David drops in on an experiment to test weightlessness.
Segment length: 9:30
- Are all objects equally affected by gravity?
- Does a bullet that's been shot actually fall?
- Does anything get in the way of gravity?
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 orbit experience weightlessness
just like objects in the falling aircraft. A space shuttle in orbit is actually in a state of free-fall as it travels
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?
acceleration change in speed during a certain
period of time
ascent going up
descent going down
force that which, when acting alone on an object, causes
a change in the motion of the object
gravity force on Earth which pulls all objects toward
orbit falling around and around Earth
resistance a force opposing the motion of an object
or opposing the forces trying to set an object in motion
weightlessness feeling or being observed as
having no weight
- Ardley, N. (1992) The science book of gravity. Chicago: Harcourt Brace.
- Boslough, J. (1989, May) Searching for the secrets of gravity. National Geographic,
- 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
(Ask about Operation Physics)
NASA Education Division
Mail Code F
Washington, DC 20546
- Physics department at a local college or university
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.
- bathroom scale (not digital)
- notebook paper
- Divide into small groups of two or three. Choose some elevators located nearby.
- Create a data table like the one below for each of the elevators you are going to
- Record each of your weights standing still.
- 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.
- Have each student record his or her own data for each elevator tried.
- Follow steps 2 through 5 for each elevator.
- What happens to your weight when you begin your ascent? How long does the change
- What happens to your weight when you begin your descent? How long does the change
- Does a person's initial weight have anything to do with the amount of change
- What kind of change would occur if the elevator cable were to snap? This, by the
way, could never happen.
Place a half-gallon plastic milk jug upright and punch a small hole 2.5 cm (1") up
from the bottom. Placing your finger over the hole, fill the jug with water. Go to a
playground with a friend as an observer. Find a high place and make sure nobody is
standing directly below. Take your finger off the hole and drop the jug. What does
the observer see happening to the flow of water during the drop?
Sky divers are in a state of free-fall only at the beginning of the jump. Think of
ways in which a sky diver could speed up or slow down the descent. Which body positions
would be best for speed? Which would be best for hanging in the air?
Does the weight of objects affect how fast they fall? Take 14 pennies. Make two piles
of seven. Tape one pile together. Hold one pile in your right hand and the other in
your left. Reach both hands up high and cleanly drop the two piles at the same time.
The taped pile is at least seven times the weight of each penny in the other pile.
You know that water falls downhill. What if the whole waterfall falls? Place a
half-gallon plastic milk jug upright and punch a small hole 2.5 cm (1") up from
the bottom. Place your finger over the hole and fill the jug with water. Go outside
and remove your finger to watch the water flow. Now, gently toss the jug up and down
without completely letting go. What happens?
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