1.56 Beds-of-nails demonstrations
Jearl Walker www.flyingcircusofphysics.com
Dec 2008 A long time ago, I introduced two bed-of-nails demonstrations to physics education. One (very painful) example is shown in the following photograph.
You see me sandwiched shirtless between two beds of very sharp nails while Peter Wiley (of my publisher, John Wiley and Sons) stands on top of the sandwich. (The photograph is courtesy of Cynthia Spencer, also of John Wiley & Sons.) If you have enough resolution on the photograph, you can see that in spite of the rigidity of the top bed, it is bent at the edge of my rib cage.
When Peter Wiley stepped off the top bed of nails and then helped me get up, I felt (and maybe heard) a slight suction effect as I pulled my back off the nails on the lower bed. The nails had not pierced my skin but they had been on the verge of penetration, with skin tightly folded over the points. Detaching myself from the nails may have caused a slight rush of air into the indentations in my skin.
In the second type of bed-of-nails demonstration, I am again sandwiched shirtless between the two beds but now a concrete block is positioned on the top bed. An assistant then swings a heavy sledgehammer down on the block, smashing it and momentarily compressing the sandwich, that is, compressing me between the two layers of nails, in something resembling the “iron maiden” of medieval torture.
At the end here, I give two links where you can see old videos of me performing the two types of bed-of-nails demonstrations.
The physics of someone standing on top
When one or two people stand on me, their weight is spread over enough nails in the top bed that the force on me from each nail is insufficient to pierce my skin. The force from the nails on my back is larger, because they must also support my weight. By experimenting I discovered how much weight the people can have before I am pierced. I can roughly calculate the pressure on me from each of the bottom nails by dividing the total weight by the number of nails on my back and by the area of each point on the nails. However, the calculation is only suggestive because my body is not uniform and thus I support more weight on some nails than on others. Don't think that I go without pain, because the demonstration hurts a great deal.
Here are some more photos. In the first two, student Amanda Beach is standing on me while being steadied by M. J. Saunders, the provost here at Cleveland State University. Amanda was quite worried about hurting me (but I don’t think the provost was). The next two photos show me doing push ups on a bed of nails while wearing gloves lined with Kelvar (sold by the Duluth Trading Company http://www.duluthtrading.com). I could feel the points on the nails but because they did not penetrate the Kelvar, the pushups were not painful. (The photos are courtesy of T-Fiz.) The fifth photo shows Kyle Taljan of Oberlin College standing on me while holding some of the books I have written. At that moment I was wishing I had written less weighty books. Dan Styer, a physics professor at Oberlin, looks on with mild concern. (Photo courtesy of Professor Jason Stalnaker, also of Oberlin College.)
The physics of smashing a concrete block
When a concrete block is smashed on the top bed, the large block not only adds a theatrical flare to the demonstration because it shatters so nicely, but it also increases the safety in three subtle ways.
1. If I am to be squeezed hard, then the block and top bed must accelerate rapidly downward; a larger block diminishes the acceleration because of its greater mass.
2. Much of the energy in the sledgehammer goes into rupturing the block rather than into the bed's motion.
3. The fact that the block disintegrates means that the collision time (from the start of the collision to the end) is longer than if the sledgehammer hit the top bed directly. That means that the force in the collision is smaller than with a direct hit.
Both bed-of-nails demonstrations hurt and are potentially harmful, especially the one with the sledgehammer because the face can be damaged by the debris from the concrete block. I have had some scary and also some silly moments with the demonstrations. Here are some of the stories.
First time with the smashing demonstration
The first time was in a classroom. I asked one of my students to swing the sledgehammer, but quite unwisely I had him place a common small brick on top of the sandwich of nails. I told him to hit the brick hard, covered my face with one hand, and then gave him a count of 1…2…3 so that I could brace my abdomen for the impact. The student hit the brick very hard with the sledgehammer, so hard that I lay stunned on the floor for several minutes. The students in my class were shocked, but my primary thought was that this was a stupid way to die. However, the advantage of using a large concrete block was suddenly very obvious to me. Pain has a way of rapidly clarifying the physics principles.
The smashing demonstration at Oxford University
One summer I gave the Flying Circus of Physics talk at Oxford University in England to a meeting of education experts from around the world. Unfortunately, only few in the audience spoke English, much less understood my Texas-brand of humor. So, as the talk went on and I generate only slight laughter at my jokes, I became more nervous and less cautious.
When I got to the bed-of-nails demonstration at the end of the talk, I discovered that I had to perform the stunt on a bench so that everyone could see it. My assistant put the concrete block on top of the bed sandwich and then swung the sledgehammer down on the block. Because I knew that, with this unusual arrangement, the angle of swing was awkward for the assistant, I tried to steady the top bed by grabbing it firmly with one hand. When the sledgehammer hit the block, the impact drove a nail across my hand, cutting it.
I didn't realize that I had been cut until I stood up to give my closing remarks, but then the blood flow was noticeable to both me and the audience. The audience was impressed with the demonstration and especially the blood --- they required no grasp of English to know that I had hurt myself.
After packing up the equipment, I met my host at a local pub for a pint of ale, feeling somewhat relieved that at least the final demonstration of the talk had impressed the audience. Then my host told me that the disease tetanus ("lock-jaw") was a real concern in that part of England. I had not minded the pain of the cut, but the thought of contracting tetanus worried me. (The bacteria causing tetanus enters the body through a cut from, say, a dirty nail. If the bacteria are not stopped, the victim soon dies while every muscle in the body in full contraction, unable to breathe.) I put down the pint of ale and hurried over to the local hospital for a tetanus shot.
There I had to explain to the nurse how I had cut myself. As she drove the needle into my rump, she was laughing so hard that she shook. I had traveled across the Atlantic Ocean to impress educators and had ended up dropping my pants in front of a nurse laughing at me.
Demonstration at a girl’s school
Equally embarrassing was the time I gave the bed-of-nails demonstration at an all-girls high school. I figured that the woman who invited me did not want to see the sledge-hammer part of the demonstration, so I planed to do only the part where I am sandwiched in the two beds of nails with a person standing on top. Over the phone she agreed to be that person.
What I did not think about during the phone conversation was the clothing that she should wear. I did not think about that feature until I was sandwiched in the two beds with the woman about to get on top. She was wearing a short skirt and, while standing immediately next to my head, decided to talk to the audience about what she was going to do. I did my very best to turn my face to the audience instead of looking upward; the girls in the audience went wild with laughter; the woman never understood what the problem was; and I was unable to straighten out my neck for a week.
Demonstration at IBM conventions
I used the bed-of-nails demonstration not only in class and in my Flying Circus talk, but also in a series of motivational talks I gave to sales people of IBM. I began the motivational talks by being the stereotype physics professor (talking of grand things, being as boring as possible) and then slowly dropping the talk into slap-stick and then ending with the bed-of-nails demonstration. My message was that I sell a product (physics) to people (students) who often do not initially want the product, much like the sales people try to sell their IBM product to customers who may not want the product.
In part of the slapstick I did a pratfall on the stage and fell over the edge of the stage to the floor. This stunt looked like a huge mistake, and every one of the 1000 people in each audience froze with tension when I did it because mistakes are never ever supposed to happen at an IBM presentation. Only gradually would the audience realize that the fall was planned, and then for the rest of my talk they would laugh and even cheer at my stunts.
Before each of these talks I met the leading IBM executive that was there, because he was to be the one who would stand on me when I was in the bed-of-nails sandwich. Each of these executives was concerned about hurting me. I told each, "Hey, look, this demonstration is OK. Sure, your weight is going to hurt me by pressing the nails into me, but the pain is something I can endure while you are up on top. Don't worry; I've got this figured out."
At one talk the executive was especially worried because he weighed about 230 pounds, which would put me at the limit of the pain I could endure. Nevertheless, I went through my calming words. Unfortunately, when I did my pratfall during that talk, I broke a rib as I hit the floor below the stage. At the time, I did not know that a rib was broken; I only knew that my chest hurt like crazy. I continued the talk, doing the rest of the slapstick while not breathing very well. Then came the bed-of-nails demonstration and the 230-pound executive. When he stood up on the top bed of nails, the pain in my chest went ballistic and, though I could hardly breathe, I said my routine words about the stunt.
I was back in Cleveland and at my doctor's office later that day. She told me that I had broken a rib and that I should take life easy for a month. I tried to laugh (but couldn't) and said, "You've got to be kidding. I have to be back at the IBM talks next week." And I was. But thankfully the next several executives that stood on me each weighed less than 230 pounds.
Blood all over me
Once when I gave the Flying Circus talk at Western Illinois University, my assistant could not make the trip and I asked my host if he would swing the sledgehammer down on me during the final demonstration. I told him to not be timid about the swing because that would disappoint the audience. I wanted a dramatic ending, and he gave me one. He swung that sledgehammer down hard, really jarring me. However, he came in at angle that sent most of the chunks of the concrete block across my face. I had one hand guarding my teeth and eyes, but one of the large chunks cut across my exposed chin.
When I climbed out of the beds of nails and stood up to give my closing remarks, blood poured from my chin onto my pants and shoes. My host was pale with worry, but the audience was crazy with applause. That ending was the best ending I ever had with the Flying Circus talks, and at every later Flying Circus talk I secretly hoped to be cut like that again. As I said once, a long time ago, "There is no better demonstration than one in which the teacher may be hurt or killed."
To see my bed of nails demonstrations on video, go to
and click on videos (under the opening photo).
The video that immediately begins to run shows the first demonstration, beginning at about 8.0 minutes into the video. By repeatedly clicking on the slide bar, you can quickly advance to that point.
To see both demonstrations, scroll down to Episode 1. The demonstrations occur about 19.5 minutes into the video.