Physics in Action: A Bed of Nails

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In some physical situations, force is the more relevant concept; but in other physical situations, it's the pressure, which is the force divided by the area over the force acts. We can illustrate that with this nectarine. If I exert a force on this nectarine, it doesn't do much to the nectarine, because it may accelerate the nectarine, but it doesn't puncture it in anyway; because that force is spread over a large area, that's the contact area between my hand and the bottom of the nectarine.
But suppose I would exert the same force, not acting directly through my hand, but indirectly through this nail. Now, the entire force that my hand is going to exert will act over a very, very small distance, a small surface area-that's the surface area of this nail. Now, as for as puncturing the nectarine is concerned, it's not the force, which I exert, but the pressure, which determines whether the nectarine gets punctured. Let's see what happens. I'm going to exert the same force on the nectarine, obviously the nail is going to go in. That tells you there's a difference as far as determining what happens to the nectarine; if it's the force or the pressure, it's actually the pressure which is the more relevant concept over here.
Now we can illustrate this dramatically with this bed of nails. This is a real bed of nails, which contains approximately two thousand nails banged into this board. To show you these are real nails, we're going to let some fruit drop on it and see what happens. Here's a tomato and we'll toss this tomato up...and you can see the tomato has been smashed. Here is another nectarine...again, the nectarine has been punctured. And now we can try an apple...and now we've produced a fruit salad and certainly, we've punctured the apple.
This is real bed of nails and shows you the demonstration that we're about ready to do, which we'll use Andrew as our intended victim. He will be facing the problem, experiencing the difference between force and pressure. What exactly is going to happen and what is his role? Stay tuned.
We've talked Andrew into being the tomato of this demonstration. We see, in fact that Andrew is lying on this bed of nails without any evidence of discomfort. And the reason again, is that his weight is spread out over, now almost two thousand nails, so that each nail exerts only a small pressure on his body. If we had his weight supported by a single nail, then the pressure would be enormous and surely that nail would puncture his body. But, he's a little bit too comfortable. Let's see if we can put him under a little more pressure.
"Do you have any last words?" Now, what I'm going to do is smash this block on top of this block on top of Andrew. If everything works as it's supposed to (and that's a big "if"), all the force that I exert and all the force that these blocks exert on him, will be spread out over these two thousand nails, and Andrew will survive. Let's see if that happens.
"Are you alive?" Notice what happened. All the force that I used to smash this cinder block did not hurt Andrew. He's still not bleeding, at least I don't think so.
So, what happened to Andrew? Evidently, he's not here, but don't worry. He's also not in the hospital. He's completely all right. And that's of course, the point of this demonstration. Andrew's weight, when spread out over almost two thousand nails on this board, did not cause sufficient pressure on any one nail to puncture his body. His total weight spread out over so many nails caused every one nail, any one nail to support just a small amount of his weight, exerting a very, very small pressure. By contrast, if we tried to support Andrew on a single nail like this, the pressure would have been enormous, because the surface area of this nail is very, very small and surely he'd be bleeding and in a lot of trouble.
So, what have we learned from this. You've learned the difference between force and pressure. In some situations, force is the more relevant, the more important concept. Sometimes it's pressure, which is the force divided by the area. In a situation like this, we want to ask the question whether the nails will puncture his body and cause him to bleed if the pressure-the force divided by the area, which is the relevant concept-and that's what we've seen demonstrated by Andrew in this bed of nails.
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