Here you will find a description of the apparatus used, as well as detailed instructions to build your own.

 

The main concept of the lesson is that pressure is equal to the force applied per unit area. Place a balloon inside the apparatus. Ask a volunteer to apply a mild, evenly distributed force to the top piece of plywood. Notice the balloon does not pop! Now, ask the volunteer to increase the pressure. At this point we are able to look at the bottom of the balloon and see each of the pressure points, where the nails are sticking up into the balloon (as seen in the project overview). Ask them to keep increasing pressure until the balloon pops. Note how a significantly larger force was necessary to pop the balloon via the nail bed versus the small force required when there were just one or two nails. This is when the equation is introduced: P=F/A. As we increase the area, assuming the same force, the pressure decreases, and a larger force is necessary to reach the pressure required to pop the balloon. A diagram of the setup can be seen below:

 

 

 

 

 

 

 

 

 

 

 

 

 

In this image, the balloon is placed inside the apparatus. No force has been applied yet.

 

 

 

 

 

 

 

 

 

 

 

 

 

Now, a downward force is applied. Notice how the nails do not pop the balloon!

 

 

 

Necessary materials to build the apparatus: 

3 20x30 centimeter pieces of ¼ inch plywood

approximately 500 nails

4 screws

4 30 centimeter dowels

drill press

hammer

electric drill

pencil

ruler

 

The nail bed is built by cutting a sheet of ¼ inch plywood 20 centimeters wide by 30 centimeters long. Three boards of this size will be required. With one board, draw horizontal and vertical lines spaced 1 centimeter apart. Allow a 3-centimeter border on each side. Using the drill press, drill a hole at each intersection, using a drill bit slightly smaller than the diameter of the nails used. Next, on this board plus a second, drill a hole, slightly larger than the diameter of the dowel, at each of the four corners. Now we need to hammer nails through the drilled board. It is easiest to do this by propping the board up a distance greater than the length of the nails. Approximately 500 nails are required for this step, depending on the size and spacing chosen. To secure the nails, screw the third, undrilled board to the bottom of the first board. Now, flip the nail bed upright. Place dowels in each of the four corner holes. Carefully flip the apparatus upside down again. Now, using an electric drill, screw the dowels into place by using one screw in each of the four corners beneath the dowels. Now the apparatus can be flipped upright once more. Aligning each of the four dowels up with the holes drilled in the remaining board, slide the board down on top of the nail bed. A balloon can be placed on top of the nails, and this moveable board is where we can apply a downward force to compress the balloon. Pictures of the fully constructed apparatus can be seen below:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A fun in class activity: have a set of weights, each around 5-10 pounds, totalling at least 50 pounds. With a balloon in place in the bed of nails, ask students how much weight they think can be placed on the top plywood plate before the balloon pops. Write down the students guesses, then add weights incrementally until the balloon pops. The student with the closest guess wins a prize. Then, the pressure can be calculated that popped the balloon - just calculate the force from the weights, and estimate the surface area of the nail heads.