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“This video takes a serious and humorous look at life in the low gravity environment of space flight. The video also includes onboard activities from Skylab to Space Shuttle missions. ”
Public domain film from NASA, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied.
Weightlessness (or zero-g) is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall. The physical path of an inertial trajectory depends only on the direction and strength of the sum of the gravitational attractions outside of the inertial reference frame.
Accelerations that are not due to gravity are called “proper accelerations” and it is only these forces (such as a push from a floor or seat) that cause g-forces. Weight is the product of mass and the g-force acceleration. Weightlessness is therefore always produced by the absence of g-forces. Accelerometers, which can measure only accelerations that produce g-force and weight, cannot detect acceleration in weightless conditions, including free fall.
The definition and use of ‘weightlessness’ is difficult unless it is understood that the sensation of “weight” is only indirectly produced by gravity, and results not from gravitation acting alone (which is not felt), but instead by the mechanical forces that resist gravity. An example is an object sitting on a scale, which does not feel gravity per se, but instead experiences only the force due to the scale that resists a gravitational fall. Once it is clear that weight is not a force exerted by gravity (but rather the ground or scale), then weightlessness in the absence of all forces except gravitation becomes easy to understand. Thus, an object in a straight free fall (as in an elevator in vacuum), or in a more complex inertial trajectory of free fall (such as within a reduced gravity aircraft or inside a space station), all experience weightlessness, since they do not experience the mechanical forces that cause the sensation of weight, or that allow the direct measurement of weight.
If objects are far from a star, planet, moon, or other such massive body, so that they experience very little gravitational interaction with them, they experience close to zero gravitation, and are also weightless. If they are close to a massive object, but are freely accelerating towards the mass by gravitational acceleration only, they are in free fall and are (as in the case near the Earth) also weightless. Objects in an inertial path (one affected no mechanical forces, but possibly affected by gravity) follow Newton’s first law of motion within the reference frame. This law describes linear motion, and with regard to an inertial frame, inertial objects do describe such a motion (this can be seen in a space station or other satellite). Such a situation, except for microgravity effects and the inhomogeneity of the gravitational field, cannot be distinguished from weightlessness due to the absence of gravity from a body nearby…
Long periods of weightlessness occur on spacecraft outside a planet’s atmosphere, provided no propulsion is applied and the vehicle is not rotating. Weightlessness does not occur when a spacecraft is firing its engines or when re-entering the atmosphere, even if the resultant acceleration is constant. The thrust provided by the engines acts at the surface of the rocket nozzle rather than acting uniformly on the spacecraft, and is transmitted through the structure of the spacecraft via compressive and tensile forces to the objects or people inside.
Weightlessness in an orbiting spacecraft is physically identical to free-fall, with the difference that gravitational acceleration causes a net change in the direction, rather than the magnitude, of the spacecraft’s velocity. This is because the acceleration vector is perpendicular to the velocity vector.
In typical free-fall, the acceleration of gravity acts along the direction of an object’s velocity, linearly increasing its speed as it falls toward the Earth, or slowing it down if it is moving away from the Earth. In the case of an orbiting spacecraft, which has a velocity vector largely perpendicular to the force of gravity, gravitational acceleration does not produce a net change in the object’s speed, but instead acts centripetally, to constantly “turn” the spacecraft’s velocity as it moves around the Earth. Because the acceleration vector turns along with the velocity vector, they remain perpendicular to each other. Without this change in the direction of its velocity vector, the spacecraft would move in a straight line, leaving the Earth altogether…
