Relationship Between the Force and the Area

The Idea

Would you sit down on a bunch of nails that are sticking out of a wooden board with the sharp pointy sides upward? This is your chance to try. This is not nearly as painful as it may seem because the large number of nails spreads the force over a larger area.

What You Need

• 144 nails, 1½ to 2 inches long. Whatever length you use, make sure the nails are nearly all the same length
• piece of plywood 14 inches × 14 inches × ¾ inches thick (or larger)
• electric drill
• drill bit whose diameter is equal to or just slightly smaller than the diameter of the nails
• inflated balloon

Method

Assembling the bed

1. Draw evenly spaced lines at 1-inch intervals running parallel with each of the edges of the board.
2. Drill a hole at the intersection of each of the holes.
3. Insert the nails in the holes. They should be snug enough not to fall out. Some may have to be driven in with a hammer (Figure 42-1).

Testing the bed

1. Press the balloon on one of the nails on the corner.
2. Press (another) balloon in the center of the bed (Figure 42-2).
3. Place the nails on a chair and sit down on it.

Expected Results

With one nail isolated, the balloon will burst. However, with several nails in contact with the balloon, the balloon does not burst, even with substantial pressure applied. Sitting on the bed of nails is surprisingly painless.

Why It Works

Pressure is force divided by area. For a given force pressing down on the balloon, the pressure is much greater with the single nail than with the large group of nails. When you sit on the group of nails, the force is the result of your weight, but the pressure is spread out over the large number of nails.

Razor blades that use multiple blades apply the principle of spreading out the force exerted by any one blade over a large surface area.

Other Things to Try

Instead of just a seat, why not build a nice comfortable bed to sleep on?

You can also show how pressure can be spread out over a larger area by using several cups to support a person. Set up a board on a row of paper cups, as shown in Figure 42-3. Then, have a person stand on the board. If you have enough cups, you should be able to stand on the board supported by the paper cups. Some cups are stronger than others, so you may have to experiment to determine how many you need. Placing them every 2 inches or so, however, is a good place to start.

The Point

Pressure is force divided by area. The larger the area a force is applied over, the smaller the pressure experienced.

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