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Human Sling Shot

 

Overview

Life is full of ups and downs. Sometimes, though, moving between these extremes can be quite entertaining. Just check out the human slingshot-or ejector seat-ride. The ejector seat ride has its roots in bungee jumping. Both depend on the elasticity of bungee cords to provide a force. In bungee jumping, the elastic force is used to negatively accelerate (decelerate) and halt a jumper's body as it plummets toward the ground. The ejector seat works in reverse. Elasticity overcomes gravity by yanking upward and positively accelerating (propelling) the riders sky-high. Riders are strapped into a cage that has been pulled down to the ground, stretching the bungee cords connected high above to maximize potential energy. Then the entire unit is released skyward, converting the potential energy of the bungee cords into the kinetic energy of the riders, shooting them 45 meters (150 feet) into the air. At the moment of release, the rider feels the maximum acceleration. Of course, energy is dissipated with each pass as heat is generated by friction between the bungee cords and air and within the cords themselves. The g-forces caused by the bungee acceleration are the very same forces that affect race car drivers and astronauts. Even when we're just standing around, our bodies experience the force of 1g, as gravity pulls our bodies toward the center of Earth. On the ejector seat, force due to the ride alone may be up to 2g, but the force your body feels is actually 3g, once you add in gravity as well.The sudden shifting of your body's position upsets the fluid in your inner ear, affecting your sense of balance and triggering responses such as an increased heart rate and tightening of your stomach muscles. Your body seems incredibly heavy as g-forces push you and the blood in your body back in the direction you started from. Your heart works harder to get the oxygen- and nutrient-bearing blood back to the brain and overworked stomach muscles. This creates a temporary chemical imbalance resulting in faintness or queasiness.At the moment acceleration stops and the bungee cords stop pulling on you, the only force from then on is gravity-you actually experience free fall. Somehow, your body adapts and often you even enjoy the experience.

Activity

Take a spin on a roller coaster to find out how gravity comes into play. In the name of science, and for you to completely understand the difference between positive and negative g-forces, Newton's Apple respectfully requests that you make a sacrifice to collect data for this very important experiment. Unfortunately, you'll have to do it at an amusement park. Materials
  • amusement park with a roller coaster
  • paper
  • regular and colored pencils
  • calculator
1. First, go for a couple of rides. Pay attention to when you feel heavy and when you feel light, when you feel your body pressed hard against the seat, and when you feel disoriented or dizzy. 2. Next, sit where you can watch the roller coaster go through its ride. Sketch out as best you can a side view and an overhead view. 3. On your overhead view, color in red the parts of the ride where you felt heavy, coloring darker and heavier at the most forceful parts. This is where you were experiencing positive g-forces. 4. Color in blue the parts of the ride where you felt light, as if the restraining bar on your car was the only thing keeping you from flying out of your seat. This is where you were experiencing negative g-forces. 5. How does this color map compare to your side view? Do positive and negative g-forces correlate with altitude? Do they seem to be associated with certain parts of a turn or an incline or descent? Now mark some dizzy or disoriented spots on your color map. Are they associated with the positive or with the negative g-force experiences? 6. Extra credit: If you hold a penny in the flat palm of your hand (facing toward what is originally upward) throughout the entire ride, it never falls out. Why? Is this due to g-forces or others?

Resources

  • Chaos for fun and profit. (1994, Feb 26) Science News, p. 143.
  • Epstein, L.C.
    (1994) Thinking physics is Gedanken physics. San Francisco: Insight Press.
  • Frase, N. (1992) Bungee jumping for fun and profit. Merrillville, IN: ICS
    Books.
  • Freeman, I.M. & Durden, W.J. (1990) Physics made simple. New York:
    Doubleday.
  • Gardner, R. (1990) Famous experiments you can do. New York:
    Franklin Watts.
  • Pearce, F. (1992, Aug 29) Licensed to thrill. New Scientist,
    p. 23.
  • Wellnitz, W.R. (1993) Be a kid physicist. Blue Ridge Summit, PA: Tab
    Books.
  • Whitelaw, I. (1992) Eyewitness science: Force and motion. New York:
    Dorling Kindersley.
    Additional resources

    1. 3M Learning Software: What's the
      secret? CD-ROMs for Macintosh or Windows. (800) 219-9022.
    2. NEWTON'S APPLE
      Multimedia Collection: Physical Sciences (roller coasters). Videodisc and
      software for Macintosh and Windows. National Geographic: (800) 368-2728.
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