Getting Bent

April 3, 2009 10:28 PM

How can two people bend rebar against their throats? In addition to a video link, there's

A friend has been searching the web for practical explanations about how two people can put the ends of a length of rebar against their throats and bend it (see YouTube video here. Admittedly this looks like pretty flimsy rebar, but there are other videos where the rebar looks thicker. Her pastor apparently has done this and has nothing to say about it that isn't spiritual/mystical. Does anyone have a rational explanation of how this works?

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

There's no trick. The bar just has to be long enough. Not sure what the technical term is. Torque, maybe? The couldn't do it if the bar was two feet long.

If you don't want to risk your throat, just get a long piece, put it between a wall and a piece of wood which you can hold in your hands and then walk towards the wall. It's done in motivational camps/cults all the time as if it's miraculous. It's not. It's schtick.
posted by You Should See the Other Guy at 10:44 PM on April 3

It looks like either #3 or #4 rebar which is either 3/8 or 1/2 inch in diameter. Rebar is mild steel that is easy to bend and is done on construction sites by hand all the time. I certainly have never done it with the soft part of my throat, but it can be easily done by pushing with the palm of the hand. You would have no problem at all bending it around the back of your neck using your outstretched arms. As stated above, the only trick is that you need a long enough piece that works like two long levers that multiply the force as it bends in the middle.
posted by JackFlash at 12:09 AM on April 4

What YSStOG said. My mechanics textbook says the critical force to induce bending is inversely proportional to the square of the length, so scaling the rod is probably an easy recipe to success.
posted by themel at 12:20 AM on April 4

The word you're all looking for is "leverage"
posted by randomstriker at 1:03 AM on April 4

Actually what you're looking for is Buckling. See especially Euler's formula for buckling.
posted by Comrade_robot at 4:35 AM on April 4

The trick is also that they are helping the bend with their wrists.
posted by gjc at 7:10 AM on April 4

The throat itself doesn't seem to be doing much, actually. Once the bar has curved even a hair away from straight, the force on the ends is going to be enough to curve it.

I suppose one could cheat by using almost straight, but very slightly precurved rebar to begin with, then be sure to have it spun the right way around to start with.
posted by rokusan at 11:03 AM on April 4

This is also commonly found in Martial Arts demos (you can see it here about two minutes in). The soft part of the throat is actually not so soft.
posted by P.o.B. at 5:31 PM on April 4

Strangely enough, today I caught the episode of Bullshit! with Penn & Teller which includes this.
posted by peanut_mcgillicuty at 5:50 PM on April 4

Even though that part of your throat seems soft, it would still take a lot of force to push a blunt piece of steel through it from a standing start like that. The force required to buckle rebar is far less than the force required to push that blunt bar through those fellows' throats.
posted by chazlarson at 4:02 PM on April 5

Actually what you're looking for is Buckling. See especially Euler's formula for buckling.

Yep, buckling. If you look at the denominator Euler's equation the force required is going to be very small due to the very long unsupported length. Also, the rebar sags in the middle, making it even easier.
posted by electroboy at 7:30 AM on April 6