Would someone die a gruesome death?

September 10, 2009 8:17 AM

A handstand on someone's chest while lying on a bed of nails?

Ok, so this came up in conversation with a street performer friend of mine. It's been a while since I've done any physics and figured that some of you might have been wondering the same as myself. So I am inclined to ask, dear AskMe! (In case you're wondering, I'm not planning on doing this. I'm just trying to settle an idle curiosity.)

Imagine, if you will, a young so-and-so lying on his back on a bed of nails. This bed of nails (as most do) extends the length of man's back - from the lower back, just above his buttocks, to his shoulders and the base of his neck. The nails' heads are approximately 1mm^2 in area at the point (we're talking longish ten-penny nails.) These nails are spread one per square inch on the bed (as are most standard beds-of-nails.) This fellow in particular weighs about 160lb, as does his friend, who now places his hands on the first young daredevil's chest and presses up into a handstand. Let's also assume that these young men (or women!) are shirtless. It's difficult to imagine people being so daring with clothing on.

Now for the million dollar question - did the first performer just meet a gruesome death? I imagine that the jolt of the handstand-er pressing up onto his friend's chest with his hands would add a considerable amount more force than would his simple bodyweight and gravity. Would that be enough to impale his friend? If that would be, what would the effect be like if the man on the bed of nails were to support the handstander's shoulders with his arms, while the handstand-er used his arms to brace himself on the lying-down man's knees? (knees which are connected to his legs, which are pressing against the ground, not touching any nails at all. From what I've researched, this pose is called the candlestick.) Would that take enough force away from the man's back that he wouldn't get impaled?

Thanks for humoring me, dear Metafilter. Inquiring minds simply *have* to know.

posted by ThomThomThomThom to science & nature (11 comments total)

People with bed of nails acts regularly have audience members standing on their chests. Presumably these audience members do not always have perfect posture and balance. I would imagine that a handstanding acrobat would be easier to support than an audience member, since the handstander will hopefully be able to avoid any sudden clumsy moves.
posted by emilyw at 8:29 AM on September 10

To answer this you would want to know the pressure at which a nail punctures the skin, and compute how close you come to it during the different phases of the performance. But the answer is probably "no, no impalement."
posted by fantabulous timewaster at 8:31 AM on September 10

Let's see....

I get a puncture pressure for human skin of about 460 psi from here.

Let's first look at the total weight of two people spread over the surface area of one back for simplicity. Total surface area for a 160 lb man at, say, 5'8" (from here) is about 1.9 square meters or 2945 square inches. Divide by two to get a half-body (only lying on your back) and a rough approximation is about 1470 square inches. Which means that you'd have about the same number of nails supporting your weight, which are 1 square millimeter each (0.0016 square inches), or 2.35 square inches total contact area with the nails. Double the weight, and 320 pounds divided by 2.35 square inches is roughly 136 psi. You're safe.

Now if you're doing a handstand on someone's torso... The force will spread out through the body so that the extra weight is "felt" on the back over a larger surface area than just the area of the stander's hands. From here, the contact area of a hand is about 24 square inches (times two, for two hands). Let's be conservative and say that the "shoring" effect of the body roughly doubles that to a total of 96 square inches. So you have 160 lbs plus 160 * (96/1470) (roughly assuming even body weight distribution) and you have only 170 pounds acting on 96 square inches of back, which equates to 0.15 square inches of nails. That's about 1133 psi, which seems... high, so I think my shoring assumption is too conservative but I have to leave for a meeting now.

Anecdotally, we broke cinder blocks with sledgehammers over people lying on beds of nails in high school and no one was ever skewered.
posted by backseatpilot at 8:32 AM on September 10 [1 favorite]

I imagine that the jolt of the handstand-er pressing up onto his friend's chest with his hands would add a considerable amount more force than would his simple bodyweight and gravity.

You're really just adding that handstander's weight to the equation. How well that weight would be spread out over the bed of nails would have to do with the composition of a human body, but it should be pretty similar to any other method of adding weight to the person lying on the nails.

From this link about a bed of nails with the nails 1'' apart:

A human's skin can endure approximately 2 pounds of force from an ordinary nail before feeling any pain. With the added protection of clothing, the skin can stand even more.

An average human of 150 pounds, when spread out among approximately 600 nails, will feel only about 0.25 pounds of force per nail.

So a normal person can add around 8 times their weight before feeling any pain from the nails, and one new normal sized person's weight shouldn't make much of a difference.
posted by burnmp3s at 8:41 AM on September 10

The distribution of force through the human body is an important factor that I believe you're underplaying. If people were just filled with jello or sand, it would be different; when I pressed my hands into your back (say, by doing a handstand on you) hand-shaped impressions would be pushed out of your front. That would be fatal, because my whole weight would be pushing your flesh onto a much smaller number of nails, and those lumps where my hands were pushing through your body would get skewered.

But we're not filled with jello; and when I press against your back, there is no specific point on your front where that pressure has a noticeable effect; it's distributed through the bones in your back.

Weight distribution is a fantastic thing.
posted by koeselitz at 8:58 AM on September 10

So when people are doing these tricks on beds of nails, do they stiffen up their whole frame to distribute the weight better? Because otherwise, unless I'm missing something, the weight would be distributed rather locally.
posted by creasy boy at 9:02 AM on September 10

Guessing about 3 feet from the bottom of your butt to the top of your head, and 2 feet wide, and taking some off because your head isn't two feet wide (OK, I don't know you) a reasonable guess about 5 square feet of contact area. (backseatpilot, I don't think total surface area or even half that is what matters here.) At one nail per square inch there will be about 700 nails and they will be supporting about 300 pounds. So one-half pound per nail is in the ballpark. If you take a nail and put a smallish book on it you can get an idea of what it is going to feel like. I just did that, and while I don't think I could fall asleep on that bed, I am not currently bleeding onto my keyboard. This assumes an even distribution of pressure, and it probably won't be evenly distributed, especially when the handstander first tries to balance. So I tried again with 3 smallish books. Ow. No blood though.
posted by Killick at 9:05 AM on September 10

creasy boy: So when people are doing these tricks on beds of nails, do they stiffen up their whole frame to distribute the weight better? Because otherwise, unless I'm missing something, the weight would be distributed rather locally.

I actually don't believe that stiffening will distribute your bodyweight any better. The bones and sinews which do most of the distribution are there doing that whether you stiffen or not. Stiffening might be helpful at some very, very high degree of pressure pushing down on you - that is, for example, the amount of pressure that would impale you if it was just your back on the nails, but not if it was your back + the redistributed force through your legs as well - but by then I don't think anybody would be capable of exerting enough stiffening pressure to make a difference.
posted by koeselitz at 9:31 AM on September 10

... in other words: if our bodies aren't filled with jello, they're also not really filled with muscle. It's more the bones and the sinews doing the work here, stuff that we couldn't break if we wanted to unless we had some pretty powerful outside tools.
posted by koeselitz at 9:33 AM on September 10

Awesome guys - thanks so much. I knew I could count on good 'ol Green! You're the best.
posted by ThomThomThomThom at 11:47 AM on September 10

... in other words: if our bodies aren't filled with jello, they're also not really filled with muscle. It's more the bones and the sinews doing the work here, stuff that we couldn't break if we wanted to unless we had some pretty powerful outside tools.

Yeah but they don't have to break in order to distribute the weight unevenly. Look, when I lie on the ground, my ass, my upper back, and my feet are basically touching the ground. Imagine if you had a 6 foot board, and you attached pillows to both ends and to the middle, representing shoulders, ass and feet, then laid that board on nails, and put a 180 pound stack of concrete on one end of the board...wouldn't the nails be more likely to puncture the pillow that's under the concrete? Or would the board really perfectly distribute the weight? And now imagine a hinge in the middle of the board, corresponding to the human waist. Wouldn't the end of the board under concrete be bearing more of the weight than the rest? Bones are solid, but they are in fact connected to each other on hinges.
posted by creasy boy at 2:37 PM on September 10