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Weightlessness is the experience of apparently having no weight.
This condition is also known as microgravity (see below). Weightlessness is not due to
an increased distance to the earth; the acceleration due to gravity at an altitude of, say, 100 km is only 3% less than at the surface of the earth.
Weightlessness means a zero g-force or zero apparent weight; acceleration on you is equal to
the acceleration due to gravity.
Overview
What humans experience as weight is not actually the force due to gravity (even though
that is the technical definition of weight). What we feel as weight is actually the normal reaction force of the ground (or
whatever surface we are in contact with) pushing upwards against us to counteract the force due to gravity.
For example, a wood block in a container in free-fall experiences
weightlessness. This is because there is no reaction to the wood block's weight from the container, as it is being pulled down
with the same acceleration. The acceleration of the container equals the acceleration of the block, which equals the acceleration
caused by gravity. When the container is at rest on the ground, however, the force on each piece of the block is not uniform.
Because the block is not accelerating, there is also a force upward that arises because the block is a solid. Each horizontal
cross section of the block experiences not only the force due to gravity on it, but also the weight of whatever portion of the
block is above it. Part of feeling weight, then, is actually experiencing a pressure gradient within one's own body.
There is another aspect of the feeling of weight that a pressure gradient does not account for, an example of which is the way
that our arms are pulled downward with respect to our body. This effect comes from the fact that something hanging is not
supported directly via a pressure from the ground. In fact the effect is almost the exact opposite of a pressure gradient, it is
a tension gradient. It occurs because each cross section of a hanging object, a rope for instance, must support the weight of
every piece below it.
Hence, in short, weightlessness has nothing to do with whether we are under the influence of a gravitational force, but has to
do with whether there are force gradients across our body. In free-fall, all parts of an object accelerate uniformly (assuming
that there are no tidal forces), and thus a human would experience no
weight.
Microgravity
The term microgravity is also used because weightlessness in e.g. a spaceship or other container is not perfect.
Causes include:
- Gravity decreases 1 ppm for every 3m increase in height.
- In a spaceship in orbit the required centripetal force is
higher at the upper side.
- Though very thin, there is some air at the level of the orbit.
The "weight" caused by the first two items is vertically outward (similar to the tides),
for the last one forward.
The microgravity symbol, µg, was used on the insignia of the Space
Shuttle flight STS-107, because this flight was devoted to microgravity research
(see picture in that article).
NASA's KC-135 Reduced Gravity Aircraft
NASA's KC-135 Reduced Gravity Aircraft is
based at Lyndon B. Johnson Space Center
and affectionately called the "vomit comet". It is an airplane that NASA flies in 6 mile
long parabolic arcs, first climbing in altitude, then falling, in such a way that
the flight path and speed correspond to that of an object without propulsion and not experiencing air friction. This is realised by propulsion and steering such that air friction is compensated and nothing
else. The result is that people inside are not pushed towards the bottom or any other side of the plane, i.e. they are
temporarily weightless, each time for a period of 25 seconds. Typically one flight lasts about two hours, in which 40 parabolas
are flown.
NASA's Zero-Gravity Research Facility
NASA's Zero-Gravity Research Facility in Cleveland, Ohio is a
145-meter vertical shaft, largely below the ground, with an integral vacuum drop chamber, in which an experiment vehicle can have
a free fall during 5 seconds, and then is decelerated at ca. 32g by pellets of expanded polystyrene. Alternatively, the vehicle starts at the bottom and is projected upward by a high-pressure
pneumatic accelerator, after which it falls down again. Then the total duration
of the weightlessness is 10 seconds.
Nearby there is also the 2.2 Second Drop Tower.
These are not for people, just for packages: the deceleration, and in the upward mode, also the acceleration, are too high,
and the vehicle is too small.
Weightlessness in a spaceship
Weightlessness for a more extended period of time occurs in a spaceship
outside the earth's atmosphere, as long as no propulsion is applied (provided that it does not rotate about its axis); orbiting
the earth this is the case except when rockets are on for flight path corrections, and until re-entry into the atmosphere.
A rocket ship that is accelerating by firing its rockets is a very different matter.
Even if the rocket is accelerating uniformly, the force is applied to the back end of the rocket by the gas escaping out the
back. This force must be transferred to each part of the ship through either pressure or tension, and thus weightlessness is not
experienced.
Weightlessness in the centre of a planet
In the centre of a planet a person would feel weightless because the pull of the surrounding mass of the planet would cancel
out. More generally, within a hollow spherically symmetrical planet, there is no gravitational field.
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