mattusmaximus said

      You speak of “it really works” and “pretty amazing results”, but what, specifically, are those? Is deep relaxation the result you’re speaking of, and if so how is it any different from the deep relaxation I get from listening to jazz music?

    • David Willey said

      What “energy channels”? I don’t remember anyone mentioning energy channels in the 5 years of Biology classes I took.

  4. […] And, like last year, there was a talent show in which I participated.  Last year I performed the bed of nails demonstration and explained the physics involved, but this year I decided to do the hitting, so I gave another […]

  5. […] Bed of Nails – I’ve made a very detailed blog post already on the subject (at the link).  However, […]

  6. […] abilities? No, he simply cheated. Such trickery is a mainstay of professional gurus, the bed of nails and fire walking are more familiar examples, another famous trick is the levitating guru performed […]

  7. […] The Physics of the Bed of Nails: No Woo Required […]

  8. billy said

    Physics question here: I understand that the force of the sledge hammer is distributed over the area of the cinder block… then the body… then all the nails. But It would seem that if one were just laying on the ground (no nails) the area would be even greater…. and thus the pressure would be even less. But intuitively, it seems like it would hurt a lot MORE to have a cinder block broken over your chest while lying on the ground. What’s the deal here? Thanks!

    • mattusmaximus said

      I’m thinking your intuition is incorrect. It’s just basic physics: the more area of contact you have to spread out the force of impact, the less the pressure you’ll feel. Please note that this assumes that you don’t just put the cinder block by itself on your chest – if you put it on top of a nail bed which is atop your chest and then lay down on the normal flat ground then I think you’ll feel little pressure.

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