I vividly remember the first time I saw the Bed of Nails science demonstration. I was in an entry level physics class at college and the instructor, who was known for doing insane science demonstrations, laid down on a bed of nails, put a cinder block on his chest, and had someone hit the block with a sledge hammer. And, while the demonstration was cool, what caught my attention the most was that the professor got up off the bed of nails and had red blood spots all over his shirt where he had laid on the bed of nails. And, come to find out, the blood wasn't actually blood - it was red paint from where he had put small dots on his shirt to simulate blood!
To this day, the blood spots on the shirt still stand out to me more than the demonstration! Why? I had never seen a teacher go to so much trouble to capture his student's attention, draw them in, and convince them to learn. This professor thought of every detail to help persuade his students that science and learning should be amazing and should be fun. In fact, this is one of the moments in my college years that eventually led me to become a science teacher.
So, let's fast forward to present day. I teach high school science (Chemistry mostly) but wanted to make a bed of nails for the STEM Magic Shows I perform at elementary schools. And, through a lot of trial and error, I've come up with the easiest way to make the Bed of Nails and have shared the instructions so others can perform this classic physics demonstration safely, inexpensively, and effectively. Because I agree with my former physics professor that everyone should find science to be "amazing and fun"!
How to make a Bed of Nails
Supplies (Click the link to find the supplies on Amazon.com)
Plywood 1/4 inch (check your local hardware store)
Plywood 1/2 inch (check your local hardware store)
Pegboard (boards with evenly spaced holes)
Cinder Block (check your local hardware store)
Building the Bed of Nails Directions
Lay on a sheet of 1/2 inch plywood and mark a distance that is as wide or wider than your shoulders.
While laying on the plywood, mark a distance from above your shoulders to the middle of your butt. These two measurements allow your entire back to be on the bed of nails which means that more surface area of your back will be in contact with more nails. This is what makes this demonstration safe enough to perform.
Using these measurements, cut the sheet of 1/4 inch plywood to the same dimensions.
Using these measurements, cut two pieces of pegboard to the same dimensions.
Stack the two pieces of pegboard on top of each other and insert a nail into each hole of the pegboards.
Lay the 1/4 inch sheet of plywood on the side of the pegboard with the nail heads to keep them from falling out.
Lay the piece of plywood on top of the 1/2 inch sheet of plywood.
Screw all of the pieces of pegboard and plywood together to make sure the nails remain tight and secure.
With the remaining plywood, cut out a piece that is big enough to fit on your chest and big enough to hold a cinder block.
How to Perform the Bed of Nails Science Demonstration
**Warning: Perform the Bed of Nails Demonstration at your own risk. It is very dangerous and severe injury is possible**
Carefully lay down on the bed of nails making sure you keep as much of your back in contact with the nails as possible.
Have an assistant place a piece of plywood on your chest and hold it securely.
Have an assistant place a cinder block on top of the plywood sitting on your chest.
Have an assistant hit the cinder block with a sledge hammer hard enough to break the block but not so hard that it will slam into the board and injury you. (**Note: It only takes one search on the internet to see how people laying on the bed of nails have been injured by someone attempting to do this demonstration. When I do this, I only have a person hitting the cinder block who has practiced multiple times and who understands the injury they could cause to me if they hit it incorrectly, too hard, or if they miss.**)
How does the Bed of Nails Demonstration work?
This physics demonstration is all about surface area, pressure and Newton's Second Law of Motion.
First, let's examine what happens to a balloon when one nails is pushed into the balloon. As the nail begins to poke the balloon, the latex fibers of the balloon start to stretch. Once the nail has pushed into the balloon far enough, the stretched latex (which is not under pressure from the nail and from the air inside that is pushing out) breaks. The pressures rip apart the latex and the air escapes.
Why then, if we press on the balloon with several nails evenly spaced apart, the balloon doesn't pop? When several nails are pushing the latex, the pressure exerted by each nail on the balloon is decreased because the force that you are pushing with is spread out to all of the nails as opposed to one nail. Because the force is spread out over a greater area (across all the nails and across a greater area of the latex on the balloon) the balloon does not rip apart as easily. This models why we use a lot of nails on the "Bed of Nails" and not just a few. If we only used a few nails, spread far apart, the force and pressure would be great enough to allow the nails to poke into our back.
When laying on the bed of nails, gravity pulls your body toward the ground which exerts pressure on the bed of nails and the nails "push" up into your body. Recall Newton's Third Law of Motion which states that for every action there is an equal and opposite reaction - meaning that the force pushing down, due to your mass being "pulled" toward the earth by gravity is equal to the force of the nails "pushing" up into your back. But, just like when we pushed the balloon onto the bed of nails, the pressure (Pressure = Force/Area) is spread out over a greater amount of surface area so any one nail is not exerting as much pressure into the body, it is spread out across multiple nails across your entire back.
So, why doesn't the breaking of a cinder block with a sledge hammer cause injury to your back? The cinder block is massive (it has a lot of mass) and when it is hit by the sledge hammer, the force from the hit is spread out into the block. And, when the block breaks, much of the energy is absorbed by the block and the breaking of the block. Therefore, very little of the energy from the hit is actually transferred into your body and then into the points of the nails.
And that is how physics saves your life during the Bed of Nails Demonstration.
Keep on Learning! Craig Beals