Bed of Nails

Introduction

How do performance artists lie “comfortably” on a bed made of nails? They are protected by the “pressure” of their work environment.

Concepts

• Pressure
• Surface area
• Force
• Bed of nails “magic”

Materials

Safety Precautions

The nail tips are sharp. Use care when placing the nails into the multi-holed block. Do not place hands, feet or other body parts on the bed of nails. Do not attempt to scale up this demonstration. To store the block, remove the nails from the block or cover the bed of nails with a block of wood or other rigid object. Latex (in balloons) may be an allergen. Wear safety glasses. Follow all laboratory safety guidelines.

Disposal

The materials can be used indefinitely.

Prelab Preparation

1. Obtain the board with multiple holes and approximately 100 nails. Assemble the Bed of Nails according to Figure 1. Insert a nail into each hole.

Inflate one balloon until it is nearly full. Before tying it off, pinch it shut and test to make sure it fits between the rods and underneath the “floating” board. Tie the balloon off only when you have a good fit. Repeat for the second balloon.

Procedure

1. Carefully place one inflated balloon onto the Bed of Nails resting on the base with rods (see Figure 2).
2. Place the floating board onto the rods and gently rest it on top of the balloon.
3. Carefully add a 500-g mass to the center of the floating board. Make sure the mass is balanced and the floating board is not tilted significantly. Does the balloon pop? (The balloon should not pop.)
4. Remove the 500-g mass and carefully add a 1000-g mass to the center of the floating board. Does the balloon pop? (Again, the balloon should not pop.)
5. Continue to carefully add masses to the floating board in 50- or 100-g increments, making sure the floating board remains balanced on the balloon. Add mass until the balloon pops, up to a maximum of 1500 g total. Caution: When the balloon pops, the masses will come crashing down along with the board. Be prepared to catch or block any masses that may bounce off the board so they do not fall to the floor.
6. Record or make a note of the total mass that was added to the floating board to pop the balloon. If the balloon did not pop, record the mass as greater than 1500 g.
7. Remove the Bed of Nails from the base. Carefully remove the nails from around the center of the board. Remove about four center rows of nails but leave one nail in the center of the board.
8. Repeat the procedure using a Bed of Nails with only one nail in the center of the block. The balloon may pop simply by placing the floating board on top of the balloon. If not, carefully add a 50-g mass to the floating board. The balloon should pop when the 50-g mass is added.
9. Compare the results obtained with the two different bed-of-nails designs.

Teacher Tips

• {12671_Tips_Figure_3}
  Fifty 5" balloons are included with this kit, enough to perform the demonstration more than 25 times, including extras for “Murphy’s law.” Additional 5" balloons may be purchased through Flinn Scientific, Catalog No. AP6420. Approximately 120 nails are provided. The Bed of Nails holds up to 98 nails.
• To safely remove the nails from the Bed of Nails, turn the bed over so the nail points are face down on the base. Carefully press down on the board by its edges. The board should slide down the nail shafts. Then, pull the nails out of the board by their heads.
• If the nails are not removed from the Bed of Nails, safely store by sandwiching the Bed of Nails between the base and floating board. Wrap a rubber band around the wood blocks to secure them in place (see Figure 3).
• To remove individual nails from the board, use the flat end of a pen to push the nail out of the board. Then, pull on the flat head of the nail to remove it completely.
• A variety of nail patterns can be created using the wood block. See Figure 4 for useful patterns to try.
  
  {12671_Tips_Figure_4}

Discussion

Pressure is a common term used every day to describe weather conditions. For example, barometric pressure is a measurement of the local air pressure and is used to help predict the weather. But outside of measuring changes in air pressure, observing and quantifying pressure between objects is difficult. Pressure is defined as the amount of force exerted on a given area or a force per unit area (F/A). Some common units of pressure are pounds per square inch (lb/in²), Newtons per square meter (N/m²) [also known as a Pascal (Pa)], atmospheres (atm) and millimeters of mercury (mm Hg) (millimeters of mercury is the height of a small column of mercury that is held up by a pressure).

Pressure exists whenever two objects are in contact with each other. The points of pressure only occur at the points of contact. The amount of pressure depends on the magnitude of the force and on the surface area over which the force acts. The larger the area, the smaller the pressure will be for a given amount of force. As shown as an example in Figure 5, a 10-lb ball and a 10-lb wood block resting on a tabletop both exert the same force due to gravity on the tabletop. The 10-lb ball, however, will exert more pressure on the tabletop than the 10-lb block of the same material because the round surface of the ball has a very small contact area with the tabletop compared to the rectangular surface of the 10-lb block. The force feels stronger when it acts on a smaller area because the force is more “concentrated.” The same amount of force on a larger surface area is spread out and “diluted” over a larger area so it feels weaker, even though it is the same force (weight). 

This demonstration dramatically shows the relationship between surface area and pressure. Each nail corresponds to one point of contact on the inflated balloon. The more points of contact there are, the lower the force is on each point. With more nails, the force from the weight of the mass is spread out over a larger surface area of the balloon and none of the nails exert enough force to puncture the balloon. When the Bed of Nails is completely filled with nails, the balloon does not pop even as 1000 g or more is added to the floating board. Removing nails from the Bed of Nails decreases the points of contact. Therefore, the weight is distributed over less surface area, meaning each nail exerts more force on the balloon. When the Bed of Nails is only half full, the weight of a 500-g mass will be great enough to puncture the balloon.