Taking it up a notch: nail beds

About a month ago, I had an extraordinary experience:

Bill Nye standing on me

Bill Nye standing on me

It was Bill Nye standing on me while I laid on a nail bed. Lots of fun, for sure, and I pointed out to the audience that it was the one shot I wanted from the whole gig. The way we set that up was to first have him stand on my chest without the nail bed, but we cautioned that it would be much safer to use the plywood on top. We talked about how we’d be able to spread Bill’s weight over several contact points on me: my thighs, my chest, and my hands. That’s when we decided to take it up a notch with the nail bed.

Now I’ve been using the nail bed for over 10 years and it’s usually a big hit. Sometimes I do a decent job of talking about the physics, but I’ll be honest that sometimes the show’s running long on time and we just do it (and the breaking of the cinder block) for show. What I wanted to talk about in this post was an idea I had about talking even more about the physics of distributing force. What’s funny is that I’ve talked to a few different people about this (including my partner and my sons) and, while they agree that it adds to the physics, they don’t think it’s appropriate for the show. So I’m looking for your second opinion. Thanks in advance 🙂

The reason the nail bed works (and why I can handle Bill (we’re on a first name basis now, you see) standing on my chest) is NOT that I’m reducing the total force on my body, but rather than I’m distributing that force over a larger area. No single nail has enough force to puncture my skin, for example. This got me thinking about pillows.

When I was in school, I was always a little confused about pillows. My free body diagrams kept telling me that it didn’t matter what I rested my head on, something had to provide a force equal and opposite to my head’s weight to keep me from smashing my head on my bedroom floor. This made sense to me from a free body diagram perspective, and certainly I could do all the homework, but it always bothered me how that perspective didn’t seem to explain the softness of pillows. What I mean is that feather pillows seem much more comfortable than, say, cement pillows but they both must be providing the same force.

I know now that really pillows deform to your head, providing a very large surface area to spread the force out. What I figured on doing for my show was to concentrate on adding more and more pillowy-type substances between me and the plywood while people are standing up there. We’d slowly go from all the weight on my thighs/chest/hands all the way up to the weight on every part of my upward-facing body, using foam or something.

The reason my family and friends think this isn’t a good idea is that people would say “oh, he just has a pillow up there, of course he’s ok” instead of “ahh, I see, the pillow disperses the weight so that no single part of his body has to support a lot.”

So what do you think?

Some comment starters for you:

  1. I’m a student in this class and, wait, nevermind
  2. I saw that show and Bill was great.
  3. I’m not sure I follow what you mean by the free body diagram. Would you, by chance, happen to have a new Surface Pro that would allow you to draw a nifty picture?
  4. Wait, I thought feather pillows were better than cement pillows because . . .
  5. Wait, they make cement pillows?
  6. Sure, do this in your show, I’d love it if you got crushed.
  7. Don’t put this in your show, but for this reason instead . . .


About Andy Rundquist

Professor of physics at Hamline University in St. Paul, MN
This entry was posted in fun, physics, teaching. Bookmark the permalink.

7 Responses to Taking it up a notch: nail beds

  1. John Burk says:

    So you’re putting a pillow between you and the top sheet of plywood?

    Is there anything to “measure” that will help students to see that the force is still the same but the pressure of the pillow is less than the pressure of the plywood on you? Maybe you could put an egg between a couple of pillows?

    • Andy "SuperFly" Rundquist says:

      I don’t tend to have a lot of measurements in the show, since most of the time they’re hard to see. I use a very large 100 lb spring scale to help with our tug-of-war act, but I guess for this I was thinking that the measure would be my pain 🙂

  2. Joss Ives says:

    Hi Andy. I think I agree with the “oh, he just has a pillow up there, of course he’s ok” concern. With the pillow situation, you are trying to explain something using an idea (a pillow) loaded with preconceptions. So you need to get their thinking about the pillow on the right track and then use that to explain what is going on with you and Bill (I’m not on a first-name basis with him, but I’m not one to follow social protocols). Although all the ideas are intertwined, it seems like a lot to ask.

    • Andy "SuperFly" Rundquist says:

      Yeah, I’m coming around to that as well. Certainly when I teach this stuff, I take the time to get my students to think about these questions. I think in the show it might be too subtle.

  3. bretbenesh says:

    I agree that you should not do the pillows, although I should add that I don’t completely understand your motivations for considering this. I DO understand the feather-pillow-versus-concrete-pillow idea; I just don’t understand how making the situation more pillowy helps explain anything.

    What I would understand is using the pillow analogy to explain why your back does not end up with a bunch of nail holes once you are done.

    If you want to use foam to make things more pillowy, I would recommend starting off with the foam (people won’t be surprised), and slowly removing the pillow-y-ness until you are lying on a bed of nothing but nails. That could create some tension and end in a nice climax. But I think that the way you are already doing the show is probably still better than this paragraph’s idea—just jump straight to the nails (and use the pillow analogy to explain it, if you like).

    • Andy "SuperFly" Rundquist says:

      I wonder if it makes a difference which side I do this on. I was thinking of foam between me and the plywood on top of me (and then someone or lots of people standing on that). The notion of putting it between me and the nails is another way to think about the same thing.

  4. Pingback: Making it meaningful | SuperFly Physics

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