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Bomb Squad

 

Overview

We often see the negative side of explosives on the news--images of twisted metal on damaged buildings or debris and rubble in a war zone. But explosives also help us construct tunnels, mine underground, remove obstacles during road construction, extinguish oil well fires, inflate automobile air bags, and destroy hazardous wastes. An explosive is a stable material that, upon stimulation, very rapidly changes from a solid or liquid into a hot, expanding gas. The sudden release of energy and the accompanying pressure exerted on the surrounding materials by the expanding gas constitute an explosion. If you confined an explosive, you would increase the likelihood of the detonation by increasing the speed of the reaction. For example, when gunpowder is confined within the paper wrapping of a firecracker, it explodes when ignited. However, the same powder sprinkled in the open simply burns when ignited. In the confined state, the heat generated at the point of ignition will immediately ignite a large amount of surrounding material. The expanding gases from all the "burning" gun powder will act together, resulting in an explosion. Explosives fall into two categories: detonating and propellant. Detonating explosives are further classified as initiating (primary) or secondary. Initiating explosives, the more sensitive of the two, must be handled with extreme care. They require only a low energy stimulus, such as being touched with a hot wire or tapped with a hammer, to explode. For that reason, they are put into blasting caps. Secondary explosives are less sensitive and can burn without producing an explosion. They only detonate by means of a severe shock, delivered by another explosive (usually a blasting cap) placed in or near them. Since they are relatively stable, large amounts can be moved and handled safely. The velocity at which explosives detonate determines their function. Explosives of low detonating velocity supply a slow push or heave. Explosives with a high detonating velocity have a bursting or shattering effect. Propellant explosives are used for firearms, rockets, general engineering, and demolition work. They differ from detonating explosions in that an avenue of escape is provided for the expanding gases. For example, when the explosive propellant in the space shuttle solid rocket boosters is ignited, all the gases from the resulting continuous explosion escape out the bottom, providing upward thrust. Unlike a detonating explosive, not all the explosive material is burned at once in a propellant explosive. Industries rely on small explosive charges for a number of purposes. In metallurgy, for example, metals can be pressed into dies, extruded, or welded together by means of such explosions. New metal alloys have been created that way.

Activity

Explosives have different detonation velocities. Some shatter and others push. With water balloons, you will simulate the range of a blast. Materials
  • meter stick
  • stepladder
  • 3 water balloons
  • thumb tack
  1. Fill three balloons with water so they are all about the same size.
  2. Move outside into a large parking lot or paved playground. 3. Place the thumb tack on the ground so that the point is facing up.
  3. Place the meter stick next to the thumb tack and measure a height of one meter from the ground.
  4. Position your balloon at arm's length over the tack at that height.
  5. Drop the balloon and measure the diameter of the water blast range from the two farthest points.
  6. Using the same procedure, climb the stepladder and drop your second balloon from a height of three meters. (Be careful on the stepladder. Make sure someone holds it steady and another person spots you.)
  7. With the third balloon, move into a clear area and throw the balloon up as high as you can. Then watch out!
  8. Record the distances dropped and the diameters of the wet areas for all three balloons.
  9. Construct a graph showing the results of the investigation. Questions

Resources

  • Bahrani, A. (1986, July 10) Engineering with a bang. New Scientist, pp. 44-47.
  • WGBH-TV Frontline videotape: Tracking the PanAm Bombers. PBS Video: (800) 328-7271.
  • Watson, R. (1989, Jan 2) An explosion in the sky. Newsweek, pp. 28-30.
  • Watson, R. (1989, Jan 9) A trail of terror. Newsweek, pp. 16-19.