O příspěvku
Public Demonstration Shows
Public physics demonstration shows have many benefits. They can get children excited about science, teach members of the general public some physics, and give people a more positive view of science. This paper discusses the type of demonstration show we have used and also describes two inexpensive demonstrations that have worked well at our institution.
Methods and Goals for our Physics Demonstration Shows
Physics demonstrations are used in many contexts; in classes, for small groups, and for large public shows. At a secondary school or college, producing a large public physics demonstration show can reach many people, including children and members of the public who have little experience with physics. In addition, having physics students help with producing the show gives them great experience in building equipment, troubleshooting, doing publicity, and public speaking. It can be both “project-based learning” and also outreach. Finally, it yields good publicity for the college or school and can be used to raise money if desired.
The type of show that we have produced at our college in the last two years can be described as follows. The general public was invited, through notices in newspapers and on the radio, and several individual secondary and elementary schools were contacted. The audience this spring was 170 people at least half of whom were children. The show lasted nearly 1,5 hours but could be shortened if desired; it was held in a large lecture hall on a Saturday evening.
A goal of our shows is to involve the audience as much as possible. Thus, we have focused on demonstrations where people can come up and actually do something. My personal experience has been that children in the audience become very excited by the demonstrations, more than I would have believed. Part of producing a good show is thinking about the presentation to make the demonstration more effective. Although foreign to many of us science people, a bit of showmanship can be very helpful.
Example 1: Children on balloons
One demonstration that has worked very well for us uses a sheet of “plywood”, a wood sheet which is approximately 1,25m x 2,5m x 2cm. We also take about 80 balloons and inflate them with air. The demonstration begins by putting the balloons on the floor and the wood on top of them. The audience is then asked what will happen if a person steps onto the wood sheet. The usual response is that the balloons will all pop. At this point, it is useful to take one balloon and step on it to show that it does indeed pop. Next, one must ask for volunteers to come forward to stand on the wood. Children are usually quite eager to do this, although many will cover their ears in anticipation of balloons popping.
When the first person steps onto the sheet, the audience is surprised that no balloons pop. Let other people step onto the wood sheet one by one, making sure that they are distributed evenly – don’t put them all at one end. A surprisingly large number of children, perhaps 20, may be supported in this manner. Eventually, the balloons will pop, although this may not happen unless the participants jump.
At the end of the demonstration, let the participants return to their seats and discuss the principle. For the general public we use no equations as they frighten people; but one can discuss pressure and how snowshoes work (large area), why high-heeled shoes can dent floors, etc.
Example 2: Bed of nails
A second demonstration that we use is the well-known “bed of nails”. Our bed of nails (built by the students!) is a sheet of plywood about 1m x 2m x 2cm thick with many nails driven into one side. We space the nails approximately 2 cm apart, and use galvanized nails which will not rust. It is important to pre-drill holes before hammering in the nails; the holes will guide the nails and keep them straight and even. A further refinement we use is to cut the “bed” in half and put two hinges on it, so that it may be folded up for easy storage. This is very practical and makes storage much safer.
This demonstration can be made more effective by creating a bit of drama. At the beginning, we show the bed of nails upright. We then throw an apple at it. The apple is pierced by the nails and convinces the audiences that the nails are real. Next, place the bed on the floor, and bring out the demonstrator. For this demonstration, we do not use a volunteer, although with practice it is quite safe. The person must be careful when first getting onto the bed; we always place something below the head to prevent scratching, and wear jeans and a T-shirt.
The audience is generally impressed when seeing the person lying on the bed of nails. To make it more dramatic, though, we place a concrete block on the person’s chest, then bring out a sledgehammer. It is important to wear safety goggles for this part of the demonstration, as pieces of concrete may strike the eye. After letting the audience look at this for a minute, and taking practice swings with the sledgehammer to create excitement, we then light the concrete block on fire. This is done by using an alcohol-based gel, called “firestarter” gel in the US. Alcohol is much safer than petroleum products as the flame may be doused very easily. One can now let the audience observe the flaming concrete atop the person on the bed of nails, and the sledgehammer preparing to strike it. For extra drama we sometimes have a student roast some food in the flame! Finally, the sledgehammer is raised and strikes the concrete, shattering it. The audience reaction is again usually excellent and one can make a few comments about pressure at the end of the demonstration.
Conclusions
These two demonstrations work well and are inexpensive – the needed materials are two sheets of wood, some balloons, many nails, hinges, and a concrete block. It takes some time to hammer in all the nails for the bed, but makes a good project for students to build.
We highly recommend trying a demonstration show if you have not done so; in addition to the other benefits, they are a lot of fun. It can work well to do a small practice show first, with just a few friends in the audience, to work out any problems that may occur.