Bed of Nails

A 2 cm (3/4") thick, 45 cm by 75 cm (1.5' × 2.5') plywood board has approximately 1,000 16-penny nails pounded into it so that they extend through the board. The nails are spaced about 2 cm apart. A similar board, but with twice the thickness (actually, just two boards together) of wood, is used for the top of the sandwich. One or two thick books (preferably physics books) serve as a pillow under the head.

The forces (weight of the body, cinder block, etc.) are distributed over the total area of all the nail points so that the pressure is not high enough for the nails to puncture the skin. Of course we know what would happen if one were to lie on just one nail! In this case, the pressure is extremely high because the area of just one nail point is quite small.

For more details on our setup see http://sciencedemonstrations.fas.harv...

Special thanks to Rob and Nils.

• Category

  Education

[moshe72]

CRC as a pillow. that's genius or sacrilege.

[waggs179]

1:05 trippy

[Carson Garrett]

It's how a 90+ ton battle ship floats on water. Not a miracle and good god you would think 30+ years old "students" wouldn't have to make a video. Go get a job fellas now in day's kids are smarter then you.

[snoopdiego9]

My physics teacher did this experiment in class,except he put ketchup packets under his white shirt without the class knowing. You know what happened

[DarthZeww]

When it faded out after the slowmotion bit they replaced the dead guy with someone else

[C24B9]

Did you have trouble finding people to assist in this video? :-) I understand how the 1000-nail boards spreads the force around. Did you record the experiment w/ nails? Curious how that went... ;-)

[AldiebMeowchaussen]

That was a little Giles Corey for your Aunt Laurie.

[TheGrinningSkull]

Ahh, okay, it makes sense a bit, out of curiousity (as I'm still learning :) ), what is inertia? I have looked it up but a human explanation I think would be better to help me understand it.

But wait, how did you apply the first law here? That you did not answer :)

[NatSciDemos]

The hammer certainly applies an external force, and we can see in the high speed footage that the top bed of nails accelerates a bit as a result of this applied force. However if the combination of cinder block and top bed had less inertia (imagine replacing them with a thin sheet of plywood for example), then the resulting acceleration would be a lot greater and Allen would probably need to go to the hospital.

[TheGrinningSkull]

Excuse me but what does the first law have to do with anything here?

The first law of motion states that bodies at rest or constant motion tend to stay at rest or constant motion unless acted on by a resultant force.

I'm seeing a resultant force being applied so I don't understand how Newton's first law is significant here.

[Ryan Kell]

He aimed and missed, haha. Cool demo!