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Gravity

 

Overview

Your fuel gauge is below empty. Both engines of the cargo plane you're piloting have just sputtered and gone silent. The nose of the plane points down and you begin a terrifying dive toward Earth. In a panic, you make your way out of the cockpit and into the back of the plane where your parachute is stored. But a 2,000-kilogram crate is blocking your path. What do you do? No problem! Since the weight of the crate on the plane's floor is actually zero, you would not have to lift it in opposition to gravity or slide it in opposition to its friction with the floor. The force required to overcome the inertia of the crate would be small enough to allow you to move it by pushing hard with your feet braced against a wall. How is this so? Let's look at the crate under normal flight conditions. The weight of the crate pushes down against the floor of the plane. What you might not realize is that the floor, which is supported by the airplane's wings and the forces that keep the airplane aloft, also pushes up against the crate. It pushes up with a force equal to the weight of the crate, so inside the plane, you're aware of how heavy the crate is. When your plane goes into free-fall, the crate is still pulled by gravity just as during a normal flight. But the floor is no longer pushing up on the crate, since it and the crate are now falling freely toward the earth. Gravity is still acting on both the crate and the plane, but inside the airplane, without the upward push from the floor, the crate now seems to be weightless. Both the crate and the pilot will float freely inside the airplane until something--like Earth--stops them. Astronauts in orbitexperience weightlessness just like objects in the falling aircraft. A space shuttle in orbit is actually in a state of free-fall as it travels around Earth. Hard to imagine? Picture yourself in a small spaceship a few meters above the ground. Now face the setting sun and go in a straight line for about 100 kilometers (62 miles). If you go in a perfectly straight line, you should notice that Earth is curving away from you. A shuttle in orbit goes so fast that Earth curves "away" just as much as the shuttle falls. The shuttle falls, but never hits the ground!
  • Falling appears to be different for different objects. For instance, which falls faster, a pen or a piece of paper? Why might one fall faster than the other?
  • In real life, when do you experience something like free-fall? For how long?
  • Which falls faster, a one-ton plane or a ten-ton plane?

Activity

You have probably noticed an empty feeling in your stomach when an elevator starts its descent. That feeling is a result of a decrease of pressure against your feet and a corresponding change in the tightness of the muscles in your abdomen. Your feet feel less pressure, because the floor of the elevator is going out from under you momentarily. Find out how you could measure this feeling in more concrete terms, and learn which elevator has the fastest acceleration.

Materials

  • bathroom scale (not digital)
  • notebook paper
  • pencil
  1. Divide into small groups of two or three. Choose some elevators located nearby.
  2. Create a data table like the one below for each of the elevators you are going to test.
  3. Record each of your weights standing still.
  4. Take the scale into the first elevator. Then one student at a time should get the maximum reading on the scale when the elevator starts its ascent and the minimum reading when the elevator starts its descent. You must have a quick eye and should be prepared for approximate results.
  5. Have each student record his or her own data for each elevator tried.
  6. Follow steps 2 through 5 for each elevator.

    Questions

    1. What happens to your weight when you begin your ascent? How long does the change last?
    2. What happens to your weight when you begin your descent? How long does the change last?
    3. Does a person's initial weight have anything to do with the amount of change recorded?
    4. What kind of change would occur if the elevator cable were to snap? This, by the way, could never happen.

Resources

  • Covault, C. (1991, June) Columbia crew launched to gather unique zero-g life sciences
    data. Aviation Week and Space Technology, pp. 68-69.
  • Epstein, L. (1989) Thinking physics. San Francisco: Insight Press.
  • Lafferty, P. (1992) Eyewitness science: Force and motion. New York: Dorling Kindersley.
  • March, R. (1992) Physics for poets (3rd ed.). New York: McGraw-Hill.
  • Zee, A. (1989) An old man's toy: Gravity at work and play in Einstein's universe.
    New York: Macmillan.

Additional sources of information



American Institute of Physics
One Physics Ellipse
College Park, MD 20740-3843
(301) 209-3000
(Ask about Operation Physics)

NASA Education Division
Mail Code F
Washington, DC 20546
(202) 453-1000

Community resources



  • Physics department at a local college or university