Bed Of Nails

Physics:	Pressure
Grade Range:	Elementary School, Middle School, High School
Format:	Stage

This demonstration is a big and flashy way to show how pressure works. Be sure to not let anyone in the audience to lie on the bed of nails, especially not children! The bed and blanket of nails are very much real, so be safe while handling and using this demonstration.

Materials

• Blast shield
• Bed and blanket of nails
• Yellow wooden rear and head supports
• Two black wood boards
• Thick gloves
• Sweatshirt
• Sledgehammer
• Metal cinder block holders (x2)
• Dixie Cups
• Plastic tarp
• Cinder block
• Black bin

Safety Precautions

Please read the Physical Demonstration and Flying Particles sections of the Demonstration Safety Page before using this demonstration.

This demonstration requires: Goggles or Safety Glasses, Blast Shields

Demonstration

Preparation: Set out at least 30 paper cups in a square, and place one of the black wood boards on top of the cups. Both presenters during this performance must wear goggles or safety glasses the entire time.

1. Ask for two volunteers, and try to get one young student and either a second, older student, or a teacher. Have the young student try to stand on a single cup without breaking it. Inevitably, it will break. Now have the older student or teacher stand on the wood board on top of the paper cups, making sure to step into the center. The cups will hold them up!
2. Explain to the audience why the cups could hold up the volunteer, and let the volunteers return to their seat. Have a co-presenter pick up the bed of nails and show it to the audience, and state that you will be lying on it.
3. Set up the Bed: Put the bed of nails on top of the tarp, and place the support boards down on either side of the bed. Although not required, it is recommended that you put on the Science Theatre sweater before lying on the bed, so to make it more comfortable. Have your co-presenter give you the gloves to wear. Make sure you are wearing goggles or safety glasses! Then, your co-presenter places the blanket of nails on top of you.
4. Have your co-presenter place the wooden boards across your legs, then the bucket over your head, with the edge on the blanket of nails. Then, have them place the blast shield(s) in front of the bed of nails, between the demonstration and the audience.
5. Co-Presenter: Show the audience the cinder block. place the metal bars on top of the blanket of nails, and rest to cinder block across the bars, making sure the bars are not underneath the holes in the cinder block. Then, show the audience the sledgehammer, and state that you will be smashing that cinder block with the sledge hammer. Have the audience give you a countdown, and at the end of the countdown swing the sledgehammer down onto the cinder block.
   • You do not need to hit the block hard; letting the hammer drop from a close to vertical point will give it enough energy to break the cinder block. Make sure it hits the cinder block either between the two bars or on either side of them, otherwise it might not break, and you will have to do a second swing.
6. After the cinder block is broken, clear any debris off of the blanket of nails and help the presenter get up. Have the presenter show the audience that they are uninjured by spinning slowly to show no holes or injuries. They can now remove the sweater if it was worn.

Why This Works

Short Explanation

When we had the single volunteer stand on a single paper cup, it was crushed. This is because all of their weight is on the single cup, which is not able to support them. However, the volunteer is able to stand on several cups, because their weight is spread out over all of them. This is the idea behind Pressure: Pressure is when you take a force, such as someone's weight, and divide it over an area. If the amount of force is constant, then you can decrease the pressure by increasing the area. This is why several of the cups can support a person, with each one taking a fraction of the weight!

This works just as well with the Bed and Blanket of Nails. Each nail in the bed will take a small fraction of the person's weight, so each nail has a low amount of pressure on it. The amount of pressure on each of these nails doesn't increase much when we add the blanket of nails on top, or even with the cinder block! When we smash the cinder block, the force from smashing it is spread out over the nails, and results in a very small change in pressure for each. This keeps the presenter on the bed safe from harm. This would be very different if, instead of the several hundred nails we have here, it was only a dozen nails, or even just one. By increasing the number of nails, and therefore the area to spread the force across, we decrease the amount of pressure on each "point", and the presenter is safe from harm.

Additional Information

• This demonstration is commonly performed with the presenter on the bed being the "Sleeper". After finishing the part with the paper cups, the "Sleeper" will act tired, and will then take a "nap" on the bed of nails. The other presenter will then wake them up by smashing the cinder block on top of them!
• This demonstration is a part of the Pressure Show.