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Hollywood Stunts

 

Overview

Hardly a movie made today is without some kind of amazing stunt work. We all are held breathless when a person falls out of a 20-story building or when a heart-stopping car chase ends in a spectacular crash. While these may seem to be spontaneous events, every moment of every stunt is carefully planned and controlled by the scene directors, who have an understanding of the basic scientific principles at work. In falls, bodies obtain their speed because of the net acceleration due to the forces of gravity and air drag . On the earth, the acceleration rate of a free-falling body is 32 feet per second per second of fall (9.8 meters per second squared). This means that for each second the body is falling, its velocity increases 32 feet per second, up to a limiting velocity of approximately 125 miles per hour. As the velocity of a falling body increases, so does its momentum. Momentum is calculated by taking the mass of a body and multiplying that number by its velocity. When two things crash, it's the rate of change of momentum that determines the force (or "wallop") and does the damage. To reduce the chances of damage or injury, stunt designers use devices that stretch out the time it takes to stop a body's momentum. These devices "soften the blow," so to speak, both figuratively and literally. The longer the period of time used in changing the momentum , the less force will be released upon impact. Air bags are one of the devices used because they slow down the impact of the falling body by allowing the body to displace a large volume of air. The greater the displacement, the slower the final impact, and the less chance of injury. For very long falls of several hundred feet, stunt people often use decelerators: long elastic ropes that serve as a brake to limit the maximum velocity. In addition to having the proper equipment, stunt people must know about human anatomy; they must practice a lot; and they must wear the right protective gear. Even with all this knowledge and training, however, stunt people can get injured -- the scientific principles they use to their advantage can be just as effective against them.

Activity

Find out how insulation can keep heat in and out. Test several different materials to see which are the best insulators of heat energy. Find the material that will take the longest amount of time to heat up and the least amount of time to cool down. Materials
  • 3 cups polystyrene-foam packing peanuts
  • 2 wool socks
  • 3 cups sand
  • empty 1-lb. coffee can with lid
  • long Celsius lab thermometer
  • watch with a second hand OR a stopwatch
  • paper and pencil
  • scissors
  • portable 1000-watt hair dryer
  1. Set up a log sheet as suggested here:
    MaterialTemp up 10deg.Temp down 10deg.
    Wool Polystyrene foam Sand
  2. Stuff the woolen socks inside the can. Put the lid back on the can.
  3. Using scissors, poke a small hole in the lid. Slip the thermometer through the hole until it is about half-way into the can.
  4. Allow the can to sit for five minutes. Then read the temperature on the thermometer and record this as your "starting temperature."
  5. Begin heating the outside of the can by moving the hair dryer slowly around all sides of the can. Using a watch or stopwatch, start timing the experiment now. See how many seconds it takes for the thermometer to go up 10 degrees. Record this on your chart as "time up."
  6. When you hit the 10-degree mark, turn off the hair dryer and time how many seconds it takes for the temperature to go back down to the starting point. Record this on your chart as "time down." (The temperature may continue to rise after removing the heat source. Record this on your chart as the "maximum temperature.")
  7. Repeat the experiment again using the sand and foam peanuts in place of the wool socks.

Resources

    Baur, T. (1989) Special effects and stunt guide. Beverly Hills, CA: Lone
    Eagle.
    Dittman, R., and G. Schmieg. (1979) Physics in everyday life. New York:
    McGraw-Hill.
    Ehrlich, R. (1990) Turning the world inside out. Princeton, NJ: Princeton
    University Press.
    Grant, C. (1990) Stunts. New York: Tor Books.
    Hewitt, P. (1991) Conceptual physics, 2d ed. New York: Addison-Wesley.
    Additional sources of information:
    Universal Studios Hollywood
    100 Universal City Pl.
    Universal City, CA 91608
    (818) 777-3801
    Hollywood's Stuntman's Hall of Fame and Museum
    P.O. Box 277
    Moab, UT 84532
    (801) 259-6100